Loading
Airframe Part 3
11-62 illustrates an electronic circuit diagram that
performs the logic NOT gate function. Any input, either a
no voltage or voltage condition, yields the opposite output.
This gate is built with bipolar junction transistors, resistors,
and a few diodes. Other designs exist that may have different
components.
When examining and discussing digital electronic circuits, the
electronic circuit design of a gate is usually not presented. The
symbol for the logic gate is most often used. [Figure 11-61]
The technician can then concentrate on the configuration of
the logic gates in relation to each other. A brief discussion of
the other logic gates, their symbols, and truth tables follow.
Buffer Gate
Another logic gate with only one input and one output is the
buffer. It is a gate with the same output as the input. While
this may seem redundant or useless, an amplifier may be
considered a buffer in a digital circuit because if there is
voltage present at the input, there is an output voltage. If
there is no voltage at the input, there is no output voltage.
When used as an amplifier, the buffer can change the values
of a signal. This is often done to stabilize a weak or varying
signal. All gates are amplifiers subject to output fluctuations.
The buffer steadies the output of the upstream device while
maintaining its basic characteristic. Another application of a
buffer that is two NOT gates, is to use it to isolate a portion
of a circuit. [Figure 11-63]
AND Gate
Most common logic gates have two inputs. Three or more
inputs are possible on some gates. When considering the
characteristics of any logic gate, an output of Logic 1 is
sought and a condition for the inputs is stated or examined.
For example, Figure 11-64 illustrates an AND gate. For an
AND gate to have a Logic 1 output, both inputs have to be
Logic 1. In an actual electronic circuit, this means that for a
voltage to be present at the output, the AND gate circuit has to
receive voltage at both of its inputs. As pointed out, there are
different arrangements of electronic components that yield
this result. Whichever is used is summarized and presented
as the AND gate symbol. The truth table in Figure 11-64
illustrates that there is only one way to have an output of
Logic 1 or voltage when using an AND gate.
OR Gate
Another useful and common logic gate is the OR gate. In an
OR gate, to have an output of Logic 1 (voltage present), one
of the inputs must be Logic 1. As seen in Figure 11-65, only
one of the inputs needs to be Logic 1 for there to be an output
of Logic 1. When both inputs are Logic 1, the OR gate has
a Logic 1 output because it still meets the condition of one
of the inputs being Logic 1.
11-28
Input Output
InputA
InputB
Output
A
0
0
1
1
B
0
1
0
1
1
1
1
The NAND gate 0
Figure 11-66. A NAND gate symbol and its truth table illustrating
that the NAND gate is an inverted AND gate.
Input Output
InputA
InputB
Output
A
0
0
1
1
B
0
1
0
1
1
0
0
The NOR gate 0
Figure 11-67. A NOR gate symbol and its truth table illustrating
that the NOR gate is an inverted OR gate.
Input Output
InputA
InputB
Output
A
0
0
1
1
B
0
1
0
1
0
1
1
The Exclusive OR gate 0
Figure 11-68. An EXCLUSIVE OR gate symbol and its truth table,
which is similar to an OR gate but excludes output when both inputs
are the same.
Input Output
InputA
InputB
Output
A
0
0
1
1
B
0
1
0
1
0
1
1
The OR gate 1
Figure 11-65. An OR gate symbol and its truth table.
NAND Gate
The AND, OR, and NOT gates are the basic logic gates. A
few other logic gates are also useful. They can be derived
from combining the AND, OR, and NOT gates. The NAND
gate is a combination of an AND gate and a NOT gate.
This means that AND gate conditions must be met and then
inverted. So, the NAND gate is an AND gate followed by
a NOT gate. The truth table for a NAND gate is shown in
Figure 11-66 along with its symbol. If a Logic 1 output is
to exist from a NAND gate, inputs A and B must not both
be Logic 1. Or, if a NAND gate has both inputs Logic 1, the
output is Logic 0. Stated in electronic terms, if there is to be
an output voltage, then the inputs cannot both have voltage
or, if both inputs have voltage, there is no output voltage.
NOTE: The values in the output column of the NAND gate
table are exactly the opposite of the output values in the
AND gate truth table.
NOR Gate
A NOR gate is similarly arranged except that it is an inverted
OR gate. If there is to be a Logic 1 output, or output voltage,
then neither input can be Logic 1 or have input voltage. This
is the same as satisfying the OR gate conditions and then
putting output through a NOT gate. The NOR gate truth table
in Figure 11-67 shows that the NOR gate output values are
exactly the opposite of the OR gate output values.
The NAND gate and the NOR gate have a unique distinction.
Each one can be the only gate used in circuitry to produce
the same output as any of the other logic gates. While it may
be inefficient, it is testimonial to the flexibility that designers
have when working with logic gates, the NAND and NOR
gates in particular.
EXCLUSIVE OR Gate
Another common logic gate is the EXCLUSIVE OR gate.
It is the same as an OR gate except for the condition where
both inputs are Logic 1. In an OR gate, there would be Logic
1 output when both inputs are Logic 1. This is not allowed
in an EXCLUSIVE OR gate. When either of the inputs is
Logic 1, the output is Logic 1. But, if both inputs are logic
1, the Logic 1 output is excluded or Logic 0. [Figure 11-68]
Negative Logic Gates
There are also negative logic gates. The negative OR and
the negative AND gates are gates wherein the inputs are
inverted rather than inverting the output. This creates a unique
set of outputs as seen in the truth tables in Figure 11-69.
The negative OR gate is not the same as the NOR gate as
is sometimes misunderstood. Neither is the negative AND
gate the same as the NAND gate. However, as the truth tables
reveal, the output of a negative AND gate is the same as a
NOR gate, and the output of a negative OR gate is the same
as a NAND gate.
In summary, electronic circuits use transistors to construct
logic gates that produce outputs related to the inputs shown
in the truth tables for each kind of gate. The gates are then
11-29
Input Output
A
0
0
1
1
B
0
1
0
1
1
0
0
0
Input Output
A
0
0
1
1
B
0
1
0
1
1
1
1
0
The Negative AND gate
The Negative OR gate
InputA
InputB
Output
InputA
InputB
Output
A.
B.
Figure 11-69. The NEGATIVE AND gate symbol and its truth table
(A) and the NEGATIVE OR gate symbol and truth table (B). The
inputs are inverted in the NEGATIVE gates.
N-S E-W
XPDR 5537 IDNT LCL23:00:34
VOR 1
270°
2
1
1
2
4300
4200
4100
4000
3900
3800
4300
3600
3500
3400
3300
3200
3100
60
20
4000
4000
130
120
110
90
80
70
1
100
9
TAS 100KT
OAT 7°C
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 360°
XPDR 5537 IDNT LCL23:00:34
VOR 1
270°
2
1
1
2
4300
4200
4100
4000
3900
3800
4300
3600
3500
3400
3300
3200
3100
60
20
4000
4000
130
120
110
90
80
70
1
100
9
TAS 100KT
OAT 7°C
NAV1 108.00 113.00
NAV2 108.00 110.60
134.000 118.000 COM1
123.800 118.000 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 360°
Figure 11-70. A modern glass cockpit on a general aviation aircraft. Digital data displays replace many older instruments and indicators
of the past.
assembled with other components to manipulate data in digital
circuits. The electronic digital signals used are voltage or no
voltage representations of Logic 1 or Logic 0 conditions. By
using a series of voltage output or no voltage output gates,
manipulation, computation, and storage of data takes place.
Digital Aircraft Systems
Digital aircraft systems are the present and future of aviation.
From communication and navigation to engine and flight
controls, increased proliferation of digital technology
increases reliability and performance. Processing, storing,
and transferring vital information for the operation of an
aircraft in digital form provides a usable common language
for monitoring, control, and safety. Integration of information
from different systems is simplified. Self monitoring, built-in
test equipment (BITE) and air-to-ground data links increase
maintenance efficiency. Digital buss networking allows
aircraft system computers to interact for a coordinated
comprehensive approach to flight operations.
Digital Data Displays
Modern digital data displays are the most visible features
of digital aircraft systems. They extend the functional
advantages of state of the art digital communication and
navigation avionics and other digital aircraft systems via the
use of an enhanced interface with the pilot. The result is an
increase in situational awareness and overall safety of flight.
Digital data displays are the glass of the glass cockpit. They
expand the amount, clarity, and proximity of the information
presented to the pilot. [Figure 11-70]
Many digital data displays are available from numerous
manufacturers as original equipment in new aircraft, or
as retrofit components or complete retrofit systems for
older aircraft. Approval for retrofit displays is usually
accomplished through supplementary type certificate (STC)
awarded to the equipment manufacturer.
Early digital displays presented scale indication in digital
or integer format readouts. Today’s digital data displays
are analogous to computer screen presentations. Numerous
aircraft and flight instrument readouts and symbolic
presentations are combined with communication and
navigational information on multifunctional displays (MFD).
Often a display has a main function with potential to back-up
another display should it fail. Names, such as primary flight
display (PFD), secondary flight display, navigational display
(ND), etc., are often used to describe a display by its primary
use. The hardware composition of the displays is essentially
the same. Avionics components and computers combine to
provide the different information portrayed on the displays.
11-30
MENU
RNG
+
-
REV
_
_
_
_
_
MODE
SYNC
MODE
SYNC
160 6500
6400
6200
6100
150
140
130
110
100
90
TAS 134 kt
GS 119 kt OAT 11°C 148°/16 kt
29.92 in
120
1
9
6280
6500
6560
105 kt 8500
10
10
20
10
10
20
MIN
5520
Vle
Va
Vlo
Vlg
ILS1
CRS VLOC1 HDG
110.90 164°
CRS HDG −580
199° FPM
ILS1
TRFC
10nm
163°
RW16R
KANO VERDI
-01
+02 21
24
W
30
N 33
3
6
E
12
4SD
TAKLE
15 S
.5
2−2
LTNGNXRD
TRFC
NRM
1
2
MENU
RNG
+
-
REV
_
_
_
_
_
MODE
SYNC
MODE
SYNC
160 6500
6400
6200
6100
150
140
130
110
100
90
TAS 134 kt
GS 119 kt OAT 11°C 148°/16 kt
29.92 in
120
1
9
6280
6500
6560
105 kt 8500
10
10
20
10
10
20
MIN
5520
Vle
Va
Vlo
Vlg
ILS1
CRS VLOC1 HDG
110.90 164°
CRS HDG −580
199° FPM
ILS1
TRFC
10nm
163°
RW16R
KANO VERDI
-01
+02 21
24
W
30
N 33
3
6
E
12
4SD
TAKLE
15 S
.5
2−2
LTNGNXRD
TRFC
NRM
1
2
Figure 11-71. A retrofit digital data display.
Figure 11-72. A digital data display dedicated to the depiction of
engine and airframe system parameter status.
Controls on the instrument panel or on the display unit itself
are used for selection. Some screens have limited display
capability because they are not part of a totally integrated
system; however, they are extremely power electronic units
with wide capability. [Figure 11-71]
The basis of the information displayed on what is known as
a primary flight display (PFD), is usually an electronic flight
instrument system (EFIS) like representation of the aircraft
attitude indicator in the upper half of the display, and an
electronic horizontal situation indicator display on the lower
half. Numerous ancillary readouts are integrated or surround
the electronic attitude indicator and the horizontal situation
indicator (HSI). On full glass cockpit PFDs, all of the basic
T instrument indications are presented and much more, such
as communication and navigation information, weather data,
terrain features, and approach information. Data displays for
engine parameters, hydraulics, fuel, and other airframe systems
are often displayed on the secondary flight display or on an
independent display made for this purpose. [Figure 11-72]
As with other avionics components, repair and maintenance
of the internal components of digital data displays is reserved
for licensed repair stations only.
Digital Tuners and Audio Panels
Numerous communication and navigation devices are
described in the following sections of this chapter. Many
of these use radio waves and must be tuned to a desired
frequency for operation. As a flight progresses, retuning
and changing from one piece of equipment to another can
occur frequently. An audio panel or digital tuner consolidates
various communication and navigation radio selection
controls into a single unit. The pilot can select and use, or
select and tune, most of the aircraft’s avionics from this one
control interface. [Figure 11-73]
Radio Communication
Much of aviation communication and navigation is
accomplished through the use of radio waves. Communication
by radio was the first use of radio frequency transmissions
in aviation.
11-31
Figure 11-73. An audio panel in a general aviation aircraft integrates the selection of several radio-based communication and navigational
aids into a single control panel (left). A digital tuner (right, Image © Rockwell Collins, Inc.) does the same on a business class aircraft
and allows the frequency of each device to be tuned from the same panel as well.
10-10
1020 1019 1018 1017 1016 1015 1014 1013 1012 1011 1010 109 108 107 106
10-9 10-8 10-7 10-6 10-5 10-4 10-3 10-2 10-1 1 10 100 1,000 10,000 100,000
Gamma ray X-ray Ultraviolet Infrared Radio
UHF VHF HF MF LF VLF
Wavelength (centimeters)
Frequency (number of waves per second) Visible
Shorter
Higher
Longer
Lower
Electronic Spectrum
Figure 11-74. Radio waves are just some of the electromagnetic waves found in space.
Radio Waves
A radio wave is invisible to the human eye. It is electromagnetic
in nature and part of the electronic spectrum of wave
activity that includes gamma rays, x-rays, ultraviolet rays,
infrared waves, and visible light rays, as well all radio
waves. [Figure 11-74] The atmosphere is filled with these
waves. Each wave occurs at a specific frequency and has
a corresponding wavelength. The relationship between
frequency and wavelength is inversely proportional. A high
frequency wave has a short wave length and a low frequency
wave has a long wave length.
In aviation, a variety of radio waves are used for
communication. Figure 11-75 illustrates the radio spectrum
that includes the range of common aviation radio frequencies
and their applications.
NOTE: A wide range of frequencies are used from low
frequency (LF) at 100 kHz (100,000 cycles per second) to
super high frequency (SHF) at nearly 10gHz (10,000,000,000
cycles per second). The Federal Communications Commission
(FCC) controls the assignment of frequency usage.
AC power of a particular frequency has a characteristic
length of conductor that is resonant at that frequency. This
length is the wavelength of the frequency that can be seen on
an oscilloscope. Fractions of the wavelength also resonate,
especially half of a wavelength, which is the same as half of
the AC sign wave or cycle.
The frequency of an AC signal is the number of times the
AC cycles every second. AC applied to the center of a radio
antenna, a conductor half the wavelength of the AC frequency,
travels the length of the antenna, collapses, and travels the
length of the antenna in the opposite direction. The number
of times it does this every second is known as the radio wave
signal frequency or radio frequency as shown in Figure 11-75.
As the current flows through the antenna, corresponding
electromagnetic and electric fields build, collapse, build in
the opposite direction, and collapse again. [Figure 11-76]
To transmit radio waves, an AC generator is placed at the
midpoint of an antenna. As AC current builds and collapses in
the antenna, a magnetic field also builds and collapses around
it. An electric field also builds and subsides as the voltage
shifts from one end of the antenna to the other. Both fields,
the magnetic and the electric, fluctuate around the antenna at
the same time. The antenna is half the wavelength of the AC
signal received from the generator. At any one point along
the antenna, voltage and current vary inversely to each other.
11-32
Loran C 100 KHz
ADF 200 - 1600 KHz
NDBs 190 - 535 KHz
AM broadcast 550 - 1800 KHz
HF comm 2 - 30 MHz
Marker beacons 75 MHz
FM broadcast 88 - 108 MHz
VHF NAV (VOR) 108 - 118 MHz
VHF comm 118 - 137 MHz
Glideslope 328 - 336 MHz
DME 960 - 1215 MHz
Transponder 1030 & 1090 MHz
GPS 1.6 GHz
Radar altimeter 4.3 GHz
Doppler NAV 8.8 GHz
Weather radar 9.375 GHz
Radio Frequencies Aviation Uses
300 GHz
30 GHz
3 GHz
300 MHz
30 MHz
3 MHz
300 KHz
30 KHz
3 KHz
Very low
frequency
Very low
frequency (LF)
Medium
frequency (MF)
High frequency
(HF)
Very high
frequency (VHF)
Ultra high
frequency (UHF)
Super high
frequency (SHF)
Extremely high
frequency (EHF)
Figure 11-75. There is a wide range of radio frequencies. Only the
very low frequencies and the extremely high frequencies are not
used in aviation.
Because of the speed of the AC, the electromagnetic fields
and electric fields created around the antenna do not have time
to completely collapse as the AC cycles. Each new current
flow creates new fields around the antenna that force the nottotally-
collapsed fields from the previous AC cycle out into
space. These are the radio waves. The process is continuous
as long as AC is applied to the antenna. Thus, steady radio
waves of a frequency determined by the input AC frequency
propagate out into space.
Radio waves are directional and propagate out into space at
186,000 miles per second. The distance they travel depends on
the frequency and the amplification of the signal AC sent to the
antenna. The electric field component and the electromagnetic
field component are oriented at 90° to each other, and at 90°
to the direction that the wave is traveling. [Figure 11-77]
Types of Radio Waves
Radio waves of different frequencies have unique
characteristics as they propagate through the atmosphere.
Very low frequency (VLF), LF, and medium frequency
(MF) waves have relatively long wavelengths and utilize
correspondingly long antennas. Radio waves produced at
these frequencies ranging from 3kHz to 3mHz are known
as ground waves or surface waves. This is because they
follow the curvature of the earth as they travel from the
broadcast antenna to the receiving antenna. Ground waves are
particularly useful for long distance transmissions. Automatic
direction finders (ADF) and LORAN navigational aids use
these frequencies. [Figure 11-78]
High frequency (HF) radio waves travel in a straight line
and do not curve to follow the earth’s surface. This would
limit transmissions from the broadcast antenna to receiving
antennas only in the line-of-sight of the broadcast antenna
except for a unique characteristic. HF radio waves bounce
off of the ionosphere layer of the atmosphere. This refraction
extends the range of HF signals beyond line-of-sight. As a
result, transoceanic aircraft often use HF radios for voice
communication. The frequency range is between 2 to 25
MHz. These kinds of radio waves are known as sky waves.
[Figure 11-78]
Above HF transmissions, radio waves are known as space
waves. They are only capable of line-of-sight transmission
and do not refract off of the ionosphere. [Figure 11-78]
Most aviation communication and navigational aids operate
with space waves. This includes VHF (30-300MHz), UHF
(300MHz-3GHz), and super high frequency (SHF) (3Ghz-
30Ghz) radio waves.
VHF communication radios are the primary communication
radios used in aviation. They operate in the frequency range
from 118.0 MHz to 136.975MHz. Seven hundred and twenty
separate and distinct channels have been designated in this
range with 25 kilohertz spacing between each channel. Further
division of the bandwidth is possible, such as in Europe
where 8.33 kilohertz separate each VHF communication
channel. VHF radios are used for communications between
aircraft and air traffic control (ATC), as well as air-to-air
communication between aircraft. When using VHF, each
party transmits and receives on the same channel. Only one
party can transmit at any one time.
Loading Information onto a Radio Wave
The production and broadcast of radio waves does not convey
any significant information. The basic radio wave discussed
above is known as a carrier wave. To transmit and receive
useful information, this wave is altered or modulated by
an information signal. The information signal contains the
unique voice or data information desired to be conveyed. The
modulated carrier wave then carries the information from the
transmitting radio to the receiving radio via their respective
11-33
Antenna
To transmit radio waves, an AC generator is placed at the
midpoint of an antenna.
As AC current builds and collapses in the antenna, a magnetic
field also builds and collapses around it.
An electric field also builds and subsides as the voltage shifts
from one end of the antenna to the other.
Both fields, the magnetic and the electric, fluctuate around the
antenna at the same time.
The antenna is ½ the wavelength of the AC signal received
from the generator.
At any one point along the antenna, voltage and current
vary inversely to each other.
Generator
I Magnetic field
Magnetic
field
Electric field
I
Electric field
Current
Voltage
2
Figure 11-76. Radio waves are produced by applying an AC signal to an antenna. This creates a magnetic and electric field around the
antenna. They build and collapse as the AC cycles. The speed at which the AC cycles does not allow the fields to completely collapse
before the next fields build. The collapsing fields are then forced out into space as radio waves.
11-34
Ionosphere
Radio station
Repeater
Receiver
Line-of-sight
Line-of-sight (VHF-UHF)
Space waves
Sky wave (HF)
Ground wave (VLF to MF)
Receiver
Figure 11-78. Radio waves behave differently in the atmosphere depending in their frequency.
Direction of propagation
Electric field
Magnetic field
Figure 11-77. The electric field and the magnetic field of a radio wave are perpendicular to each other and to the direction of propagation
of the wave.
antennas. Two common methods of modulating carrier waves
are amplitude modulation and frequency modulation.
Amplitude Modulation (AM)
A radio wave can be altered to carry useful information by
modulating the amplitude of the wave. A DC signal, for example
from a microphone, is amplified and then superimposed over
the AC carrier wave signal. As the varying DC information
signal is amplified, the amplifier output current varies
proportionally. The oscillator that creates the carrier wave does
so with this varying current. The oscillator frequency output
is consistent because it is built into the oscillator circuit. But
the amplitude of the oscillator output varies in relation to the
fluctuating current input. [Figure 11-79]
When the modulated carrier wave strikes the receiving
antenna, voltage is generated that is the same as that which
was applied to the transmitter antenna. However, the signal
is weaker. It is amplified so that it can be demodulated.
Demodulation is the process of removing the original
information signal from the carrier wave. Electronic circuits
containing capacitors, inductors, diodes, filters, etc., remove
11-35
A. 121.5 MHz carrier
B. Varying DC audio information
C. Amplitude modulated carrier leaving transmitter
+
−
0
+
−
0
Figure 11-79. A DC audio signal modifies the 121.5 MHz carrier
wave as shown in C. The amplitude of the carrier wave (A) is
changed in relation to modifier (B). This is known as amplitude
modulation (AM).
A. Amplitude modulated carrier in receiver
B. Detected modulated carrier
C. Demodulated signal
D. Audio frequency signal in speaker
+
0
+
0
+
0
+
−
0
Figure 11-80. Demodulation of a received radio signal involves
separating the carrier wave from the information signal.
all but the desired information signal identical to the original
input signal. Then, the information signal is typically
amplified again to drive speakers or other output devices.
[Figure 11-80]
AM has limited fidelity. Atmospheric noises or static alter
the amplitude of a carrier wave making it difficult to separate
the intended amplitude modulation caused by the information
signal and that which is caused by static. It is used in aircraft
VHF communication radios.
Frequency Modulation (FM)
Frequency modulation (FM) is widely considered superior
to AM for carrying and deciphering information on radio
waves. A carrier wave modulated by FM retains its constant
amplitude. However, the information signal alters the
frequency of the carrier wave in proportion to the strength of
the signal. Thus, the signal is represented as slight variations
to the normally consistent timing of the oscillations of the
carrier wave. [Figure 11-81]
Since the transmitter oscillator output fluctuates during
modulation to represent the information signal, FM bandwidth
is greater than AM bandwidth. This is overshadowed by the
ease with which noise and static can be removed from the
FM signal. FM has a steady current flow and requires less
power to produce since modulating an oscillator producing a
carrier wave takes less power than modulating the amplitude
of a signal using an amplifier.
Demodulation of an FM signal is similar to that of an AM
receiver. The signal captured by the receiving antenna is
usually amplified immediately since signal strength is lost as
11-36
Modulating
signal
FM signal
Figure 11-81. A frequency modulated (FM) carrier wave retains
the consistent amplitude of the AC sign wave. It encodes the unique
information signal with slight variations to the frequency of the
carrier wave. These variations are shown as space variations
between the peaks and valleys of the wave on an oscilloscope.
Lower sidebands Upper sidebands
AM bandwidth
Carrier
Figure 11-82. The bandwidth of an AM signal contains the carrier
wave, the carrier wave plus the information signal frequencies, and
the carrier wave minus the information signal frequencies.
the wave travels through the atmosphere. Numerous circuits
are used to isolate, stabilize, and remove the information
from the carrier wave. The result is then amplified to drive
the output device.
Single Side Band (SSB)
When two AC signals are mixed together, such as when a
carrier wave is modulated by an information signal, three
main frequencies result:
1. Original carrier wave frequency;
2. Carrier wave frequency plus the modulating
frequency; and
3. Carrier wave frequency minus the modulating
frequency.
Due to the fluctuating nature of the information signal, the
modulating frequency varies from the carrier wave up or
down to the maximum amplitude of the modulating frequency
during AM. These additional frequencies on either side of the
carrier wave frequency are known as side bands. Each side
band contains the unique information signal desired to be
conveyed. The entire range of the lower and upper sidebands
including the center carrier wave frequency is known as
bandwidth. [Figure 11-82]
There are a limited number of frequencies within the usable
frequency ranges (i.e., LF, HF, and VHF). If different
broadcasts are made on frequencies that are too close
together, some of the broadcast from one frequency interfere
with the adjacent broadcast due to overlapping side bands.
The FCC divides the various frequency bands and issues
rules for their use. Much of this allocation is to prevent
interference. The spacing between broadcast frequencies
is established so that a carrier wave can expand to include
the upper and lower side bands and still not interfere with a
signal on an adjacent frequency.
As use of the radio frequencies increases, more efficient
allocation of bandwidth is imperative. Sending information
via radio waves using the narrowest bandwidth possible is
the focus of engineering moving forward. At the same time,
fully representing all of the desired information or increasing
the amount of information conveyed is also desired. Various
methods are employed to keep bandwidth to a minimum,
many of which restrict the quality or quantity of information
able to be transmitted.
In lower frequency ranges, such as those used for ground
wave and some sky wave broadcasts, SSB transmissions
are a narrow bandwidth solution. Each side band represents
the initial information signal in its entirety. Therefore in
an SSB broadcast, the carrier wave and either the upper
or lower sidebands are filtered out. Only one sideband
with its frequencies is broadcast since it contains all of the
needed information. This cuts the bandwidth required in
half and allows more efficient use of the radio spectrum.
SSB transmissions also use less power to transmit the same
amount of information over an equal distance. Many HF longdistance
aviation communications are SSB. [Figure 11-83]
11-37
Lower sidebands
Upper sidebands
are removed
SSB bandwidth
Carrier is
removed
Figure 11-83. The additional frequencies above and below the
carrier wave produced during modulation with the information
signal are known as sidebands. Each sideband contains the unique
information of the information signal and can be transmitted
independent of the carrier wave and the other sideband.
Frequency
oscillator
Audio
microphone
Audio
processing
Frequency
multiplier
Modulator
Power
amplifier
Figure 11-84. Block diagram of a basic radio transmitter.
Radio Transmitters and Receivers
Radio transmitters and receivers are electronic devices that
manipulate electricity resulting in the transmission of useful
information through the atmosphere or space.
Transmitters
A transmitter consists of a precise oscillating circuit or
oscillator that creates an AC carrier wave frequency. This
is combined with amplification circuits or amplifiers. The
distance a carrier wave travels is directly related to the
amplification of the signal sent to the antenna.
Other circuits are used in a transmitter to accept the input
information signal and process it for loading onto the carrier
wave. Modulator circuits modify the carrier wave with the
processed information signal. Essentially, this is all there is
to a radio transmitter.
NOTE: Modern transmitters are highly refined devices with
extremely precise frequency oscillation and modulation. The
circuitry for controlling, filtering, amplifying, modulating,
and oscillating electronic signals can be complex.
A transmitter prepares and sends signals to an antenna that, in
the process described above, radiates the waves out into the
atmosphere. A transmitter with multiple channel (frequency)
capability contains tuning circuitry that enables the user to
select the frequency upon which to broadcast. This adjusts
the oscillator output to the precise frequency desired. It is
the oscillator frequency that is being tuned. [Figure 11-84]
As shown in Figure 11-84, most radio transmitters generate
a stable oscillating frequency and then use a frequency
multiplier to raise the AC to the transmitting frequency. This
allows oscillation to occur at frequencies that are controllable
and within the physical working limits of the crystal in
crystal-controlled oscillators.
Receivers
Antennas are simply conductors of lengths proportional to
the wavelength of the oscillated frequency put out by the
transmitter. An antenna captures the desired carrier wave
as well as many other radio waves that are present in the
atmosphere. A receiver is needed to isolate the desired carrier
wave with its information. The receiver also has circuitry
to separate the information signal from the carrier wave.
It prepares it for output to a device, such as speakers or a
display screen. The output is the information signal originally
introduced into the transmitter.
A common receiver is the super heterodyne receiver. As with
any receiver, it must amplify the desired radio frequency
captured by the antenna since it is weak from traveling
through the atmosphere. An oscillator in the receiver is
used to compare and select the desired frequency out of all
of the frequencies picked up by the antenna. The undesired
frequencies are sent to ground.
A local oscillator in the receiver produces a frequency that is
different than the radio frequency of the carrier wave. These
two frequencies are mixed in the mixer. Four frequencies
result from this mixing. They are the radio frequency, the
local oscillator frequency, and the sum and difference of
these two frequencies. The sum and difference frequencies
contain the information signal.
The frequency that is the difference between the local
oscillator frequency and the radio frequency carrier wave
frequency is used during the remaining processing. In VHF
aircraft communication radios, this frequency is 10.8 MHz.
Called the intermediate frequency, it is amplified before it is
sent to the detector. The detector, or demodulator, is where
the information signal is separated from the carrier wave
portion of the signal. In AM, since both sidebands contain
the useful information, the signal is rectified leaving just one
sideband with a weak version of the original transmitter input
signal. In FM receivers, the varying frequency is changed to
a varying amplitude signal at this point. Finally, amplification
occurs for the output device. [Figure 11-85]
11-38
Figure 11-86. VHF aircraft communication transceivers.
RF
amplifier
Local
oscillator
Mixer IF
amplifier
Detector/
Demodulator
AF
amplifier
Figure 11-85. The basic stages used in a receiver to produce an
output from a radio wave.
Over the years, with the development of transistors, microtransistors,
and integrated circuits, radio transmitters and
receivers have become smaller. Electronic bays were
established on older aircraft as remote locations to mount
radio devices simply because they would not fit in the flight
deck. Today, many avionics devices are small enough to be
mounted in the instrument panel, which is customary on most
light aircraft. Because of the number of communication and
navigation aids, as well as the need to present an uncluttered
interface to the pilot, most complicated aircraft retain an area
away from the flight deck for the mounting of avionics. The
control heads of these units remain on the flight deck.
Transceivers
A transceiver is a communication radio that transmits
and receives. The same frequency is used for both. When
transmitting, the receiver does not function. The push to
talk (PTT) switch blocks the receiving circuitry and allows
the transmitter circuitry to be active. In a transceiver, some
of the circuitry is shared by the transmitting and receiving
functions of the device. So is the antenna. This saves space
and the number of components used. Transceivers are half
duplex systems where communication can occur in both
directions but only one party can speak while the other
must listen. VHF aircraft communication radios are usually
transceivers. [Figure 11-86]
Antennas
As stated, antennas are conductors that are used to transmit
and receive radio frequency waves. Although the airframe
technician has limited duties in relation to maintaining and
repairing avionics, it is the responsibility of the technician to
install, inspect, repair, and maintain aircraft radio antennas.
Three characteristics are of major concern when considering
antennas:
1. Length
2. Polarization
3. Directivity
The exact shape and material from which an antenna is made
can alter its transmitting and receiving characteristics. Also
note that some non-metallic aircraft have antennas imbedded
into the composite material as it is built up.
Length
When an AC signal is applied to an antenna, it has a
certain frequency. There is a corresponding wavelength for
that frequency. An antenna that is half the length of this
wavelength is resonant. During each phase of the applied
AC, all voltage and current values experience the full range
of their variability. As a result, an antenna that is half the
wavelength of the corresponding AC frequency is able to
allow full voltage and full current flow for the positive phase
of the AC signal in one direction. The negative phase of
the full AC sign wave is accommodated by the voltage and
current simply changing direction in the conductor. Thus, the
applied AC frequency flows through its entire wavelength,
first in one direction and then in the other. This produces the
strongest signal to be radiated by the transmitting antenna. It
also facilitates capture of the wave and maximum induced
voltage in the receiving antenna. [Figure 11-87]
11-39
2
Figure 11-87. An antenna equal to the full length of the applied AC
frequency wavelength would have the negative cycle current flow
along the antenna as shown by the dotted line. An antenna that is
½ wavelength allows current to reverse its direction in the antenna
during the negative cycle. This results in low current at the ends of
the ½ wavelength antenna and high current in the center. As energy
radiates into space, the field is strongest 90° to the antenna where
the current flow is strongest.
Most radios, especially communication radios, use the same
antenna for transmitting and receiving. Multichannel radios
could use a different length antenna for each frequency,
however, this is impractical. Acceptable performance
can exist from a single antenna half the wavelength of a
median frequency. This antenna can be made effectively
shorter by placing a properly rated capacitor in series with
the transmission line from the transmitter or receiver. This
electrically shortens the resonant circuit of which the antenna
is a part. An antenna may be electrically lengthened by
adding an inductor in the circuit. Adjusting antenna length
in this fashion allows the use of a single antenna for multiple
frequencies in a narrow frequency range.
Many radios use a tuning circuit to adjust the effective
length of the antenna to match the wavelength of the
desired frequency. It contains a variable capacitor and an
inductor connected in parallel in a circuit. Newer radios use
a more efficient tuning circuit. It uses switches to combine
frequencies from crystal controlled circuits to create a
resonant frequency that matches the desired frequency. Either
way, the physical antenna length is a compromise when using
a multichannel communication or navigation device that must
be electronically tuned for the best performance.
A formula can be used to find the ideal length of a half
wavelength antenna required for a particular frequency as
follows:
Antenna Length (feet) = 468
F MHz
The formula is derived from the speed of propagation of
radio waves, which is approximately 300 million meters per
second. It takes into account the dielectric effect of the air at
the end of an antenna that effectively shortens the length of
the conductor required.
VHF radio frequencies used by aircraft communication radios
are 118–136.975 MHz. The corresponding half wavelengths
of these frequencies are 3.96 – 3.44 feet (47.5–41.2 inches).
Therefore, VHF antennas are relatively long. Antennas
one-quarter of the wavelength of the transmitted frequency
are often used. This is possible because when mounted on
a metal fuselage, a ground plane is formed and the fuselage
acts as the missing one-quarter length of the half wavelength
antenna. This is further discussed in the following antenna
types section.
Polarization, Directivity, and Field Pattern
Antennas are polarized. They radiate and receive in certain
patterns and directions. The electric field cause by the voltage
in the conductor is parallel to the polarization of an antenna.
It is caused by the voltage difference between each end of the
antenna. The electromagnetic field component of the radio
wave is at 90° to the polarization. It is caused by changing
current flow in the antenna. These fields were illustrated in
Figure 11-76 and 11-77. As radio waves radiate out from
the antenna they propagate in a specific direction and in a
specific pattern. This is the antenna field. The orientation
of the electric and electromagnetic fields remains at 90° to
each other, but radiate from antenna with varying strength in
different directions. The strength of the radiated field varies
depending on the type of antenna and the angular proximity
to it. All antennas, even those that are omnidirectional,
radiate a stronger signal in some direction compared to other
directions. This is known as the antenna field directivity.
Receiving antennas with the same polarization as the
transmitting antenna generate the strongest signal. A
vertically polarized antenna is mounted up and down. It
radiates waves out from it in all directions. To receive the
strongest signal from these waves, the receiving antenna
should also be positioned vertically so the electromagnetic
component of the radio wave can cross it at as close to a
90° angle as possible for most of the possible proximities.
[Figure 11-88]
Horizontally polarized antennas are mounted side to side
(horizontally). They radiate in a donut-like field. The
strongest signals come from, or are received at, 90° to the
length of the antenna. There is no field generated off of the
end of the antenna. Figure 11-89 illustrates the field produced
by a horizontally polarized antenna.
Many vertical and horizontal antennas on aircraft are
mounted at a slight angle off plane. This allows the antenna
to receive a weak signal rather than no signal at all when the
11-40
90°
Minimum
radiation
Maximum
radiation
Figure 11-89. A horizontally polarized antenna radiates in a
donut-like pattern. The strongest signal is at 90° to the length of
the conductor.
Figure 11-90. Many antenna are canted for better reception.
Up
Down
E W
N
S
Figure 11-88. A vertically polarized antenna radiates radio waves
in a donut-like pattern in all directions.
polarization of the receiving antenna is not identical to the
transmitting antenna. [Figure 11-90]
Types
There are three basic types of antennas used in aviation:
1. Dipole antenna
2. Marconi antenna
3. Loop antenna.
Dipole Antenna
The dipole antenna is the type of antenna referred to in the
discussion of how a radio wave is produced. It is a conductor,
the length of which is approximately equal to half the
wavelength of the transmission frequency. This sometimes is
referred to as a Hertz antenna. The AC transmission current is
fed to a dipole antenna in the center. As the current alternates,
current flow is greatest in the middle of the antenna and
gradually less as it approaches the ends. Then, it changes
direction and flows the other way. The result is that the
largest electromagnetic field is in the middle of the antenna
and the strongest radio wave field is perpendicular to the
length of the antenna. Most dipole antennas in aviation are
horizontally polarized.
A common dipole antenna is the V-shaped VHF navigation
antenna, known as a VOR antenna, found on numerous
aircraft. Each arm of the V is one-fourth wavelength creating
a half wave antenna which is fed in the center. This antenna
is horizontally polarized. For a dipole receiving antenna, this
means it is most sensitive to signals approaching the antenna
from the sides rather than head-on in the direction of flight.
[Figure 11-91]
Marconi Antenna
A Marconi antenna is a one-fourth wave antenna. It achieves
the efficiency of a half wave antenna by using the mounting
surface of the conductive aircraft skin to create the second
one-fourth wavelength. Most aircraft VHF communications
antennas are Marconi antennas. They are vertically polarized
and create a field that is omnidirectional. On fabric skinned
11-41
Figure 11-91. The V-shaped VOR navigation antenna is a common
dipole antenna.
Metal aircraft skin
ground plane
Ground plane under
skin in non-metallic
aircraft
Antenna
4
4
Figure 11-92. On a metal-skinned aircraft, a ¼ wavelength Marconi
antenna is used. The skin is the ground plane that creates the 2nd
quarter of the antenna required for resonance (left). On a nonmetallic-
skinned aircraft, wires, conductive plates or strips equal
in length to the antenna must be installed under the skin to create
the ground plane (right).
aircraft, the ground plane that makes up the second one-fourth
wavelength of the antenna must be fashioned under the skin
where the Marconi antenna is mounted. This can be done with
thin aluminum or aluminum foil. Sometimes four or more
wires are extended under the skin from the base of the vertical
antenna that serve as the ground plane. This is enough to give
the antenna the proper conductive length. The same practice
is also utilized on ground based antennas. [Figure 11-92]
Loop Antenna
The third type of antenna commonly found on aircraft is
the loop antenna. When the length of an antenna conductor
is fashioned into a loop, its field characteristics are altered
significantly from that of a straight-half wavelength antenna.
It also makes the antenna more compact and less prone to
damage.
Used as a receiving antenna, the loop antenna’s properties
are highly direction-sensitive. A radio wave intercepting
the loop directly broadside causes equal current flow in
both sides of the loop. However, the polarity of the current
flows is opposite each other. This causes them to cancel out
and produce no signal. When a radio wave strikes the loop
antenna in line with the plane of the loop, current is generated
first in one side, and then in the other side. This causes the
current flows to have different phases and the strongest signal
can be generated from this angle. The phase difference (and
strength) of the generated current varies proportionally to the
angle at which the radio wave strikes the antenna loop. This
is useful and is discussed further in the section on automatic
direction finder (ADF) navigational aids. [Figure 11-93]
Transmission Lines
Transmitters and receivers must be connected to their
antenna(s) via conductive wire. These transmission lines
are coaxial cable, also known as coax. Coax consists of a
center wire conductor surrounded by a semirigid insulator.
Surrounding the wire and insulator material is a conductive,
braided cover that runs the length of the cable. Finally, a
waterproof covering is set around the braided shield to protect
the entire assembly from the elements. The braided cover
in the coax shields the inner conductor from any external
fields. It also prevents the fields generated by the internal
conductor from radiating. For optimum performance, the
impedance of the transmission line should be equal to the
impedance of the antenna. In aviation antenna applications,
this is often approximately 50 ohms. [Figure 11-94] Special
connectors are used for coaxial cable. A variety can be seen
in Advisory Circular 43.13-1b. The technician should follow
all manufacturer’s instructions when installing transmission
lines and antenna. Correct installation is critical to radio and
antenna performance.
Radio Navigation
In the early years of aviation, a compass, a map, and dead
reckoning were the only navigational tools. These were
marginally reassuring if weather prevented the pilot from
seeing the terrain below. Voice radio transmission from
11-42
Protective plastic covering
Shielding–outer conductor
Central conductor
Dielectric–Insulator
Figure 11-94. Coaxial cable is used as the transmission line between an antenna and its transmitters and/or receiver.
Plane of loop perpendicular to direction of wave travel
Plane of loop parallel to direction of wave travel
Maximum reception loop orientation
Minimum reception loop orientation
A. B.
Figure 11-93. A loop antenna is highly direction-sensitive. A signal origin perpendicular or broadside to the loop creates a weak signal
(A). A signal origin parallel or in the plain of the loop creates a strong signal (B).
someone on the ground to the pilot indicating that the aircraft
could be heard overhead was a preview of what electronic
navigational aids could provide. For aviation to reach fruition
as a safe, reliable, consistent means of transportation, some
sort of navigation system needed to be developed.
Early flight instruments contributed greatly to flying when the
ground was obscured by clouds. Navigation aids were needed
to indicate where an aircraft was over the earth as it progressed
towards its destination. In the 1930s and 1940s, a radio
navigation system was used that was a low frequency, fourcourse
radio range system. Airports and selected navigation
waypoints broadcast two Morse code signals with finite ranges
and patterns. Pilots tuned to the frequency of the broadcasts
and flew in an orientation pattern until both signals were
received with increasing strength. The signals were received
as a blended tone of the highest volume when the aircraft
was directly over the broadcast area. From this beginning,
numerous refinements to radio navigational aids developed.
Radio navigation aids supply the pilot with intelligence
that maintains or enhances the safety of flight. As with
communication radios, navigational aids are avionics devices,
the repair of which must be carried out by trained technicians
at certified repair stations. However, installation, maintenance
and proper functioning of the electronic units, as well as their
antennas, displays, and any other peripheral devices, are the
responsibilities of the airframe technician.
VOR Navigation System
One of the oldest and most useful navigational aids is the
VOR system. The system was constructed after WWII and
11-43
Figure 11-95. A VOR ground station.
Figure 11-96. V-shaped, horizontally polarized, bi-pole antennas are commonly used for VOR and VOR/glideslope reception. All antenna
shown are VOR/glideslope antenna.
is still in use today. It consists of thousands of land-based
transmitter stations, or VORs, that communicate with radio
receiving equipment on board aircraft. Many of the VORs
are located along airways. The Victor airway system is built
around the VOR navigation system. Ground VOR transmitter
units are also located at airports where they are known as
TVOR (terminal VOR). The U.S. Military has a navigational
system known as TACAN that operates similarly to the VOR
system. Sometimes VOR and TACAN transmitters share a
location. These sites are known as VORTACs.
The position of all VORs, TVORs, and VORTACs are
marked on aeronautical charts along with the name of the
station, the frequency to which an airborne receiver must be
tuned to use the station, and a Morse code designation for
the station. Some VORs also broadcast a voice identifier
on a separate frequency that is included on the chart.
[Figure 11-95]
VOR uses VHF radio waves (108–117.95 MHz) with 50 kHz
separation between each channel. This keeps atmospheric
interference to a minimum but limits the VOR to line-ofsight
usage. To receive VOR VHF radio waves, generally a
V-shaped, horizontally polarized, bi-pole antenna is used. A
typical location for the V dipole is in the vertical fin. Other
type antennas are also certified. Follow the manufacturer’s
instructions for installation location. [Figure 11-96]
The signals produced by a VOR transmitter propagate 360°
from the unit and are used by aircraft to navigate to and from
the station with the help of an onboard VOR receiver and
display instruments. A pilot is not required to fly a pattern to
intersect the signal from a VOR station since it propagates
out in every direction. The radio waves are received as long
as the aircraft is in range of the ground unit and regardless
of the aircraft’s direction of travel. [Figure 11-97]
A VOR transmitter produces two signals that a receiver
on board an aircraft uses to locate itself in relation to the
ground station. One signal is a reference signal. The second
is produced by electronically rotating a variable signal. The
variable signal is in phase with the reference signal when at
magnetic north, but becomes increasingly out of phase as it
is rotated to 180°. As it continues to rotate to 360° (0°), the
signals become increasingly in phase until they are in phase
again at magnetic north. The receiver in the aircraft deciphers
the phase difference and determines the aircraft’s position
in degrees from the VOR ground based unit. [Figure 11-98]
Most aircraft carry a dual VOR receiver. Sometimes, the
VOR receivers are part of the same avionics unit as the
VHF communication transceiver(s). These are known as
NAV/COM radios. Internal components are shared since
frequency bands for each are adjacent. [Figure 11-99]
Large aircraft may have two dual receivers and even dual
antennas. Normally, one receiver is selected for use and the
11-44
0° Magnetic radial
90° Magnetic radial
180° Magnetic radial
270° Magnetic radial
Fixed signal Rotating signal
Figure 11-98. The phase relationship of the two broadcast VOR
signals.
COMM KY196A TSD
PULL 25K
PULL
TEST
OFF
STBY
CHAN
CHAN
Figure 11-99. A NAV/COM receiver typically found in light aircraft.
0° Magnetic north
010
350
340
330
320
310
300
290
280
270
260
250
240
230
220
210
200
190 020
030
040
050
060
070
080
090
100
110
120
130
140
150
160
170
180
315 135
Figure 11-97. A VOR transmitter produces signals for 360° radials
that an airborne receiver uses to indicate the aircraft’s location in
relation to the VOR station regardless of the aircraft’s direction of
flight. The aircraft shown is on the 315° radial even though it does
not have a heading of 315°.
second is tuned to the frequency of the next VOR station to
be encountered en route. A means for switching between
NAV 1 and NAV 2 is provided as is a switch for selecting the
active or standby frequency. [Figure 11-100] VOR receivers
are also found coupled with instrument landing system (ILS)
receivers and glideslope receivers.
A VOR receiver interprets the bearing in degrees to (or from)
the VOR station where the signals are generated. It also
produces DC voltage to drive the display of the deviation
from the desired course centerline to (or from) the selected
station. Additionally, the receiver decides whether or not
the aircraft is flying toward the VOR or away from it. These
items can be displayed a number of different ways on various
instruments. Older aircraft are often equipped with a VOR
gauge dedicated to display only VOR information. This
is also called an omni-bearing selector (OBS) or a course
deviation indicator (CDI). [Figure 11-101]
The CDI linear indicator remains essentially vertical but
moves left and right across the graduations on the instrument
face to show deviation from being on course. Each graduation
represents 2°. The OBS knob rotates the azimuth ring. When
in range of a VOR, the pilot rotates the OBS until the course
deviation indicator centers. For each location of an aircraft,
the OBS can be rotated to two positions where the CDI will
center. One produces an arrow in the TO window of the gauge
indicating that the aircraft is traveling toward the VOR station.
The other selectable bearing is 180° from this. When chosen,
the arrow is displayed in the FROM window indicating the
aircraft is moving away from the VOR on the course selected.
The pilot must steer the aircraft to the heading with the CDI
centered to fly directly to or from the VOR. The displayed
VOR information is derived from deciphering the phase
relationship between the two simultaneously transmitted
signals from the VOR ground station. When power is lost
or the VOR signal is weak or interrupted, a NAV warning
flag comes into view. [Figure 11-101]
11-45
ACTIVE NAV 1 STBY
ACTIVE NAV 2 STBY
Figure 11-100. An airliner VOR control head with two independent
NAV receivers each with an active and standby tuning circuit
controlled by a toggle switch.
Course index
Unroliable signal flag
CDI needle
2 inch dots
OBS knob
TO/FROM indicator
Figure 11-101. A traditional VOR gauge, also known as a course
deviation indicator (CDI) or an omni-bearing selector(OBS).
33
30
24
2I
I5
I2
6
3
GS NAV HDG GS
188°
G
HDG 183
CDI
“TO” indicator
CDI lateral deviation index
Azimuth scale
Actual heading of aircraft
Omnibearing selector
Figure 11-102. A mechanical HSI (left) and an electronic HSI (right) both display VOR information.
A separate gauge for the VOR information is not always
used. As flight instruments and displays have evolved,
VOR navigation information has been integrated into other
instruments displays, such as the radio magnetic indicator
(RMI), the horizontal situation indicator (HSI), an EFIS
display or an electronic attitude director indicator (EADI).
Flight management systems and automatic flight control
systems are also made to integrate VOR information to
automatically control the aircraft on its planned flight
segments. Flat panel MFDs integrate VOR information into
moving map presentations and other selected displays. The
basic information of the radial bearing in degrees, course
deviation indication, and to/from information remains
unchanged however. [Figure 11-102]
11-46
At large airports, an instrument landing system (ILS) guides
the aircraft to the runway while on an instrument landing
approach. The aircraft’s VOR receiver is used to interpret the
radio signals. It produces a more sensitive course deviation
indication on the same instrument display as the VOR CDI
display. This part of the ILS is known as the localizer and is
discussed below. While tuned to the ILS localizer frequency,
the VOR circuitry of the VOR/ILS receiver is inactive.
It is common at VOR stations to combine the VOR
transmitter with distance measuring equipment (DME) or a
nondirectional beacon (NDB) such as an ADF transmitter and
antenna. When used with a DME, pilots can gain an exact fix
on their location using the VOR and DME together. Since the
VOR indicates the aircraft’s bearing to the VOR transmitter
and a co-located DME indicates how far away the station is,
this relieves the pilot from having to fly over the station to
know with certainty his or her location. These navigational
aids are discussed separately in the following sections.
Functional accuracy of VOR equipment is critical to the safety
of flight. VOR receivers are operationally tested using VOR
test facilities (VOT). These are located at numerous airports
that can be identified in the Airport Facilities Directory for
the area concerned. Specific points on the airport surface are
given to perform the test. Most VOTs require tuning 108.0
MHz on the VOR receiver and centering the CDI. The OBS
should indicate 0° showing FROM on the indicator or 180°
when showing TO. If an RMI is used as the indicator, the test
heading should always indicate 180°. Some repair stations
can also generate signals to test VOR receivers although not
on 108.0 MHz. Contact the repair station for the transmission
frequency and for their assistance in checking the VOR
system. A logbook entry is required.
NOTE: Some airborne testing using VOTs is possible by
the pilot.
An error of ±4° should not be exceeded when testing a VOR
system with a VOT. An error in excess of this prevents the
use of the aircraft for IFR fight until repairs are made. Aircraft
having dual VOR systems where only the antenna is shared
may be tested by comparing the output of each system to
the other. Tune the VOR receivers to the local ground VOR
station. A bearing indication difference of no more than ±4°
is permissible.
Automatic Direction Finder (ADF)
An automatic direction finder (ADF) operates off of a ground
signal transmitted from a NDB. Early radio direction finders
(RDF) used the same principle. A vertically polarized antenna
was used to transmit LF frequency radio waves in the 190 kHz
to 535 kHz range. A receiver on the aircraft was tuned to the
transmission frequency of the NDB. Using a loop antenna,
the direction to (or from) the antenna could be determined
by monitoring the strength of the signal received. This
was possible because a radio wave striking a loop antenna
broadside induces a null signal. When striking it in the plane
of the loop, a much stronger signal is induced. The NDB
signals were modulated with unique Morse code pulses that
enabled the pilot to identify the beacon to which he or she
was navigating.
With RDF systems, a large rigid loop antenna was installed
inside the fuselage of the aircraft. The broadside of the
antenna was perpendicular to the aircraft’s longitudinal
axis. The pilot listened for variations in signal strength of
the LF broadcast and maneuvered the aircraft so a gradually
increasing null signal was maintained. This took them to
the transmitting antenna. When over flown, the null signal
gradually faded as the aircraft became farther from the station.
The increasing or decreasing strength of the null signal was
the only way to determine if the aircraft was flying to or from
the NDB. A deviation left or right from the course caused the
signal strength to sharply increase due to the loop antenna’s
receiving properties.
The ADF improved on this concept. The broadcast frequency
range was expanded to include MF up to about 1800 kHz.
The heading of the aircraft no longer needed to be changed
to locate the broadcast transmission antenna. In early model
ADFs, a rotatable antenna was used instead. The antenna
rotated to seek the position in which the signal was null.
The direction to the broadcast antenna was shown on an
azimuth scale of an ADF indicator in the flight deck. This
type of instrument is still found in use today. It has a fixed
card with 0° always at the top of a non-rotating dial. A
pointer indicates the relative bearing to the station. When
the indication is 0°, the aircraft is on course to (or from) the
station. [Figure 11-103]
As ADF technology progressed, indicators with rotatable
azimuth cards became the norm. When an ADF signal is
received, the pilot rotates the card so that the present heading
is at the top of the scale. This results in the pointer indicating
the magnetic bearing to the ADF transmitter. This is more
intuitive and consistent with other navigational practices.
[Figure 11-104]
In modern ADF systems, an additional antenna is used to
remove the ambiguity concerning whether the aircraft is
heading to or from the transmitter. It is called a sense antenna.
The reception field of the sense antenna is omnidirectional.
When combined with the fields of the loop antenna, it forms
a field with a single significant null reception area on one
side. This is used for tuning and produces an indication in the
11-47
Radio station
N-S E-W
33
30
24
21
15
12
6
3
W
S
E
N
Magnetic bearing to station
Relative bearing
Magnetic heading
Magnetic North
Figure 11-103. Older ADF indicators have nonrotating azimuth
cards. 0° is fixed at the top of the instrument and the pointer always
indicates the relative bearing to the ADF transmission antenna. To
fly to the station, the pilot turns the aircraft until the ADF pointer
indicates 0°.
33
30
24
21
15
12
6
3
S
W
E
N
HDG
Figure 7-4. Relative bearing (RB) on a movable-card indicator.
Figure 11-104. A movable card ADF indicator can be rotated to put
the aircraft’s heading at the top of the scale. The pointer then points
to the magnetic bearing the ADF broadcast antenna.
Pattern of sense antenna
Pattern of loop
Loop antenna
Combined pattern of loop and sense antenna
Tx
Figure 11-105. The reception fields of a loop and sense antenna
combine to create a field with a sharp null on just one side. This
removes directional ambiguity when navigating to an ADF station.
direction toward the ADF station at all times. The onboard
ADF receiver needs only to be tuned to the correct frequency
of the broadcast transmitter for the system to work. The loop
and sense antenna are normally housed in a single, low profile
antenna housing. [Figure 11-105]
Any ground antenna transmitting LF or MF radio waves in
range of the aircraft receiver’s tuning capabilities can be
used for ADF. This includes those from AM radio stations.
Audible identifier tones are loaded on the NDB carrier waves.
Typically a two-character Morse code designator is used.
With an AM radio station transmission, the AM broadcast is
heard instead of a station identifier code. The frequency for
an NDB transmitter is given on an aeronautical chart next
to a symbol for the transmitter. The identifying designator
is also given. [Figure 11-106]
ADF receivers can be mounted in the flight deck with the
controls accessible to the user. This is found on many general
aviation aircraft. Alternately, the ADF receiver is mounted in
a remote avionics bay with only the control head in the flight
deck. Dual ADF receivers are common. ADF information
can be displayed on the ADF indicators mentioned or it can
be digital. Modern, flat, multipurpose electronic displays
usually display the ADF digitally. [Figure 11-107] When
ANT is selected on an ADF receiver, the loop antenna is
cut out and only the sense antenna is active. This provides
better multi-directional reception of broadcasts in the ADF
frequency range, such as weather or AWAS broadcasts.
11-48
Figure 11-107. A cockpit mountable ADF receiver used on general
aviation aircraft.
33
30
24
21
15
12
6
3
W
S
E
N
HDG
Motor
ADF indicator
From loop-drive amplifier
To loop
input of
the ADF
receiver
Fixed loop
Goniometer
Figure 11-108. In modern ADF, a rotor in a goniometer replaces a
the rotating loop antenna used in earlier models.
Figure 11-106. Nondirectional broadcast antenna in the LF and
medium frequency range are used for ADF navigation.
When the best frequency oscillator (BFO) is selected on an
ADF receiver/controller, an internal beat frequency oscillator
is connected to the IF amplifier inside the ADF receiver. This
is used when an NDB does not transmit a modulated signal.
Continued refinements to ADF technology has brought it to
its current state. The rotating receiving antenna is replaced
by a fixed loop with a ferrite core. This increases sensitivity
and allows a smaller antenna to be used. The most modern
ADF systems have two loop antennas mounted at 90° to
each other. The received signal induces voltage that is sent
to two stators in a resolver or goniometer. The goniometer
stators induce voltage in a rotor that correlates to the signal
of the fixed loops. The rotor is driven by a motor to seek the
null. The same motor rotates the pointer in the flight deck
indicator to show the relative or magnetic bearing to the
station. [Figure 11-108]
Technicians should note that the installation of the ADF
antenna is critical to a correct indication since it is a
directional device. Calibration with the longitudinal axis of
the fuselage or nose of the aircraft is important. A single null
reception area must exist in the correct direction. The antenna
must be oriented so the ADF indicates station location when
the aircraft is flying toward it rather than away. Follow all
manufacturer’s instructions.
Radio Magnetic Indicator (RMI)
To save space in the instrument panel and to consolidate related
information into one easy to use location, the radio magnetic
indicator (RMI) has been developed. It is widely used. The
11-49
33
30
24
2I
I5
I2
6
3
N
S
W
E
Figure 3-26. IThe compass card in this RMI is driven by signals
from a flux valve and it indicates the heading of the aircraft
opposite the upper center index mark.
Figure 11-109. A radio magnetic indicator (RMI) combines a
magnetic compass, VOR, and ADF indications.
RMI combines indications from a magnetic compass, VOR,
and ADF into one instrument. [Figure 11-109]
The azimuth card of the RMI is rotated by a remotely located
flux gate compass. Thus, the magnetic heading of the aircraft
is always indicated. The lubber line is usually a marker or
triangle at the top of the instrument dial. The VOR receiver
drives the solid pointer to indicate the magnetic direction TO
a tuned VOR station. When the ADF is tuned to an NDB,
the double, or hollow pointer, indicates the magnetic bearing
to the NDB.
Since the flux gate compass continuously adjusts the
azimuth card so that the aircraft heading is at the top of the
instrument, pilot workload is reduced. The pointers indicate
where the VOR and ADF transmission stations are located
in relationship to where the aircraft is currently positioned.
Push buttons allow conversion of either pointer to either ADF
or VOR for navigation involving two of one type of station
and none of the other.
Instrument Landing Systems (ILS)
An ILS is used to land an aircraft when visibility is poor. This
radio navigation system guides the aircraft down a slope to the
touch down area on the runway. Multiple radio transmissions
are used that enable an exact approach to landing with an
ILS. A localizer is one of the radio transmissions. It is used to
provide horizontal guidance to the center line of the runway.
A separate glideslope broadcast provides vertical guidance of
the aircraft down the proper slope to the touch down point.
Compass locator transmissions for outer and middle approach
marker beacons aid the pilot in intercepting the approach
navigational aid system. Marker beacons provide distancefrom-
the-runway information. Together, all of these radio
signals make an ILS a very accurate and reliable means for
landing aircraft. [Figure 11-110]
Localizer
The localizer broadcast is a VHF broadcast in the lower
range of the VOR frequencies (108 MHz–111.95 MHz) on
odd frequencies only. Two modulated signals are produced
from a horizontally polarized antenna complex beyond the
far end of the approach runway. They create an expanding
field that is 21⁄2° wide (about 1,500 feet) 5 miles from the
runway. The field tapers to runway width near the landing
threshold. The left side of the approach area is filled with a
VHF carrier wave modulated with a 90 Hz signal. The right
side of the approach contains a 150 MHz modulated signal.
The aircraft’s VOR receiver is tuned to the localizer VHF
frequency that can be found on published approach plates
and aeronautical charts.
The circuitry specific to standard VOR reception is inactive
while the receiver uses localizer circuitry and components
common to both. The signals received are passed through
filters and rectified into DC to drive the course deviation
indicator. If the aircraft receives a 150 Hz signal, the CDI of
the VOR/ILS display deflects to the left. This indicates that
the runway is to the left. The pilot must correct course with
a turn to the left. This centers course deviation indicator on
the display and centers the aircraft with the centerline of the
runway. If the 90 Hz signal is received by the VOR receiver,
the CDI deflects to the right. The pilot must turn toward the
right to center the CDI and the aircraft with the runway center
line. [Figure 11-111]
Glideslope
The vertical guidance required for an aircraft to descend for
a landing is provided by the glideslope of the ILS. Radio
signals funnel the aircraft down to the touchdown point on
the runway at an angle of approximately 3°. The transmitting
glideslope antenna is located off to the side of the approach
runway approximately 1,000 feet from the threshold. It
transmits in a wedge-like pattern with the field narrowing
as it approaches the runway. [Figure 11-112]
The glideslope transmitter antenna is horizontally polarized.
The transmitting frequency range is UHF between 329.3
MHz and 335.0 MHz. The frequency is paired to the localizer
frequency of the ILS. When the VOR/ILS receiver is tuned
for the approach, the glideslope receiver is automatically
tuned. Like the localizer, the glideslope transmits two signals,
one modulated at 90 Hz and the other modulated at 150
11-50
OBS
N
E
S
W
3
33
24
21
15
12
30
6
GS
NAV
OBS
N
E
S
W
3
33
24
21
15
12
30
6
GS
NAV
OBS
N
E
S
W
3
33
24
21
12
30
6
GS
NAV
90
II0
I30
I50
I60
–
15
–
OBS
N
E
S
W
3
33
24
21
15
12
30
6
GS
NAV
90Hz 150Hz
OBS
N
E
S
W
3
33
24
21
15
12
30
6
GS
NAV
Figure 11-110. Components of an instrument landing system (ILS).
11-51
Figure 11-111. An ILS localizer antenna.
GS aerial
Glideslope
150 Hz
90 Hz
-1,000 feet
50 feet
Figure 11-112. A glideslope antenna broadcasts radio signals to
guide an aircraft vertically to the runway.
Figure 11-113. A traditional course deviation indicator is shown on the left. The horizontal white line is the deviation indicator for the
glideslope. The vertical line is for the localizer. On the right, a Garmin G-1000 PFD illustrates an aircraft during an ILS approach. The
narrow vertical scale on the right of the attitude indicator with the “G” at the top is the deviation scale for the glideslope. The green
diamond moves up and down to reflect the aircraft being above or below the glidepath. The diamond is shown centered indicating the
aircraft is on course vertically. The localizer CDI can be seen at the bottom center of the display. It is the center section of the vertical
green course indicator. LOC1 is displayed to the left of it.
Hz. The aircraft’s glideslope receiver deciphers the signals
similar to the method of the localizer receiver. It drives a
vertical course deviation indicator known as the glideslope
indicator. The glideslope indicator operates identically to the
localizer CDI only 90° to it. The VOR/ILS localizer CDI and
the glideslope are displayed together on whichever kind of
instrumentation is in the aircraft. [Figure 11-113]
The UHF antenna for aircraft reception of the glideslope
signals comes in many forms. A single dipole antenna
mounted inside the nose of the aircraft is a common option.
Antenna manufacturers have also incorporated glideslope
reception into the same dipole antenna used for the VHS
VOR/ILS localizer reception. Blade type antennas are also
used. [Figures 11-114] Figure 11-115 shows a VOR and a
glideslope receiver for a GA aircraft ILS.
Compass Locators
It is imperative that a pilot be able to intercept the ILS to
enable its use. A compass locator is a transmitter designed
for this purpose. There is typically one located at the outer
marker beacon 4-7 miles from the runway threshold. Another
may be located at the middle marker beacon about 3,500 feet
from the threshold. The outer marker compass locator is a
25 watt NDB with a range of about 15 miles. It transmits
omnidirectional LF radio waves (190 Hz to 535 Hz) keyed
with the first two letters of the ILS identifier. The ADF
11-52
Figure 11-116. Various marker beacon instrument panel display
lights.
Figure 11-114. Glideslope antennas—designed to be mounted inside
a non-metallic aircraft nose (left), and mounted inside or outside
the aircraft (right).
Figure 11-115. A localizer and glideslope receiver for a general
aviation aircraft ILS.
receiver is used to intercept the locator so no additional
equipment is required. If a middle marker compass locator is
in place, it is similar but is identified with the last two letters
of the ILS identifier. Once located, the pilot maneuvers the
aircraft to fly down the glidepath to the runway.
Marker Beacons
Marker beacons are the final radio transmitters used in the
ILS. They transmit signals that indicate the position of the
aircraft along the glidepath to the runway. As mentioned, an
outer marker beacon transmitter is located 4–7 miles from the
threshold. It transmits a 75 MHz carrier wave modulated with
a 400 Hz audio tone in a series of dashes. The transmission
is very narrow and directed straight up. A marker beacon
receiver receives the signal and uses it to light a blue light on
the instrument panel. This, plus the oral tone in combination
with the localizer and the glideslope indicator, positively
locates the aircraft on an approach. [Figure 11-115]
A middle marker beacon is also used. It is located on approach
approximately 3,500 feet from the runway. It also transmits
at 75 MHz. The middle marker transmission is modulated
with a 1300 Hz tone that is a series of dots and dashes so as
to not be confused with the all dash tone of the outer marker.
When the signal is received, it is used in the receiver to
illuminate an amber-colored light on the instrument panel.
[Figure 11-116]
Some ILS approaches have an inner marker beacon that
transmits a signal modulated with 3000 Hz in a series of dots
only. It is placed at the land-or-go-around decision point of
the approach close to the runway threshold. If present, the
signal when received is used to illuminate a white light on the
instrument panel. The three marker beacon lights are usually
incorporated into the audio panel of a general aviation aircraft
or may exist independently on a larger aircraft. Electronic
display aircraft usually incorporate marker lights or indicators
close to the glideslope display near attitude director indicator.
[Figure 11-117]
ILS radio components can be tested with an ILS test unit.
Localizer, glideslope, and marker beacon signals are
generated to ensure proper operation of receivers and correct
display on flight deck instruments. [Figure 11-118]
Distance Measuring Equipment (DME)
Many VOR stations are co-located with the military version
of the VOR station, which is known as TACAN. When this
occurs, the navigation station is known as a VORTAC station.
Civilian aircraft make use of one of the TACAN features
not originally installed at civilian VOR stations–distance
measuring equipment (DME). A DME system calculates the
distance from the aircraft to the DME unit at the VORTAC
ground station and displays it on the flight deck. It can also
display calculated aircraft speed and elapsed time for arrival
when the aircraft is traveling to the station.
11-53
Figure 11-117. An outer marker transmitter antenna 4 –7 miles from the approach runway transmits a 75 MHz signal straight up (left).
Aircraft mounted marker beacon receiver antennas are shown (center and right).
FUNCTION
ATTENUATOR
T-30D
RAMP
TEST SET
CAT III
DELETE
150
AC POWER
1020
LOC
DELETE
90
RF VOR MB
OUTPUT
108.10
334.70
108.15
VAR 334.55
OFF
ON
SIMULTANEOUS
MB
VAR
VAR
GS
−2 +2
−1 +1
OC
LOC
L2 R2
L1
270 1300
225 400 3000
180
135
90
45
0
VOR
30 Hz
VAR 0
D
E
L
E
T
E
D
REF 0
R1 315
OC
LOC
ILS MB
GS VOR
1020
VOR
108.05
TEST SET
POWER
TEST/FAIL
STATUS
5A
32V
5A
32V
ON
DC
50-400 Hz, SINGLE PHASE, OFF
120 - 220VAC, 25 WATTS
120 VAC-0.25 FTT, 220 VAC - 0.125 FTT
FUSE•FUSE•FUSE•
FUSE•FUSE•FUSE•
10
2030
40
50
60
70
80
90
100
1
2
3
4
5
6
7
8
9
10
0 −0
Figure 11-118. An ILS test unit.
Figure 11-119. A VOR with DME ground station.
Figure 11-120. Distance information from the DME can be displayed
on a dedicated DME instrument or integrated into any of the
electronic navigational displays found on modern aircraft. A dual
display DME is shown with its remote mounted receiver.
DME ground stations have subsequently been installed at
civilian VORs, as well as in conjunction with ILS localizers.
These are known as VOR/DME and ILS/DME or LOC/DME.
The latter aid in approach to the runway during landings.
The DME system consists of an airborne DME transceiver,
display, and antenna, as well as the ground based DME unit
and its antenna. [Figure 11-119]
The DME is useful because with the bearing (from the VOR)
and the distance to a known point (the DME antenna at the
VOR), a pilot can positively identify the location of the
aircraft. DME operates in the UHF frequency range from
962 MHz to 1213 MHz. A carrier signal transmitted from
the aircraft is modulated with a string of integration pulses.
The ground unit receives the pulses and returns a signal to
the aircraft. The time that transpires for the signal to be sent
and returned is calculated and converted into nautical miles
for display. Time to station and speed are also calculated and
displayed. DME readout can be on a dedicated DME display
or it can be part of an EHSI, EADI, EFIS, or on the primary
flight display in a glass cockpit. [Figure 11-120]
11-54
Slant distance = 13.0 N.M.
Reply pulse
Interogation pulse
Altitude (approx.
12,000 feet)
Actual distance over ground = 12.8 N.M.
DME station
M
u else
DME Display 1 3 . 0 N.M.
Figure 11-122. Many DME’s only display the slant distance, which
is the actual distance from the aircraft to the DME station. This
is different than the ground distance due to the aircraft being at
altitude. Some DMEs compute the ground distance for display.
Figure 11-121. A typical aircraft mounted DME antenna.
The DME frequency is paired to the co-located VOR or
VORTAC frequency. When the correct frequency is tuned
for the VOR signal, the DME is tuned automatically. Tones
are broadcast for the VOR station identification and then
for the DME. The hold selector on a DME panel keeps the
DME tuned in while the VOR selector is tuned to a different
VOR. In most cases, the UHF of the DME is transmitted
and received via a small blade-type antenna mounted to the
underside of the fuselage centerline. [Figure 11-121]
A traditional DME displays the distance from the DME
transmitter antenna to the aircraft. This is called the slant
distance. It is very accurate. However, since the aircraft is
at altitude, the distance to the DME ground antenna from a
point directly beneath the aircraft is shorter. Some modern
DMEs are equipped to calculate this ground distance and
display it. [Figure 11-122]
Area Navigation (RNAV)
Area navigation (RNAV) is a general term used to describe
the navigation from point A to point B without direct over
flight of navigational aids, such as VOR stations or ADF nondirectional
beacons. It includes VORTAC and VOR/DME
based systems, as well as systems of RNAV based around
LORAN, GPS, INS, and the FMS of transport category aircraft.
However, until recently, the term RNAV was most commonly
used to describe the area navigation or the process of direct
flight from point A to point B using VORTAC and VOR/DME
based references which are discussed in this section.
All RNAV systems make use of waypoints. A waypoint is
a designated geographical location or point used for route
definition or progress-reporting purposes. It can be defined
or described by using latitude/longitude grid coordinates or,
in the case of VOR based RNAV, described as a point on a
VOR radial followed by that point’s distance from the VOR
station (i.e., 200/25 means a point 25 nautical miles from the
VOR station on the 200° radial).
Figure 11-123 illustrates an RNAV route of flight from
airport A to airport B. The VOR/DME and VORTAC
stations shown are used to create phantom waypoints that are
overflown rather than the actual stations. This allows a more
direct route to be taken. The phantom waypoints are entered
into the RNAV course-line computer (CLC) as a radial and
distance number pair. The computer creates the waypoints
and causes the aircraft’s CDI to operate as though they
are actual VOR stations. A mode switch allows the choice
between standard VOR navigation and RNAV.
VOR based RNAV uses the VOR receiver, antenna, and
VOR display equipment, such as the CDI. The computer
in the RNAV unit uses basic geometry and trigonometry
calculations to produce heading, speed, and time readouts
for each waypoint. VOR stations need to be within line-of
sight and operational range from the aircraft for RNAV use.
[Figure 11-124]
RNAV has increased in flexibility with the development of
GPS. Integration of GPS data into a planned VOR RNAV
flight plan is possible as is GPS route planning without the
use of any VOR stations.
Radar Beacon Transponder
A radar beacon transponder, or simply, a transponder,
provides positive identification and location of an aircraft
on the radar screens of ATC. For each aircraft equipped
with an altitude encoder, the transponder also provides the
pressure altitude of the aircraft to be displayed adjacent to the
on-screen blip that represents the aircraft. [Figure 11-125]
11-55
Station
Radial
Waypoint
Distance
VOR flightpath
RNAV flightpath
Airport A
VORTAC XYZ
ZYX
108/15
ABC 348/19 ABC 015/30 W
1
XYZ 105/25
XYZ 167/16
VOR/DME ABC
VOR/DME ZYX
Airport B
Phantom waypoints created by RNAV CLC computer
Figure 11-123. The pilot uses the aircraft’s course deviation indicator to fly to and from RNAV phantom waypoints created by computer.
This allows direct routes to be created and flown rather than flying from VOR to VOR.
VOR R.NAV HOLD USE DSP DATA
PAR ENR
NM KT MIN FRQ RAD DST
APR
VOR RNV HLD ILS USE DSP
MODE DME WAYPOINT FREQ-RAD-DST
NAV SYSTEM
PULL
OFF ID
Figure 11-124. RNAV unit from a general aviation aircraft.
Radar capabilities at airports vary. Generally, two types of
radar are used by air traffic control (ATC). The primary radar
transmits directional UHF or SHF radio waves sequentially
in all directions. When the radio waves encounter an aircraft,
part of those waves reflect back to a ground antenna.
Calculations are made in a receiver to determine the direction
and distance of the aircraft from the transmitter. A blip or
target representing the aircraft is displayed on a radar screen
also known as a plan position indicator (PPI). The azimuth
direction and scaled distance from the tower are presented
giving controllers a two dimensional fix on the aircraft.
[Figure 11-126]
A secondary surveillance radar (SSR) is used by ATC to
verify the aircraft’s position and to add the third dimension of
altitude to its location. SSD radar transmits coded pulse trains
that are received by the transponder on board the aircraft.
Mode 3/A pulses, as they are known, aid in confirming
the location of the aircraft. When verbal communication is
established with ATC, a pilot is instructed to select one of
4,096 discrete codes on the transponder. These are digital
octal codes. The ground station transmits a pulse of energy
at 1030 MHz and the transponder transmits a reply with
the assigned code attached at 1090 MHz. This confirms the
aircraft’s location typically by altering its target symbol
on the radar screen. As the screen may be filled with many
confirmed aircraft, ATC can also ask the pilot to ident. By
pressing the IDENT button on the transponder, it transmits
in such a way that the aircraft’s target symbol is highlighted
on the PPI to be distinguishable.
To gain altitude clarification, the transponder control must
be placed in the ALT or Mode C position. The signal
transmitted back to ATC in response to pulse interrogation
is then modified with a code that places the pressure altitude
of the aircraft next to the target symbol on the radar screen.
The transponder gets the pressure altitude of the aircraft
11-56
Range marks
g
Rotating sweep
Echoes or returns from aircraft
Figure 11-126. A plan position indicator (PPI) for ATC primary
radar locates target aircraft on a scaled field.
A
B
C
Figure 11-125. A traditional transponder control head (A), a lightweight digital transponder (B), and a remote altitude encoder (C) that
connects to a transponder to provide ATC with an aircraft’s altitude displayed on a PPI radar screen next to the target that represents
the aircraft.
from an altitude encoder that is electrically connected to
the transponder. Typical aircraft transponder antennas are
illustrated in Figure 11-127.
The ATC/aircraft transponder system described is known
as Air Traffic Control Radar Beacon System (ATCRBS).
To increase safety, Mode S altitude response has been
developed. With Mode S, each aircraft is pre-assigned a
unique identity code that displays along with its pressure
altitude on ATC radar when the transponder responds to SSR
interrogation. Since no other aircraft respond with this code,
the chance of two pilots selecting the same response code on
the transponder is eliminated. A modern flight data processor
computer (FDP) assigns the beacon code and searches flight
plan data for useful information to be displayed on screen next
to the target in a data block for each aircraft. [Figure 11-128]
Mode S is sometimes referred to as mode select. It is a
data packet protocol that is also used in onboard collision
avoidance systems. When used by ATC, Mode S interrogates
one aircraft at a time. Transponder workload is reduced by
not having to respond to all interrogations in an airspace.
Additionally, location information is more accurate with
Mode S. A single reply in which the phase of the transponder
reply is used to calculate position, called monopulse, is
sufficient to locate the aircraft. Mode S also contains capacity
11-57
Figure 11-127. Aircraft radar beacon transponder antennas transmit and receive UHF and SHF radio waves.
Figure 11-128. Air traffic control radar technology and an onboard radar beacon transponder work together to convey and display air
traffic information on a PPI radar screen. A modern approach ATC PPI is shown. Targets representing aircraft are shown as little aircraft
on the screen. The nose of the aircraft indicates the direction of travel. Most targets shown above are airliners. The data block for each
target includes the following information either transmitted by the transponder or matched and loaded from flight plans by a flight data
processor computer: call sign, altitude/speed, origination/destination, and aircraft type/ETA (ZULU time). A “C” after the altitude indicates
the information came from a Mode C equipped transponder. The absence of a C indicates Mode S is in use. An arrow up indicates the
aircraft is climbing. An arrow down indicates a descent. White targets are arrivals, light blue targets are departures, all other colors are
for arrivals and departures to different airports in the area.
11-58
A1 A2 A4 B1 B2
B4 C1 C2 C4 D4
PIN 1 PIN 4 PIN 7
PIN 8 PIN 14
POWER MOTION
PIN 15
Figure 11-129. A handheld transponder test unit.
Figure 11-130. Modern altitude encoders for general aviation
aircraft.
for a wider variety of information exchange that is untapped
potential for the future. At the same time, compatibility with
older radar and transponder technology has been maintained.
Transponder Tests and Inspections
Title 14 of the Code of Federal Regulations (CFR) part 91,
section 91.413 states that all transponders on aircraft flown
into controlled airspace are required to be inspected and
tested in accordance with 14 CFR part 43, Appendix F, every
24 calendar months. Installation or maintenance that may
introduce a transponder error is also cause for inspection and
test in accordance with Appendix F. Only an appropriately
rated repair station, the aircraft manufacturer (if it installed
transponder), and holders of a continuous airworthy program
are approved to conduct the procedures. As with many radioelectronic
devices, test equipment exists to test airworthy
operation of a transponder. [Figure 11-129]
Operating a transponder in a hangar or on the ramp does not
immunize it from interrogation and reply. Transmission of
certain codes reserved for emergencies or military activity
must be avoided. The procedure to select a code during
ground operation is to do so with the transponder in the OFF
or STANDBY mode to avoid inadvertent transmission. Code
0000 is reserved for military use and is a transmittable code.
Code 7500 is used in a hijack situation and 7600 and 7700
are also reserved for emergency use. Even the inadvertent
transmission of code 1200 reserved for VFR flight not under
ATC direction could result in evasion action. All signals
received from a radar beacon transponder are taken seriously
by ATC.
Altitude Encoders
Altitude encoders convert the aircraft’s pressure altitude
into a code sent by the transponder to ATC. Increments of
100 feet are usually reported. Encoders have varied over the
years. Some are built into the altimeter instrument used in the
instrument panel and connected by wires to the transponder.
Others are mounted out of sight on an avionics rack or similar
out of the way place. These are known as blind encoders. On
transport category aircraft, the altitude encoder may be a large
black box with a static line connection to an internal aneroid.
Modern general aviation encoders are smaller and more
lightweight, but still often feature an internal aneroid and
static line connection. Some encoders use microtransistors
and are completely solid-state including the pressure sensing
device from which the altitude is derived. No static port
connection is required. Data exchange with GPS and other
systems is becoming common. [Figure 11-130]
When a transponder selector is set on ALT, the digital pulse
message sent in response to the secondary surveillance
radar interrogation becomes the digital representation of
the pressure altitude of the aircraft. There are 1280 altitude
codes, one for each 100 feet of altitude between 1200 feet
mean sea level (MSL) and 126,700 feet MSL. Each altitude
increment is assigned a code. While these would be 1280 of
the same codes used for location and IDENT, the Mode C
(or S) interrogation deactivates the 4096 location codes and
causes the encoder to become active. The correct altitude
code is sent to the transponder that replies to the interrogation.
The SSR receiver recognized this as a response to a Mode C
(or S) interrogation and interprets the code as altitude code.
Collision Avoidance Systems
The ever increasing volume of air traffic has caused a
corresponding increase in concern over collision avoidance.
Ground-based radar, traffic control, and visual vigilance are
11-59
2.1 NM
3.3 NM
25 seconds
40 seconds
20 NM
RA issued for 300 KT
TA issed Closure
Surveillance Range
Targets displayed
on-screen (TCAS I & II)
Traffic advisory (TA)
region (TCAS I & II)
TA Region
RA Region (TCAS II only)
Resolution advisory (RA)
region (TCAS II only)
Pilot commanded to
take evasive action
Pilot alerted to traffic in range
Range criterion
Intruder
Intruder
Altitude criterion
1200'
850'
1200'
850'
Figure 11-131. Traffic collision and avoidance system (TCAS) uses an aircraft’s transponder to interrogate and receive replies from
other aircraft in close proximity. The TCAS computer alerts the pilot as to the presence of an intruder aircraft and displays the aircraft on
a screen in the cockpit. Additionally, TCAS II equipped aircraft receive evasive maneuver commands from the computer that calculates
trajectories of the aircraft to predict potential collisions or near misses before they become unavoidable.
no longer adequate in today’s increasingly crowded skies.
Onboard collision avoidance equipment, long a staple in
larger aircraft, is now common in general aviation aircraft.
New applications of electronic technology combined with
lower costs make this possible.
Traffic Collision Avoidance Systems (TCAS)
Traffic collision avoidance systems (TCAS) are transponder
based air-to-air traffic monitoring and alerting systems.
There are two classes of TCAS. TCAS I was developed to
accommodate the general aviation community and regional
airlines. This system identifies traffic in a 35–40 mile range
of the aircraft and issues Traffic Advisories (TA) to assist
pilots in visual acquisition of intruder aircraft. TCAS I is
mandated on aircraft with 10 to 30 seats.
TCAS II is a more sophisticated system. It is required
internationally in aircraft with more than 30 seats or weighing
more than 15,000 kg. TCAS II provides the information
of TCAS I, but also analyzes the projected flightpath of
approaching aircraft. If a collision or near miss is imminent, the
TCAS II computer issues a Resolution Advisory (RA). This is an
aural command to the pilot to take a specific evasive action (i.e.,
DESCEND). The computer is programmed such that the pilot in
the encroaching aircraft receives an RA for evasive action in the
opposite direction (if it is TCAS II equipped). [Figure 11-131]
11-60
Figure 11-132. TCAS information displayed on an electronic vertical
speed indicator.
TCAS/ATC
ABOVE ABS
BELOW REL
ABOVE ABS
BELOW
FAIL
REL
N N
XPDR
STBY TA
TEST TA/RA
0 0 0 0
L R
Figure 11-134. This control panel from a Boeing 767 controls the
transponder for ATC use and TCAS.
15 5 21 24
197
GS 356 TAS386
195° 17 HDG MAG
TRAFFIC
+30
+13
+02
−12
Figure 11-133. TCAS information displayed on a multifunction
display. An open diamond indicates a target; a solid diamond
represents a target that is within 6 nautical miles of 1,2000 feet
vertically. A yellow circle represents a target that generates a TA
(25-48 seconds before contact). A red square indicates a target
that generates an RA in TCAS II (contact within 35 seconds). A (+)
indicates the target aircraft is above and a (-) indicates it is below.
The arrows show if the target is climbing or descending.
The transponder of an aircraft with TCAS is able to
interrogate the transponders of other aircraft nearby using
SSR technology (Mode C and Mode S). This is done with
a 1030 MHz signal. Interrogated aircraft transponders
reply with an encoded 1090 MHz signal that allows the
TCAS computer to display the position and altitude of each
aircraft. Should the aircraft come within the horizontal or
vertical distances shown in Figure 11-131, an audible TA is
announced. The pilot must decide whether to take action and
what action to take. TCAS II equipped aircraft use continuous
reply information to analyze the speed and trajectory of target
aircraft in close proximity. If a collision is calculated to be
imminent, an RA is issued.
TCAS target aircraft are displayed on a screen on the
flight deck. Different colors and shapes are used to depict
approaching aircraft depending on the imminent threat
level. Since RAs are currently limited to vertical evasive
maneuvers, some stand-alone TCAS displays are electronic
vertical speed indicators. Most aircraft use some version
of an electronic HSI on a navigational screen or page to
display TCAS information. [Figure 11-132] A multifunction
display may depict TCAS and weather radar information on
the same screen. [Figure 11-133] A TCAS control panel
[Figure 11-134] and computer are required to work with
a compatible transponder and its antenna(s). Interface with
EFIS or other previously installed or selected display(s) is
also required.
TCAS may be referred to as airborne collision avoidance
system (ACAS), which is the international name for the same
system. TCAS II with the latest revisions is known as Version
7. The accuracy and reliability of this TCAS information is
such that pilots are required to follow a TCAS RA over an
ATC command.
ADS-B
Collision avoidance is a significant part of the FAA’s
NextGen plan for transforming the National Airspace
System (NAS). Increasing the number of aircraft using the
same quantity of airspace and ground facilities requires the
implementation of new technologies to maintain a high level
of performance and safety. The successful proliferation of
global navigation satellite systems (GNSS), such as GPS,
has led to the development of a collision avoidance system
11-61
ADS-B signal
ADS-B signal
Ground transceiver
Conventional data networks
GNSS position data
Aircraft broadcast position, Altitude, Speed, etc.
Figure 11-136. ADS-B OUT uses satellites to identify the position aircraft. This position is then broadcast to other aircraft and to ground
stations along with other flight status information.
Figure 11-135. Low power requirements allow remote ADS-B
stations with only solar or propane support. This is not possible
with ground radar due to high power demands which inhibit remote
area radar coverage for air traffic purposes.
known as automatic dependant surveillance broadcast
(ADS-B). ADS-B is an integral part of NextGen program.
The implementation of its ground and airborne infrastructure
is currently underway. ADS-B is active in parts of the United
States and around the world. [Figure 11-135]
ADS-B is considered in two segments: ADS-B OUT
and ADS-B IN. ADS-B OUT combines the positioning
information available from a GPS receiver with on-board
flight status information, i.e. location including altitude,
velocity, and time. It then broadcasts this information
to other ADS-B equipped aircraft and ground stations.
[Figure 11-136]
Two different frequencies are used to carry these broadcasts
with data link capability. The first is an expanded use of the
1090 MHz Mode-S transponder protocol known as 1090 ES.
The second, largely being introduced as a new broadband
solution for general aviation implementation of ADS-B, is at
978 MHz. A 978 universal access transceiver (UAT) is used
to accomplish this. An omni-directional antenna is required
in addition to the GPS antenna and receiver. Airborne
receivers of an ADS-B broadcast use the information to plot
the location and movement of the transmitting aircraft on a
flight deck display similar to TCAS. [Figure 11-137]
Inexpensive ground stations (compared to radar) are
constructed in remote and obstructed areas to proliferate
ADS-B. Ground stations share information from airborne
ADS-B broadcasts with other ground stations that are part of
the air traffic management system (ATMS). Data is transferred
with no need for human acknowledgement. Microwave and
satellite transmissions are used to link the network.
For traffic separation and control, ADS-B has several
advantages over conventional ground-based radar. The first is
the entire airspace can be covered with a much lower expense.
The aging ATC radar system that is in place is expensive to
maintain and replace. Additionally, ADS-B provides more
accurate information since the vector state is generated
from the aircraft with the help of GPS satellites. Weather is
11-62
Figure 11-137. A cockpit display of ADS-B generated targets (left) and an ADS-B airborne receiver with antenna (right).
a greatly reduced factor with ADS-B. Ultra high frequency
GPS transmissions are not affected. Increased positioning
accuracy allows for higher density traffic flow and landing
approaches, an obvious requirement to operate more aircraft
in and out of the same number of facilities. The higher degree
of control available also enables routing for fewer weather
delays and optimal fuel burn rates. Collision avoidance is
expanded to include runway incursion from other aircraft
and support vehicles on the surface of an airport.
ADS-B IN offers features not available in TCAS. Equipped
aircraft are able to receive abundant data to enhance
situational awareness. Traffic information services-broadcast
(TIS-B) supply traffic information from non-ADS-B aircraft
and ADS-B aircraft on a different frequency. Ground radar
monitoring of surface targets, and any traffic data in the
linked network of ground stations is sent via ADS-B IN
to the flight deck. This provides a more complete picture
than air-to-air only collision avoidance. Flight information
services-broadcast (FIS-B) are also received by ADS-B
IN. Weather text and graphics, ATIS information, and
NOTAMS are able to be received in aircraft that have 987
UAT capability. [Figure 11-138]
ADS-B test units are available for trained maintenance
personnel to verify proper operation of ADS-B equipment.
This is critical since close tolerance of air traffic separation
depends on accurate data from each aircraft and throughout
all components of the ADS-B system. [Figure 11-139]
Radio Altimeter
A radio altimeter, or radar altimeter, is used to measure the
distance from the aircraft to the terrain directly beneath it. It
is used primarily during instrument approach and low level
or night flight below 2500 feet. The radio altimeter supplies
the primary altitude information for landing decision height.
It incorporates an adjustable altitude bug that creates a
visual or aural warning to the pilot when the aircraft reaches
that altitude. Typically, the pilot will abort a landing if the
decision height is reached and the runway is not visible.
Using a transceiver and a directional antenna, a radio
altimeter broadcasts a carrier wave at 4.3 GHz from the
aircraft directly toward the ground. The wave is frequency
modulated at 50 MHz and travels at a known speed. It strikes
surface features and bounces back toward the aircraft where
a second antenna receives the return signal. The transceiver
processes the signal by measuring the elapsed time the signal
traveled and the frequency modulation that occurred. The
display indicates height above the terrain also known as
above ground level (AGL). [Figure 11-140]
A radar altimeter is more accurate and responsive than an air
pressure altimeter for AGL information at low altitudes. The
transceiver is usually located remotely from the indicator.
Multifunctional and glass cockpit displays typically integrate
decision height awareness from the radar altimeter as a digital
number displayed on the screen with a bug, light, or color
change used to indicate when that altitude is reached. Large
aircraft may incorporate radio altimeter information into a
ground proximity warning system (GPWS) which aurally
alerts the crew of potentially dangerous proximity to the
terrain below the aircraft. A decision height window (DH)
displays the radar altitude on the EADI in Figure 11-141.
11-63
UAT + MFD UAT + MFD
Aircraft “See” each other
AWOS
Text weather radar weather
VHF
Wind
barometer
temp/DP etc.
Visibility
Ceiling
Weather data
A/C position
Figure 11-138. ADS-B IN enables weather and traffic information to be sent into the flight deck. In addition to AWOS weather, NWS can
also be transmitted.
Figure 11-139. An ADS-B test unit.
Figure 11-140. A digital display radio altimeter (top), and the two
antennas and transceiver for a radio/radar altimeter (bottom).
11-64
10 10
20 20
DH200
-4
80
110
60
4
45
6
SPD
LIM
VI
INOP
Figure 11-141. The decision height, DH200, in the lower right
corner of this EADI display uses the radar altimeter as the source
of altitude information.
Figure 11-142. A dedicated weather radar display (top) and a
multifunctional navigation display with weather radar overlay (bottom).
KGRN
KBUC
KBJC
L89
308
KHEF KCOS
KCEN
NORTH KFTC
H82
KBJC
KCLN
L89
KGHR
308
KLAR
KLVL
KHEF KCOS
LAR
HCT
KFTC
H82
KDEN
KFTR
KBUC
KCEN
W GPH
25
NORTH UP
V SIG / AIR
MAP WPT AUX NRST
NEXRAD
AGE: 5min
RAIN
MIX
SNOW
L
I
G
H
T
H
E
A
V
Y
5 30NM
338
1
1652
200
46
13.7
2300
23.0
NAV1 117.95 115.40
NAV2 108.00 117.95
123.750 119.925 COM1
135.975 120.050 COM2
GS 123kts DIS 53.2NM ETE 25:58 ESA 16800
ENGINE NEXRAD ECHO TOP CLD TOP LTNG CELL MOV SIG / AIR METAR LEGEND MORE WX
MAP WPT AUX NRST
MAP - NAVIGATION MAP
UP
30NM
Weather Radar
There are three common types of weather aids used in an
aircraft flight deck that are often referred to as weather radar:
1. Actual on-board radar for detecting and displaying
weather activity;
2. Lightning detectors; and
3. Satellite or other source weather radar information that
is uploaded to the aircraft from an outside source.
On-board weather radar systems can be found in aircraft
of all sizes. They function similar to ATC primary radar
except the radio waves bounce off of precipitation instead
of aircraft. Dense precipitation creates a stronger return than
light precipitation. The on-board weather radar receiver
is set up to depict heavy returns as red, medium return as
yellow and light returns as green on a display in the flight
deck. Clouds do not create a return. Magenta is reserved to
depict intense or extreme precipitation or turbulence. Some
aircraft have a dedicated weather radar screen. Most modern
aircraft integrate weather radar display into the navigation
display(s). Figure 11-142 illustrates weather radar displays
found on aircraft.
Radio waves used in weather radar systems are in the
SHF range such as 5.44 GHz or 9.375 GHz. They are
transmitted forward of the aircraft from a directional antenna
usually located behind a non-metallic nose cone. Pulses of
approximately 1 micro-second in length are transmitted.
A duplexer in the radar transceiver switches the antenna
to receive for about 2500 micro seconds after a pulse is
transmitted to receive and process any returns. This cycle
repeats and the receiver circuitry builds a two dimensional
image of precipitation for display. Gain adjustments control
the range of the radar. A control panel facilitates this and
other adjustments. [Figure 11-143]
Severe turbulence, wind shear, and hail are of major concern
to the pilot. While hail provides a return on weather radar,
wind shear and turbulence must be interpreted from the
movement of any precipitation that is detected. An alert is
annunciated if this condition occurs on a weather radar system
so equipped. Dry air turbulence is not detectable. Ground
clutter must also be attenuated when the radar sweep includes
any terrain features. The control panel facilitates this.
Special precautions must be followed by the technician
during maintenance and operation of weather radar systems.
The radome covering the antenna must only be painted with
approved paint to allow the radio signals to pass unobstructed.
Many radomes also contain grounding strips to conduct
lightning strikes and static away from the dome.
11-65
Figure 11-143. A typical on-board weather radar system for a high performance aircraft uses a nose-mounted antenna that gimbals. It is
usually controlled by the inertial reference system (IRS) to automatically adjust for attitude changes during maneuvers so that the radar
remains aimed at the desired weather target. The pilot may also adjust the angle and sweep manually as well as the gain. A dual mode
control panel allows separate control and display on the left or right HSI or navigational display.
Figure 11-144. A receiver and antenna from a lightning detector
system.
WX
RDR
LEFT MODE
RIGHT MODE
GAIN
UCAL
GAIN
UCAL
0
15
15
UP
DOWN
5
5
0
15
15
UP
DOWN
5
5
TFR WX/T WX MAP GCS
TFR WX/T WX
TEST
MAP GCS
When operating the radar, it is important to follow all
manufacturer instructions. Physical harm is possible from
the high energy radiation emitted, especially to the eyes and
testes. Do not look into the antenna of a transmitting radar.
Operation of the radar should not occur in hangars unless
special radio wave absorption material is used. Additionally,
operation of radar should not take place while the radar is
pointed toward a building or when refueling takes place.
Radar units should be maintained and operated only by
qualified personnel.
Lightning detection is a second reliable means for identifying
potentially dangerous weather. Lightning gives off its own
electromagnetic signal. The azimuth of a lightning strike can
be calculated by a receiver using a loop type antenna such as
that used in ADF. [Figure 11-144] Some lightning detectors
make use of the ADF antenna. The range of the lightning
strike is closely associated with its intensity. Intense strikes
are plotted as being close to the aircraft.
Stormscope is a proprietary name often associated with
lightning detectors. There are others that work in a similar
manner. A dedicated display plots the location of each strike
within a 200 mile range with a small mark on the screen. As
time progresses, the marks may change color to indicate their
age. Nonetheless, a number of lightning strikes in a small
area indicates a storm cell, and the pilot can navigate around
it. Lightning strikes can also be plotted on a multifunctional
navigation display. [Figure 11-145]
A third type of weather radar is becoming more common in
all classes of aircraft. Through the use of orbiting satellite
systems and/or ground up-links, such as described with
ADS-B IN, weather information can be sent to an aircraft
in flight virtually anywhere in the world. This includes
text data as well as real-time radar information for overlay
on an aircraft’s navigational display(s). Weather radar
data produced remotely and sent to the aircraft is refined
through consolidation of various radar views from different
angles and satellite imagery. This produces more accurate
depictions of actual weather conditions. Terrain databases
are integrated to eliminate ground clutter. Supplemental
data includes the entire range of intelligence available from
the National Weather Service (NWS) and the National
11-66
METAR
Daylight: Sunrise 06:03 AM. Sunset 08:50 PM LT
Wind: 270 degrees (W) 9 knots (~10 MPH)
Variable between 220 and 310 degrees
Visibility: 6 or more miles
Clouds: broken clouds at 5,500 feet
Temperature: 59°F, dewpoint: 50°F, RH:72%
Pressure: 30.15 inches Hg
No significant changes
• METARs/TAFs/PIREPs/SIGMETs/NOTAMs
• Hundreds of web-based graphical weather charts
• Area forecasts and route weather briefings
• Wind and temperature aloft data
• “Plain language” passenger weather briefs
• Route of flight images with weather overlays
• Significant weather charts and other prognostic charts
• Worldwide radar and satellite imagery
Conditions at: 08:20 AM local time (9th)
Bern / Belp, CH (LSZB)
Satellite weather services available
VFR
Updated at 02:43 PM Source:NWS
Figure 11-146. A plain language METAR weather report received
in the cockpit from a satellite weather service for aircraft followed
by a list of various weather data that can be radioed to the cockpit
from a satellite weather service.
BRT
OFF ON
FWD
TST
CLR
25
100 200
50
0
27 9
18
NAV
MAP
BRG
VUE
SHFT SYNC WX-10
33
30
24
2I
I5
I2
6
3
M
GPS1
360
ILX 310°
5.8 nm
310°
NAV1
16.1 nm
337°
165 KT
RNG 20 nm
310°
S107
310°
3
MA34
KUTG
ILX
MD
ARBLE
FF22
AZT
T53
+++ ++++ ++
+++
+++
+
+
Figure 11-145. A dedicated stormscope lightning detector display (left), and an electronic navigational display with lightning strikes
overlaid in the form of green “plus” signs (right).
Oceanographic and Atmospheric Administration (NOAA).
Figure 11-146 illustrates a plain language weather summary
received in an aircraft along with a list of other weather
information available through satellite or ground link weather
information services.
As mentioned, to receive an ADS-B weather signal, a 1090
ES or 970 UAT transceiver with associated antenna needs to
be installed on board the aircraft. Satellite weather services
are received by an antenna matched to the frequency of
the service. Receivers are typically located remotely and
interfaced with existing navigational and multifunction
displays. Handheld GPS units also may have satellite weather
capability. [Figure 11-147]
Emergency Locator Transmitter (ELT)
An emergency locator transmitter (ELT) is an independent
battery powered transmitter activated by the excessive
G-forces experienced during a crash. It transmits a digital
signal every 50 seconds on a frequency of 406.025 MHz at 5
watts for at least 24 hours. The signal is received anywhere
in the world by satellites in the COSPAS-SARSAT satellite
system. Two types of satellites, low earth orbiting (LEOSATs)
and geostationary satellites (GEOSATs) are used with
different, complimentary capability. The signal is partially
processed and stored in the satellites and then relayed to
ground stations known as local user terminals (LUTs).
Further deciphering of a signal takes place at the LUTs, and
appropriate search and rescue operations are notified through
mission control centers (MCCs) set up for this purpose.
NOTE: Maritime vessel emergency locating beacons (EPIRBs)
and personal locator beacons (PLBs) use the exact same system.
The United States portion of the COSPAS-SARSAT system is
maintained and operated by NOAA. Figure 11-148 illustrates
the basic components in the COSPAS-SARSAT system.
11-67
Figure 11-147. A satellite weather receiver and antenna enable display of real-time textual and graphic weather information beyond that
of airborne weather radar. A handheld GPS can also be equipped with these capabilities. A built-in multifunctional display with satellite
weather overlays and navigation information can be found on many aircraft.
Key:
EPIRB: Emergency position indicating radio beacon
ELT: Emergency locator transmitter
PLB: Personal locator beacon
SAR: Search and rescue
SAR
GOES MSG INSAT COSPAS SARSAT
SAR
Local user terminal (LUT)
Mission control center (MCC)
Rescue coordination center (RCC)
Distressed vessel
Distressed aircraft
GEO Satellites
LEO Satellites
PLB
EPIRB
ELT
406 MHz
406 MHz
406 MHz
406 MHz
406 MHz
406 MHz
Downlink
Downlink
Figure 11-148. The basic operating components of the satellite-based COSPAS-SARSAT rescue system of which aircraft ELTs are a part.
11-68
Figure 11-149. An emergency locator transmitter (ELT) mounting
location is generally far aft in a fixed-wing aircraft fuselage in
line with the longitudinal axis. Helicopter mounting location and
orientation varies.
Figure 11-150. An ELT and its components including a cockpitmounted
panel, the ELT, a permanent mount antenna, and a
portable antenna.
ELTs are required to be installed in aircraft according to
FAR 91.207. This encompasses most general aviation
aircraft not operating under Parts 135 or 121. ELTs must be
inspected within 12 months of previous inspection for proper
installation, battery corrosion, operation of the controls and
crash sensor, and the presence of a sufficient signal at the
antenna. Built-in test equipment facilitates testing without
transmission of an emergency signal. The remainder of the
inspection is visual. Technicians are cautioned to not activate
the ELT and transmit an emergency distress signal. Inspection
must be recorded in maintenance records including the new
expiration date of the battery. This must also be recorded on
the outside of the ELT.
ELTs are typically installed as far aft in the fuselage of an
aircraft as is practicable just forward of the empennage. The
built-in G-force sensor is aligned with the longitudinal axis of
the aircraft. Helicopter ELTs may be located elsewhere on the
airframe. They are equipped with multidirectional activation
devices. Follow ELT and airframe manufacturer’s instructions
for proper installation, inspection, and maintenance of all
ELTs. Figure 11-149 illustrates ELTs mounted locations.
Use of Doppler technology enables the origin of the 406
MHz ELT signal to be calculated within 2 to 5 kilometers.
Second generation 406 MHz ELT digital signals are loaded
with GPS location coordinates from a receiver inside the
ELT unit or integrated from an outside unit. This reduces
the location accuracy of the crash site to within 100 meters.
The digital signal is also loaded with unique registration
information. It identifies the aircraft, the owner, and contact
information, etc. When a signal is received, this is used to
immediately research the validity of the alert to ensure it is
a true emergency transmission so that rescue resources are
not deployed needlessly.
ELTs with automatic G-force activation mounted in
aircraft are easily removable. They often contain a portable
antenna so that crash victims may leave the site and carry
the operating ELT with them. A flight deck mounted panel
is required to alert the pilot if the ELT is activated. It also
allows the ELT to be armed, tested, and manually activated
if needed. [Figure 11-150]
Modern ELTs may also transmit a signal on 121.5 MHz. This
is an analog transmission that can be used for homing. Prior
to 2009, 121.5 MHz was a worldwide emergency frequency
monitored by the CORPAS-SARSAT satellites. However, it
has been replaced by the 406 MHz standard. Transmission on
121.5 MHz are no longer received and relayed via satellite.
The use of a 406 MHz ELT has not been mandated by the
FAA. An older 121.5 MHz ELT satisfies the requirements
of FAR Part 91.207 in all except new aircraft. Thousands of
aircraft registered in the United States remain equipped with
ELTs that transmit a .75 watt analog 121.5 MHz emergency
signal when activated. The 121.5 MHz frequency is still an
active emergency frequency and is monitored by over-flying
aircraft and control towers.
Technicians are required to perform an inspection/test of
121.5 MHz ELTs within 12 months of the previous one and
11-69
Figure 11-151. Panel-mounted LORAN units are now obsolete as
LORAN signals are no longer generated from the tower network.
inspect for the same integrity as required for the 406MHz
ELTs mentioned above. However, older ELTs often lack the
built-in test circuitry of modern ELTs certified to TSO C-126.
Therefore, a true operational test may include activating
the signal. This can be done by removing the antenna and
installing a dummy load. Any activation of an ELT signal is
required to only be done between the top of each hour and 5
minutes after the hour. The duration of activation must be no
longer than three audible sweeps. Contact of the local control
tower or flight service station before testing is recommended.
It must be noted that older 121.5 MHz analog signal ELTs
often also transmit an emergency signal on a frequency of
243.0 MHz. This has long been the military emergency
frequency. Its use is being phased out in favor of digital ELT
signals and satellite monitoring. Improvements in coverage,
location accuracy, identification of false alerts, and shortened
response times are so significant with 406 MHz ELTs, they
are currently the service standard worldwide.
Long Range Aid to Navigation System (LORAN)
Long range aid to navigation system (LORAN) is a type of
RNAV that is no longer available in the United States. It was
developed during World War II, and the most recent edition,
LORAN-C, has been very useful and accurate to aviators as
well as maritime sailors. LORAN uses radio wave pulses
from a series of towers and an on-board receiver/computer
to positively locate an aircraft amid the tower network. There
are twelve LORAN transmitter tower “chains” constructed
across North America. Each chain has a master transmitter
tower and a handful of secondary towers. All broadcasts
from the transmitters are at the same frequency, 100 KHz.
Therefore, a LORAN receiver does not need to be tuned.
Being in the low frequency range, the LORAN transmissions
travel long distances and provide good coverage from a small
number of stations.
Precisely-timed, synchronized pulse signals are transmitted
from the towers in a chain. The LORAN receiver measures
the time to receive the pulses from the master tower and two
other towers in the chain. It calculates the aircraft’s position
based on the intersection of parabolic curves representing
elapsed signal times from each of these known points.
The accuracy and proliferation of GPS navigation has
caused the U.S. Government to cease support for the
LORAN navigation system citing redundancy and expense
of operating the towers as reasons. The LORAN chain in
the Aleutian Island shared with Russia is the only LORAN
chain at the time of printing of this handbook which had not
yet been given a date for closure. Panel-mounted LORAN
navigation units will likely be removed and replaced by GPS
units in aircraft that have not already done so. [Figure 11-151]
Global Positioning System (GPS)
Global positioning system navigation (GPS) is the fastest
growing type of navigation in aviation. It is accomplished
through the use of NAVSTAR satellites set and maintained
in orbit around the earth by the U.S. Government. Continuous
coded transmissions from the satellites facilitate locating the
position of an aircraft equipped with a GPS receiver with
extreme accuracy. GPS can be utilized on its own for en
route navigation, or it can be integrated into other navigation
systems, such as VOR/RNAV, inertial reference, or flight
management systems.
There are three segments of GPS: the space segment, the
control segment, and the user segment. Aircraft technicians
are only involved with user segment equipment such as GPS
receivers, displays, and antennas.
Twenty-four satellites (21 active, 3 spares) in six separate
plains of orbit 12, 625 feet above the planet comprise what
is known as the space segment of the GPS system. The
satellites are positioned such that in any place on earth at any
one time, at least four will be a minimum of 15° above the
horizon. Typically, between 5 and 8 satellites are in view.
[Figure 11-152]
Two signals loaded with digitally coded information are
transmitted from each satellite. The L1 channel transmission
on a1575.42 MHz carrier frequency is used in civilian aviation.
Satellite identification, position, and time are conveyed to the
aircraft GPS receiver on this digitally modulated signal along
with status and other information. An L2 channel 1227.60
MHz transmission is used by the military.
The amount of time it takes for signals to reach the aircraft
GPS receiver from transmitting satellites is combined with
each satellite’s exact location to calculate the position of
11-70
Figure 11-153. A GPS unit integrated with NAV/COM circuitry.
Figure 11-152. The space segment of GPS consists of 24 NAVSTAR
satellites in six different orbits around the earth.
an aircraft. The control segment of the GPS monitors each
satellite to ensure its location and time are precise. This
control is accomplished with five ground-based receiving
stations, a master control station, and three transmitting
antenna. The receiving stations forward status information
received from the satellites to the master control station.
Calculations are made and corrective instructions are sent
to the satellites via the transmitters.
The user segment of the GPS is comprised of the thousands
of receivers installed in aircraft as well as every other
receiver that uses the GPS transmissions. Specifically, for
the aircraft technician, the user section consists of a control
panel/display, the GPS receiver circuitry, and an antenna. The
control, display and receiver are usually located in a single
unit which also may include VOR/ILS circuitry and a VHF
communications transceiver. GPS intelligence is integrated
into the multifunctional displays of glass cockpit aircraft.
[Figure 11-153]
The GPS receiver measures the time it takes for a signal to
arrive from three transmitting satellites. Since radio waves
travel at 186,000 miles per second, the distance to each
satellite can be calculated. The intersection of these ranges
provides a two dimensional position of the aircraft. It is
expressed in latitude/longitude coordinates. By incorporating
the distance to a fourth satellite, the altitude above the surface
of the earth can be calculated as well. This results in a three
dimensional fix. Additional satellite inputs refine the accuracy
of the position.
Having deciphered the position of the aircraft, the GPS unit
processes many useful navigational outputs such as speed,
direction, bearing to a waypoint, distance traveled, time of
arrival, and more. These can be selected to display for use.
Waypoints can be entered and stored in the unit’s memory.
Terrain features, airport data, VOR/RNAV and approach
information, communication frequencies, and more can also
be loaded into a GPS unit. Most modern units come with
moving map display capability.
A main benefit of GPS use is immunity from service
disruption due to weather. Errors are introduced while the
carrier waves travel through the ionosphere; however, these
are corrected and kept to a minimum. GPS is also relatively
inexpensive. GPS receivers for IFR navigation in aircraft
must be built to TSO-129A. This raises the price above that
of handheld units used for hiking or in an automobile. But the
overall cost of GPS is low due to its small infrastructure. Most
of the inherent accuracy is built into the space and control
segments permitting reliable positioning with inexpensive
user equipment.
The accuracy of current GPS is within 20 meters horizontally
and a bit more vertically. This is sufficient for en route
navigation with greater accuracy than required. However,
departures and approaches require more stringent accuracy.
Integration of the wide area augmentation system (WAAS)
improves GPS accuracy to within 7.6 meters and is discussed
below. The future of GPS calls for additional accuracy
by adding two new transmissions from each satellite. An
L2C channel will be for general use in non-safety critical
application. An aviation dedicated L5 channel will provide
the accuracy required for category I, II, and III landings. It
will enable the NEXTGEN NAS plan along with ADS-B.
The first replacement NAVSTAR satellites with L2C and L5
capability have already been launched. Full implementation
is schedule by 2015.
11-71
GPS satellites
Wide area reference station
Ground
Earth
station
Communication satellites
L1
L1
Wide area
master station
Figure 11-154. The wide area augmentation system (WAAS) is used
to refine GPS positions to a greater degree of accuracy. A WAAS
enabled GPS receiver is required for its use as corrective information
is sent from geostationary satellites directly to an aircraft’s GPS
receiver for use.
SYS
OFF
TEST
TK/GS
PPOS WIND
HDG
STS
1
3 2
1
N2
3
W4
H5
E6
7
S8
9
ENT 0 CLR
OFF
IR1 IR3
DATA
DISPLAY
IR2
ADR1 ADR3 ADR2
NAV ATT OFF NAV ATT OFF NAV ATT
Figure 11-155. An interface panel for three air data and inertial
reference systems on an Airbus. The keyboard is used to initialize
the system. Latitude and longitude position is displayed at the top.
Wide Area Augmentation System (WAAS)
To increase the accuracy of GPS for aircraft navigation, the
wide area augmentation system (WAAS) was developed.
It consists of approximately 25 precisely surveyed ground
stations that receive GPS signals and ultimately transmit
correction information to the aircraft. An overview of WAAS
components and its operation is shown in Figure 11-154.
WAAS ground stations receive GPS signals and forward
position errors to two master ground stations. Time and
location information is analyzed, and correction instructions
are sent to communication satellites in geostationary orbit
over the NAS. The satellites broadcast GPS-like signals
that WAAS enabled GPS receivers use to correct position
information received from GPS satellites.
A WAAS enable GPS receiver is required to use the wide
area augmentation system. If equipped, an aircraft qualifies
to perform precision approaches into thousands of airports
without any ground-based approach equipment. Separation
minimums are also able to be reduced between aircraft that
are WAAS equipped. The WAAS system is known to reduce
position errors to 1–3 meters laterally and vertically.
Inertial Navigation System (INS)/Inertial
Reference System (IRS)
An inertial navigation system (INS) is used on some
large aircraft for long range navigation. This may also be
identified as an inertial reference system (IRS), although
the IRS designation is generally reserved for more modern
systems. An INS/IRS is a self contained system that does
not require input radio signals from a ground navigation
facility or transmitter. The system derives attitude, velocity,
and direction information from measurement of the aircraft’s
accelerations given a known starting point. The location of the
aircraft is continuously updated through calculations based on
the forces experienced by INS accelerometers. A minimum
of two accelerometers is used, one referenced to north, and
the other referenced to east. In older units, they are mounted
on a gyro-stabilized platform. This averts the introduction
of errors that may result from acceleration due to gravity.
An INS uses complex calculation made by an INS computer to
convert applied forces into location information. An interface
control head is used to enter starting location position data
while the aircraft is stationary on the ground. This is called
initializing. [Figure 11-155] From then on, all motion of
the aircraft is sensed by the built-in accelerometers and run
through the computer. Feedback and correction loops are used
to correct for accumulated error as flight time progresses. The
amount an INS is off in one hour of flight time is a reference
point for determining performance. Accumulated error of less
than one mile after one hour of flight is possible. Continuous
accurate adjustment to the gyro-stabilized platform to keep it
parallel to the Earth’s surface is a key requirement to reduce
accumulated error. A latitude/longitude coordinate system is
used when giving the location output.
11-72
CAUTION
CAUTION
Figure 11-156. A modern micro-IRS with built-in GPS.
INS is integrated into an airliner’s flight management
system and automatic flight control system. Waypoints can
be entered for a predetermined flightpath and the INS will
guide the aircraft to each waypoint in succession. Integration
with other NAV aids is also possible to ensure continuous
correction and improved accuracy but is not required.
Modern INS systems are known as IRS. They are completely
solid-state units with no moving parts. Three-ring, laser
gyros replace the mechanical gyros in the older INS
platform systems. This eliminates precession and other
mechanical gyro shortcomings. The use of three solid-state
accelerometers, one for each plane of movement, also
increases accuracy. The accelerometer and gyro output
are input to the computer for continuous calculation of the
aircraft’s position.
The most modern IRS integrate is the satellite GPS. The GPS
is extremely accurate in itself. When combined with IRS, it
creates one of the most accurate navigation systems available.
The GPS is used to initialize the IRS so the pilot no longer
needs to do so. GPS also feeds data into the IRS computer to
be used for error correction. Occasional service interruptions
and altitude inaccuracies of the GPS system pose no
problem for IRS/GPS. The IRS functions continuously and
is completely self contained within the IRS unit. Should the
GPS falter, the IRS portion of the system continues without
it. The latest electronic technology has reduced the size and
weight of INS/IRS avionics units significantly. Figure 11-156
shows a modern micro-IRS unit that measures approximately
6-inches on each side.
Installation of Communication and
Navigation Equipment
Approval of New Avionics Equipment
Installations
Most of the avionics equipment discussed in this chapter is
only repairable by the manufacturer or FAA-certified repair
stations that are licensed to perform specific work. The airframe
technician; however, must competently remove, install,
inspect, maintain, and troubleshoot these ever increasingly
complicated electronic devices and systems. It is imperative to
follow all equipment and airframe manufacturers’ instruction
when dealing with an aircraft’s avionics.
The revolution to GPS navigation and the pace of modern
electronic development results in many aircraft owner
operators upgrading flight decks with new avionics. The
aircraft technician must only perform airworthy installations.
The avionics equipment to be installed must be a TSO’d
device that is approved for installation in the aircraft in
question. The addition of a new piece of avionics equipment
and/or its antenna is a minor alteration if previously approved
by the airframe manufacturer. A licensed airframe technician
is qualified to perform the installation and return the aircraft
to service. The addition of new avionics not on the aircraft’s
approved equipment list is considered a major alteration and
requires an FAA Form 337 to be enacted. A technician with an
inspection authorization is required to complete a Form 337.
Most new avionics installations are approved and performed
under an STC. The equipment manufacturer supplies a list
of aircraft on which the equipment has been approved for
installation. The STC includes thorough installation and
maintenance instructions which the technician must follow.
Regardless, if not on the aircraft’s original equipment list,
the STC installation is considered a major alteration and an
FAA Form 337 must be filed. The STC is referenced as the
required approved data.
Occasionally, an owner/operator or technician wishes to install
an electronic device in an aircraft that has no STC for the
model aircraft in question. A field approval and a Form 337
must be filed on which it must be shown that the installation
will be performed in accordance with approved data.
Considerations
There are many factors which the technician must consider
prior to altering an aircraft by the addition of avionics
equipment. These factors include the space available, the size
and weight of the equipment, and previously accomplished
alterations. The power consumption of the added equipment
must be considered to calculate and determine the maximum
continuous electrical load on the aircraft’s electrical system.
11-73
Machine screws and self-locking nuts
Rear case support
Rivets or machine screws
and self-locking nuts
Figure 11-157. An avionics installation in a stationary instrument
panel may include a support for the avionics case.
Shock mount
Figure 11-158. A shock mounted equipment rack is often used to
install avionics.
Each installation should also be planned to allow easy access
for inspection, maintenance, and exchange of units.
The installation of avionics equipment is partially mechanical,
involving sheet metal work to mount units, racks, antennas,
and controls. Routing of the interconnecting wires, cables,
antenna leads, etc. is also an important part of the installation
process. When selecting a location for the equipment, use
the area(s) designated by the airframe manufacturer or the
STC. If such information is not available, select a location for
installation that will carry the loads imposed by the weight
of the equipment, and which is capable of withstanding the
additional inertia forces.
If an avionics device is to be mounted in the instrument panel
and no provisions have been made for such an installation,
ensure that the panel is not a primary structure prior to making
any cutouts. To minimize the load on a stationary instrument
panel, a support bracket may be installed between the rear of
the electronics case or rack and a nearby structural member
of the aircraft. [Figure 11-157]
Avionics radio equipment must be securely mounted to the
aircraft. All mounting bolts must be secured by locking
devices to prevent loosening from vibration. Adequate
clearance between all units and adjacent structure must be
provided to prevent mechanical damage to electric wiring or
to the avionic equipment from vibration, chafing, or landing
shock.
Do not locate avionics equipment and wiring near
units containing combustible fluids. When separation is
impractical, install baffles or shrouds to prevent contact of
the combustible fluids with any electronic equipment in the
event of plumbing failure.
Cooling and Moisture
The performance and service life of most avionics equipment
is seriously limited by excessive ambient temperatures. High
performance aircraft with avionics equipment racks typically
route air-conditioned air over the avionics to keep them cool.
It is also common for non-air conditioned aircraft to use a
blower or scooped ram air to cool avionics installations.
When adding a unit to an aircraft, the installation should
be planned so that it can dissipate heat readily. In some
installations, it may be necessary to produce airflow over
the new equipment either with a blower or through the use
of routed ram air. Be sure that proper baffling is used to
prevent water from reaching any electronics when ducting
outside air. The presence of water in avionics equipment areas
promotes rapid deterioration of the exposed components and
could lead to failure.
Vibration Isolation
Vibration is a continued motion by an oscillating force. The
amplitude and frequency of vibration of the aircraft structure
will vary considerably with the type of aircraft. Avionics
equipment is sensitive to mechanical shock and vibration
and is normally shock mounted to provide some protection
against in-flight vibration and landing shock.
Special shock mounted racks are often used to isolate avionics
equipment from vibrating structure. [Figure 11-158] Such
mounts should provide adequate isolation over the entire
range of expected vibration frequencies. When installing
shock mounts, assure that the equipment weight does not
exceed the weight-carrying capabilities of the mounts. Radio
equipment installed on shock mounts must have sufficient
clearance from surrounding equipment and structure to allow
for normal swaying of the equipment.
11-74
Bonding jumper
Shock mount
Figure 11-159. A bonding jumper is used to ground an equipment
rack and avionics chassis around the non-conductive shock mount
material.
Radios installed in instrument panels do not ordinarily require
vibration protection since the panel itself is usually shock
mounted. However, make certain that the added weight of any
added equipment can be safely carried by the existing mounts.
In some cases, it may be necessary to install larger capacity
mounts or to increase the number of mounting points.
Periodic inspection of the shock mounts is required and
defective mounts should be replaced with the proper type.
The following factors to observe during the inspection are:
1. Deterioration of the shock-absorbing material;
2. Stiffness and resiliency of the material; and
3. Overall rigidity of the mount.
If the mount is too stiff, it may not provide adequate
protection against the shock of landing. If the shock mount is
not stiff enough, it may allow prolonged vibration following
an initial shock.
Shock-absorbing materials commonly used in shock
mounts are usually electrical insulators. For this reason,
each electronic unit mounted with shock mounts must be
electrically bonded to a structural member of the aircraft to
provide a current path to ground. This is accomplished by
secure attachment of a tinned copper wire braid from the
component, across the mount, to the aircraft structure as
shown in Figure 11-159. Occasional bonding is accomplished
with solid aluminum or copper material where a short flexible
strap is not possible.
Reducing Radio Interference
Suppression of unwanted electromagnetic fields and
electrostatic interference is essential on all aircraft. In
communication radios, this is noticeable as audible noise.
In other components, the effects may not be audible but
pose a threat to proper operation. Large discharges of static
electricity can permanently damage the sensitive solid-state
microelectronics found in nearly all modern avionics.
Shielding
Many components of an aircraft are possible sources of
electrical interference which can deteriorate the performance
and reliability of avionics components. Rotating electrical
devices, switching devices, ignition systems, propeller
control systems, AC power lines, and voltage regulators
all produce potential damaging fields. Shielding wires to
electric components and ignition systems dissipates radio
frequency noise energy. Instead of radiating into space, the
braided conductive shielding guides unwanted current flows
to ground. To prevent the build-up of electrical potential, all
electrical components should also be bonded to the aircraft
structure (ground).
Isolation
Isolation is another practical method of radio frequency
suppression to prevent interference. This involves separating
the source of the noise from the input circuits of the affected
equipment. In some cases, noise in a receiver may be entirely
eliminated simply by moving the antenna lead-in wire just
a few inches away from a noise source. On other occasions,
when shielding and isolation are not effective, a filter
may need to be installed in the input circuit of an affected
component.
Bonding
The aircraft surface can become highly charged with static
electricity while in flight. Measures are required to eliminate
the build-up and radiation of unwanted electrical charges.
One of the most important measures taken to eliminate
unwanted electrical charges which may damage or interfere
with avionics equipment is bonding. Charges flowing in
paths of variable resistance due to such causes as intermittent
contact from vibration or the movement of a control surface
produce electrical disturbances (noise) in avionics. Bonding
provides the necessary electric connection between metallic
parts of an aircraft to prevent variable resistance in the
airframe. It provides a low-impedance ground return which
minimizes interference from static electricity charges.
All metal parts of the aircraft should be bonded to prevent
the development of electrical potential build-up. Bonding
also provides the low resistance return path for singlewire
electrical systems. Bonding jumpers and clamps are
examples of bonding connectors. Jumpers should be as short
as possible. Be sure finishes are removed in the contact area
of a bonding device so that metal-to-metal contact exists.
Resistance should not exceed .003 ohm. When a jumper
is used only to reduce radio frequency noise and is not
11-75
Figure 11-160. Static dischargers or wicks dissipate built up static energy in flight at points a safe distance from avionics antennas to
prevent radio frequency interference.
for current carrying purposes, a resistance of 0.01 ohm is
satisfactory.
Static Discharge Wicks
Static dischargers, or wicks, are installed on aircraft to
reduce radio receiver interference. This interference is
caused by corona discharge emitted from the aircraft as a
result of precipitation static. Corona occurs in short pulses
which produce noise at the radio frequency spectrum. Static
dischargers are normally mounted on the trailing edges of the
control surfaces, wing tips and the vertical stabilizer. They
discharge precipitation static at points a critical distance away
from avionics antennas where there is little or no coupling
of the static to cause interference or noise.
Flexible and semi-flexible dischargers are attached to the
aircraft structure by metal screws, rivets, or epoxy. The
connections should be checked periodically for security.
A resistance measurement from the mount to the airframe
should not exceed 0.1 ohm. Inspect the condition of all static
dischargers in accordance with manufacturer’s instructions.
Figure 11-160 illustrates examples of static dischargers.
Installation of Aircraft Antenna Systems
Knowledge of antenna installation and maintenance is
especially important as these tasks are performed by the
aircraft technician. Antennas take many forms and sizes
dependent upon the frequency of the transmitter and
receiver to which they are connected. Airborne antennas
must be mechanically secure. The air loads on an antenna
are significant and must be considered. Antennas must be
electrically matched to the receiver and transmitter which
they serve. They must also be mounted in interference
free locations and in areas where signals can be optimally
transmitted and received. Antennas must also have the same
polarization as the ground station.
The following procedures describe the installation of a
typical rigid antenna. They are presented as an example
only. Always follow the manufacturer’s instructions when
installing any antenna. An incorrect antenna installation could
cause equipment failure.
1. Place a template similar to that shown in Figure 11-161
on the fore-and-aft centerline at the desired location.
Drill the mounting holes and correct diameter hole for
the transmission line cable in the fuselage skin.
11-76
Fuselage skin
Antenna
Existing stringers
Reinforcing doubler Alclad 2024-T3
Approximately one inch spacing
of 1/8" minimum diameter rivet
A A
View A - A
1½" edge distance minimum
Figure 11-162. A typical antenna installation on a skin panel
including a doubler.
No. 18 drill
Sufficient size to accommodate transmission line cable
C/L
Figure 11-161. A typical antenna mounting template.
2. Install a reinforcing doubler of sufficient thickness to
reinforce the aircraft skin. The length and width of
the reinforcing plate should approximate the example
shown in Figure 11-162.
3. Install the antenna on the fuselage, making sure that
the mounting bolts are tightened firmly against the
reinforcing doubler, and the mast is drawn tight against
the gasket. If a gasket is not used, seal between the
mast and the fuselage with a suitable sealer, such as
zinc chromate paste or equivalent.
The mounting bases of antennas vary in shape and sizes;
however, the aforementioned installation procedure is typical
of mast-type antenna installations.
Transmission Lines
A transmitting or receiving antenna is connected directly to its
associated transmitter or receiver by a transmission line. This
is a shielded wire also known as coax. Transmission lines may
vary from only a few feet to many feet in length. They must
transfer energy with minimal loss. Transponders, DME and
other pulse type transceivers require transmission lines that
are precise in length. The critical length of transmission lines
provides minimal attenuation of the transmitted or received
signal. Refer to the equipment manufacturer’s installation
manual for the type and allowable length of transmission
lines.
To provide the proper impedance matching for the most
efficient power transfer, a balun may be used in some antenna
installations. It is formed in the transmission line connection
to the antenna. A balun in a dipole antenna installation is
illustrated in Figure 11-163.
Coax connectors are usually used with coax cable to ensure
a secure connection. Many transmission lines are part of
the equipment installation kit with connectors previously
installed. The aircraft technician is also able to install these
connectors on coax. Figure 11-164 illustrates the basic steps
used when installing a coax cable connector.
When installing coaxial cable, secure the cables firmly
along their entire length at intervals of approximately 2
feet. To assure optimum operation, coaxial cables should
not be routed or tied to other wire bundles. When bending
coaxial cable, be sure that the bend is at least 10 times the
size of the cable diameter. In all cases, follow the equipment
manufacturer’s instructions.
Maintenance Procedure
Detailed instructions, procedures, and specifications for
the servicing of avionics equipment are contained in the
manufacturer’s operating manuals. Additional instructions
for removal and installation of the units are contained in the
maintenance manual for the aircraft in which the equipment
is installed. Although an installation may appear to be a
simple procedure, many avionics troubles are attributed to
11-77
32
3
Remove ¼-inch of the outer insulation
Separate and fan out the braid
Remove 1/8-inch of the inner insulator
from the center conductor
Add the clamp to the end of the cable outer insulator
and slide the nut, washer, and gasket toward it
Neatly fold back the separated shielding strands
over the taper of the clamp and trim evenly with the
end of the taper. Slide the gasket to the clamp.
Tin the inner conductor with 60-40 resin core solder.
Slide the inner contact over the cable end until flush
with the inner insulator. Solder the contact to the
conductor. Slip the connector jack body over the end
of the cable and secure it with the nut and washer.
Nut Washer Gasket Clamp
Jack body Insulator Contact
Braid
Gasket Clamp
¼
1/8
Figure 11-164. Steps in attaching a connector to coax cable used
as antenna transmission lines.
4
To navigation receiver
Center conductor open
Twisted shield
Airframe ground
Protective outer covering
Wire wrapped and
soldered to shield
Attach to antenna dipoles
Shield removed
Figure 11-163. A balun in a dipole antenna installation provides
the proper impedance for efficient power transfer.
careless oversights during equipment replacement. Loose
cable connections, switched cable terminations, improper
bonding, worn shock mounts, improper safety wiring, and
failure to perform an operational check after installation may
result in poor performance or inoperative avionics.
11-78
12-1
Aircraft Hydraulic Systems
The word “hydraulics” is based on the Greek word for water
and originally meant the study of the physical behavior of
water at rest and in motion. Today, the meaning has been
expanded to include the physical behavior of all liquids,
including hydraulic fluid. Hydraulic systems are not new
to aviation. Early aircraft had hydraulic brake systems. As
aircraft became more sophisticated, newer systems with
hydraulic power were developed.
Hydraulic systems in aircraft provide a means for the
operation of aircraft components. The operation of landing
gear, flaps, flight control surfaces, and brakes is largely
accomplished with hydraulic power systems. Hydraulic
system complexity varies from small aircraft that require
fluid only for manual operation of the wheel brakes to large
transport aircraft where the systems are large and complex. To
achieve the necessary redundancy and reliability, the system
may consist of several subsystems. Each subsystem has a
power generating device (pump) reservoir, accumulator, heat
exchanger, filtering system, etc. System operating pressure
may vary from a couple hundred pounds per square inch (psi)
in small aircraft and rotorcraft to 5,000 psi in large transports.
Hydraulic and Pneumatic
Power Systems
Chapter 12
12-2
Heating unit
Container
Cork Reservoir
Liquid bath Thermometer
Oil
60 c.c.
Figure 12-1. Saybolt viscosimeter.
Hydraulic systems have many advantages as power sources
for operating various aircraft units; they combine the
advantages of light weight, ease of installation, simplification
of inspection, and minimum maintenance requirements.
Hydraulic operations are also almost 100 percent efficient,
with only negligible loss due to fluid friction.
Hydraulic Fluid
Hydraulic system liquids are used primarily to transmit and
distribute forces to various units to be actuated. Liquids
are able to do this because they are almost incompressible.
Pascal’s Law states that pressure applied to any part of a
confined liquid is transmitted with undiminished intensity
to every other part. Thus, if a number of passages exist in a
system, pressure can be distributed through all of them by
means of the liquid.
Manufacturers of hydraulic devices usually specify the type
of liquid best suited for use with their equipment in view of
the working conditions, the service required, temperatures
expected inside and outside the systems, pressures the liquid
must withstand, the possibilities of corrosion, and other
conditions that must be considered. If incompressibility
and fluidity were the only qualities required, any liquid that
is not too thick could be used in a hydraulic system. But a
satisfactory liquid for a particular installation must possess
a number of other properties. Some of the properties and
characteristics that must be considered when selecting a
satisfactory liquid for a particular system are discussed in
the following paragraphs.
Viscosity
One of the most important properties of any hydraulic fluid is
its viscosity. Viscosity is internal resistance to flow. A liquid
such as gasoline that has a low viscosity flows easily, while
a liquid such as tar that has a high viscosity flows slowly.
Viscosity increases as temperature decreases. A satisfactory
liquid for a given hydraulic system must have enough body
to give a good seal at pumps, valves, and pistons, but it must
not be so thick that it offers resistance to flow, leading to
power loss and higher operating temperatures. These factors
add to the load and to excessive wear of parts. A fluid that is
too thin also leads to rapid wear of moving parts or of parts
that have heavy loads. The instruments used to measure
the viscosity of a liquid are known as viscometers or
viscosimeters. Several types of viscosimeters are in use today.
The Saybolt viscometer measures the time required, in
seconds, for 60 milliliters of the tested fluid at 100 °F to
pass through a standard orifice. The time measured is used to
express the fluid’s viscosity, in Saybolt universal seconds
or Saybolt furol seconds. [Figure 12-1]
Chemical Stability
Chemical stability is another property that is exceedingly
important in selecting a hydraulic liquid. It is the liquid’s
ability to resist oxidation and deterioration for long periods.
All liquids tend to undergo unfavorable chemical changes
under severe operating conditions. This is the case, for
example, when a system operates for a considerable period
of time at high temperatures. Excessive temperatures have
a great effect on the life of a liquid. It should be noted that
the temperature of the liquid in the reservoir of an operating
hydraulic system does not always represent a true state of
operating conditions. Localized hot spots occur on bearings,
gear teeth, or at the point where liquid under pressure is
forced through a small orifice. Continuous passage of a liquid
through these points may produce local temperatures high
enough to carbonize or sludge the liquid, yet the liquid in the
reservoir may not indicate an excessively high temperature.
Liquids with a high viscosity have a greater resistance
to heat than light or low-viscosity liquids that have been
derived from the same source. The average hydraulic liquid
has a low viscosity. Fortunately, there is a wide choice of
liquids available for use within the viscosity range required
of hydraulic liquids.
Liquids may break down if exposed to air, water, salt, or
other impurities, especially if they are in constant motion or
subject to heat. Some metals, such as zinc, lead, brass, and
copper, have an undesirable chemical reaction on certain
liquids. These chemical processes result in the formation
of sludge, gums, and carbon or other deposits that clog
openings, cause valves and pistons to stick or leak, and give
12-3
poor lubrication to moving parts. As soon as small amounts
of sludge or other deposits are formed, the rate of formation
generally increases more rapidly. As they are formed, certain
changes in the physical and chemical properties of the liquid
take place. The liquid usually becomes darker in color, higher
in viscosity, and acids are formed.
Flash Point
Flash point is the temperature at which a liquid gives off vapor
in sufficient quantity to ignite momentarily or flash when a
flame is applied. A high flash point is desirable for hydraulic
liquids because it indicates good resistance to combustion and
a low degree of evaporation at normal temperatures.
Fire Point
Fire point is the temperature at which a substance gives off
vapor in sufficient quantity to ignite and continue to burn
when exposed to a spark or flame. Like flash point, a high
fire point is required of desirable hydraulic liquids.
Types of Hydraulic Fluids
To assure proper system operation and to avoid damage to
nonmetallic components of the hydraulic system, the correct
fluid must be used. When adding fluid to a system, use the
type specified in the aircraft manufacturer’s maintenance
manual or on the instruction plate affixed to the reservoir or
unit being serviced.
The three principal categories of hydraulic fluids are:
1. Minerals
2. Polyalphaolefins
3. Phosphate esters
When servicing a hydraulic system, the technician must
be certain to use the correct category of replacement fluid.
Hydraulic fluids are not necessarily compatible. For example,
contamination of the fire-resistant fluid MIL-H-83282 with
MIL-H-5606 may render the MIL-H-83282 non fire-resistant.
Mineral-Based Fluids
Mineral oil-based hydraulic fluid (MIL-H-5606) is the oldest,
dating back to the 1940s. It is used in many systems, especially
where the fire hazard is comparatively low. MIL-H-6083
is simply a rust-inhibited version of MIL-H-5606. They
are completely interchangeable. Suppliers generally ship
hydraulic components with MIL-H-6083. Mineral-based
hydraulic fluid (MIL–H-5606) is processed from petroleum.
It has an odor similar to penetrating oil and is dyed red.
Synthetic rubber seals are used with petroleum-based fluids.
Polyalphaolefin-Based Fluids
MIL-H-83282 is a fire-resistant hydrogenated polyalphaolefinbased
fluid developed in the 1960s to overcome the
flammability characteristics of MIL-H-5606. MIL-H-83282
is significantly more flame resistant than MIL-H-5606, but
a disadvantage is the high viscosity at low temperature.
It is generally limited to –40 °F. However, it can be used
in the same system and with the same seals, gaskets, and
hoses as MIL-H-5606. MIL-H-46170 is the rust-inhibited
version of MIL-H-83282. Small aircraft predominantly use
MIL-H-5606, but some have switched to MIL-H-83282 if
they can accommodate the high viscosity at low temperature.
Phosphate Ester-Based Fluid (Skydrol®)
These fluids are used in most commercial transport category
aircraft and are extremely fire-resistant. However, they are not
fireproof and under certain conditions, they burn. The earliest
generation of these fluids was developed after World War II as
a result of the growing number of aircraft hydraulic brake fires
that drew the collective concern of the commercial aviation
industry. Progressive development of these fluids occurred as
a result of performance requirements of newer aircraft designs.
The airframe manufacturers dubbed these new generations of
hydraulic fluid as types based on their performance.
Today, types IV and V fluids are used. Two distinct classes
of type IV fluids exist based on their density: class I fluids
are low density and class II fluids are standard density. The
class I fluids provide weight savings advantages versus
class II. In addition to the type IV fluids that are currently
in use, type V fluids are being developed in response to
industry demands for a more thermally stable fluid at higher
operating temperatures. Type V fluids will be more resistant
to hydrolytic and oxidative degradation at high temperature
than the type IV fluids.
Intermixing of Fluids
Due to the difference in composition, petroleum-based and
phosphate ester-based fluids will not mix; neither are the
seals for any one fluid usable with or tolerant of any of the
other fluids. Should an aircraft hydraulic system be serviced
with the wrong type fluid, immediately drain and flush the
system and maintain the seals according to the manufacturer’s
specifications.
Compatibility with Aircraft Materials
Aircraft hydraulic systems designed around Skydrol®
fluids should be virtually trouble-free if properly serviced.
Skydrol® is a registered trademark of Monsanto Company.
Skydrol® does not appreciably affect common aircraft
metals—aluminum, silver, zinc, magnesium, cadmium, iron,
stainless steel, bronze, chromium, and others—as long as the
fluids are kept free of contamination. Due to the phosphate
12-4
ester base of Skydrol® fluids, thermoplastic resins, including
vinyl compositions, nitrocellulose lacquers, oil-based
paints, linoleum, and asphalt may be softened chemically
by Skydrol® fluids. However, this chemical action usually
requires longer than just momentary exposure, and spills that
are wiped up with soap and water do not harm most of these
materials. Paints that are Skydrol® resistant include epoxies
and polyurethanes. Today, polyurethanes are the standard of
the aircraft industry because of their ability to keep a bright,
shiny finish for long periods of time and for the ease with
which they can be removed.
Hydraulic systems require the use of special accessories that
are compatible with the hydraulic fluid. Appropriate seals,
gaskets, and hoses must be specifically designated for the
type of fluid in use. Care must be taken to ensure that the
components installed in the system are compatible with the
fluid. When gaskets, seals, and hoses are replaced, positive
identification should be made to ensure that they are made of
the appropriate material. Skydrol® type V fluid is compatible
with natural fibers and with a number of synthetics, including
nylon and polyester, which are used extensively in most
aircraft. Petroleum oil hydraulic system seals of neoprene or
Buna-N are not compatible with Skydrol® and must be replaced
with seals of butyl rubber or ethylene-propylene elastoiners.
Hydraulic Fluid Contamination
Experience has shown that trouble in a hydraulic system
is inevitable whenever the liquid is allowed to become
contaminated. The nature of the trouble, whether a simple
malfunction or the complete destruction of a component,
depends to some extent on the type of contaminant. Two
general contaminants are:
• Abrasives, including such particles as core sand, weld
spatter, machining chips, and rust.
• Nonabrasives, including those resulting from oil
oxidation and soft particles worn or shredded from
seals and other organic components.
Contamination Check
Whenever it is suspected that a hydraulic system has become
contaminated or the system has been operated at temperatures
in excess of the specified maximum, a check of the system
should be made. The filters in most hydraulic systems are
designed to remove most foreign particles that are visible
to the naked eye. Hydraulic liquid that appears clean to the
naked eye may be contaminated to the point that it is unfit for
use. Thus, visual inspection of the hydraulic liquid does not
determine the total amount of contamination in the system.
Large particles of impurities in the hydraulic system are
indications that one or more components are being subjected
to excessive wear. Isolating the defective component requires
a systematic process of elimination. Fluid returned to the
reservoir may contain impurities from any part of the system.
To determine which component is defective, liquid samples
should be taken from the reservoir and various other locations
in the system. Samples should be taken in accordance with
the applicable manufacturer’s instructions for a particular
hydraulic system. Some hydraulic systems are equipped with
permanently installed bleed valves for taking liquid samples,
whereas on other systems, lines must be disconnected to
provide a place to take a sample.
Hydraulic Sampling Schedule
• Routine sampling—each system should be sampled
at least once a year, or every 3,000 flight hours, or
whenever the airframe manufacturer suggests.
• Unscheduled maintenance—when malfunctions may
have a fluid related cause, samples should be taken.
• Suspicion of contamination—if contamination is
suspected, fluids should be drained and replaced,
with samples taken before and after the maintenance
procedure.
Sampling Procedure
• Pressurize and operate hydraulic system for 10–15
minutes. During this period, operate various flight
controls to activate valves and thoroughly mix
hydraulic fluid.
• Shut down and depressurize the system.
• Before taking samples, always be sure to wear the
proper personal protective equipment that should
include, at the minimum, safety glasses and gloves.
• Wipe off sampling port or tube with a lint-free cloth.
Do not use shop towels or paper products that could
produce lint. Generally speaking, the human eye can
see particles down to about 40 microns in size. Since
we are concerned with particles down to 5 microns in
size, it is easy to contaminate a sample without ever
knowing it.
• Place a waste container under the reservoir drain
valve and open valve so that a steady, but not forceful,
stream is running.
• Allow approximately 1 pint (250 ml) of fluid to drain.
This purges any settled particles from the sampling
port.
• Insert a precleaned sample bottle under the fluid stream
and fill, leaving an air space at the top. Withdraw the
bottle and cap immediately.
• Close drain valve.
12-5
• Fill out sample identification label supplied in sample
kit, making sure to include customer name, aircraft
type, aircraft tail number, hydraulic system sampled,
and date sampled. Indicate on the sample label under
remarks if this is a routine sample or if it is being taken
due to a suspected problem.
• Service system reservoirs to replace the fluid that was
removed.
• Submit samples for analysis to laboratory.
Contamination Control
Filters provide adequate control of the contamination
problem during all normal hydraulic system operations.
Control of the size and amount of contamination entering
the system from any other source is the responsibility of the
people who service and maintain the equipment. Therefore,
precautions should be taken to minimize contamination
during maintenance, repair, and service operations. If the
system becomes contaminated, the filter element should be
removed and cleaned or replaced. As an aid in controlling
contamination, the following maintenance and servicing
procedures should be followed at all times:
• Maintain all tools and the work area (workbenches
and test equipment) in a clean, dirt-free condition.
• A suitable container should always be provided to
receive the hydraulic liquid that is spilled during
component removal or disassembly procedures.
• Before disconnecting hydraulic lines or fittings, clean
the affected area with dry cleaning solvent.
• All hydraulic lines and fittings should be capped or
plugged immediately after disconnecting.
• Before assembly of any hydraulic components, wash
all parts in an approved dry cleaning solvent.
• After cleaning the parts in the dry cleaning solution,
dry the parts thoroughly and lubricate them with the
recommended preservative or hydraulic liquid before
assembly. Use only clean, lint-free cloths to wipe or
dry the component parts.
• All seals and gaskets should be replaced during the
reassembly procedure. Use only those seals and
gaskets recommended by the manufacturer.
• All parts should be connected with care to avoid
stripping metal slivers from threaded areas. All fittings
and lines should be installed and torqued in accordance
with applicable technical instructions.
• All hydraulic servicing equipment should be kept clean
and in good operating condition.
Contamination, both particulate and chemical, is detrimental
to the performance and life of components in the aircraft
hydraulic system. Contamination enters the system through
normal wear of components by ingestion through external
seals during servicing, or maintenance, when the system is
opened to replace/repair components, etc. To control the
particulate contamination in the system, filters are installed
in the pressure line, in the return line, and in the pump case
drain line of each system. The filter rating is given in microns
as an indication of the smallest particle size that is filtered
out. The replacement interval of these filters is established
by the manufacturer and is included in the maintenance
manual. In the absence of specific replacement instructions,
a recommended service life of the filter elements is:
• Pressure filters—3,000 hours
• Return Filters—1,500 hours
• Case drain filters—600 hours
Hydraulic System Flushing
When inspection of hydraulic filters or hydraulic fluid
evaluation indicates that the fluid is contaminated, flushing
the system may be necessary. This must be done according to
the manufacturer’s instructions; however, a typical procedure
for flushing is as follows:
1. Connect a ground hydraulic test stand to the inlet and
outlet test ports of the system. Verify that the ground
unit fluid is clean and contains the same fluid as
the aircraft.
2. Change the system filters.
3. Pump clean, filtered fluid through the system, and
operate all subsystems until no obvious signs of
contamination are found during inspection of the filters.
Dispose of contaminated fluid and filter. Note: A visual
inspection of hydraulic filters is not always effective.
4. Disconnect the test stand and cap the ports.
5. Ensure that the reservoir is filled to the full line or
proper service level.
It is very important to check if the fluid in the hydraulic test
stand, or mule, is clean before the flushing operation starts.
A contaminated hydraulic test stand can quickly contaminate
other aircraft if used for ground maintenance operations.
Health and Handling
Skydrol® fluids are phosphate ester-based fluids blended with
performance additives. Phosphate esters are good solvents
and dissolve away some of the fatty materials of the skin.
Repeated or prolonged exposure may cause drying of the
skin, which if unattended, could result in complications,
such as dermatitis or even secondary infection from bacteria.
12-6
Hand
pump
Filter
Motor-driven pump
Reservoir
Actuating cylinder
Pressure
relief
valve
One-way
check
valve
Suction
Pressure
Return
Figure 12-2. Basic hydraulic system.
Reservoir
Relief
valve
Pump
Directional
control valves
Suction
Pressure
Return
A
B
Figure 12-3. Open center hydraulic system.
Skydrol® fluids could cause itching of the skin but have not
been known to cause allergic-type skin rashes. Always use the
proper gloves and eye protection when handling any type of
hydraulic fluid. When Skydrol®/Hyjet mist or vapor exposure
is possible, a respirator capable of removing organic vapors
and mists must be worn. Ingestion of any hydraulic fluid
should be avoided. Although small amounts do not appear to
be highly hazardous, any significant amount should be tested
in accordance with manufacturer’s direction, followed with
hospital supervised stomach treatment.
Basic Hydraulic Systems
Regardless of its function and design, every hydraulic system
has a minimum number of basic components in addition to a
means through which the fluid is transmitted. A basic system
consists of a pump, reservoir, directional valve, check valve,
pressure relieve valve, selector valve, actuator, and filter.
[Figure 12-2]
Open Center Hydraulic Systems
An open center system is one having fluid flow, but no
pressure in the system when the actuating mechanisms are
idle. The pump circulates the fluid from the reservoir, through
the selector valves, and back to the reservoir. [Figure 12-3]
The open center system may employ any number of
subsystems, with a selector valve for each subsystem. Unlike
the closed center system, the selector valves of the open
center system are always connected in series with each other.
In this arrangement, the system pressure line goes through
each selector valve. Fluid is always allowed free passage
through each selector valve and back to the reservoir until one
of the selector valves is positioned to operate a mechanism.
When one of the selector valves is positioned to operate an
actuating device, fluid is directed from the pump through one
of the working lines to the actuator. [Figure 12-3B] With the
selector valve in this position, the flow of fluid through the
valve to the reservoir is blocked. The pressure builds up in
the system to overcome the resistance and moves the piston
of the actuating cylinder; fluid from the opposite end of the
actuator returns to the selector valve and flows back to the
reservoir. Operation of the system following actuation of the
component depends on the type of selector valve being used.
Several types of selector valves are used in conjunction with
the open center system. One type is both manually engaged
and manually disengaged. First, the valve is manually moved
to an operating position. Then, the actuating mechanism
reaches the end of its operating cycle, and the pump output
continues until the system relief valve relieves the pressure.
The relief valve unseats and allows the fluid to flow back to
the reservoir. The system pressure remains at the relief valve
set pressure until the selector valve is manually returned to the
neutral position. This action reopens the open center flow and
allows the system pressure to drop to line resistance pressure.
The manually engaged and pressure disengaged type of
selector valve is similar to the valve previously discussed.
When the actuating mechanism reaches the end of its cycle,
the pressure continues to rise to a predetermined pressure.
The valve automatically returns to the neutral position and
to open center flow.
12-7
Figure 12-5. Hydraulic power pack.
Static
Suction
Pressure
Return
Reservoir
Pump
Relief valve
Actuating
unit
Actuating
unit
Actuating
unit
Selector valve
(closed) A
Selector valve
(operating) B
Selector valve
(operating) C
Figure 12-4. A basic closed-center hydraulic system with a variable displacement pump.
Closed-Center Hydraulic Systems
In the closed-center system, the fluid is under pressure
whenever the power pump is operating. The three actuators are
arranged in parallel and actuating units B and C are operating
at the same time, while actuating unit A is not operating. This
system differs from the open-center system in that the selector
or directional control valves are arranged in parallel and not in
series. The means of controlling pump pressure varies in the
closed-center system. If a constant delivery pump is used, the
system pressure is regulated by a pressure regulator. A relief
valve acts as a backup safety device in case the regulator fails.
If a variable displacement pump is used, system pressure
is controlled by the pump’s integral pressure mechanism
compensator. The compensator automatically varies the
volume output. When pressure approaches normal system
pressure, the compensator begins to reduce the flow output
of the pump. The pump is fully compensated (near zero flow)
when normal system pressure is attained. When the pump
is in this fully compensated condition, its internal bypass
mechanism provides fluid circulation through the pump for
cooling and lubrication. A relief valve is installed in the
system as a safety backup. [Figure 12-4] An advantage of the
open-center system over the closed-center system is that the
continuous pressurization of the system is eliminated. Since
the pressure is built up gradually after the selector valve is
moved to an operating position, there is very little shock from
pressure surges. This action provides a smoother operation of
the actuating mechanisms. The operation is slower than the
closed-center system, in which the pressure is available the
moment the selector valve is positioned. Since most aircraft
applications require instantaneous operation, closed-center
systems are the most widely used.
Hydraulic Power Systems
Evolution of Hydraulic Systems
Smaller aircraft have relatively low flight control surface
loads, and the pilot can operate the flight controls by hand.
Hydraulic systems were utilized for brake systems on early
aircraft. When aircraft started to fly faster and got larger in
size, the pilot was not able to move the control surfaces by
hand anymore, and hydraulic power boost systems were
introduced. Power boost systems assist the pilot in overcoming
high control forces, but the pilot still actuates the flight
controls by cable or push rod.
Many modern aircraft use a power supply system and fly-bywire
flight control. The pilot input is electronically sent to the
flight control servos. Cables or push rods are not used. Small
power packs are the latest evolution of the hydraulic system.
They reduce weight by eliminating hydraulic lines and large
quantities of hydraulic fluid. Some manufacturers are reducing
hydraulic systems in their aircraft in favor of electrically
controlled systems. The Boeing 787 is the first aircraft designed
with more electrical systems than hydraulic systems.
Hydraulic Power Pack System
A hydraulic power pack is a small unit that consists of an
electric pump, filters, reservoir, valves, and pressure relief
valve. [Figure 12-5] The advantage of the power pack is that
12-8
ACMP
C2
ACMP
C1
Left
G&P
Nose landing
gear actuation
Nose landing
gear steering
TE flaps
primary prive
LE slats
primary prive
Thrust
reversers
Main landing
gear steering
Wing flight
(controls)
Tail flight
(controls)
Wing flight
controls
Tail flight
controls
Wing flight
controls
Tail flight
controls
Main landing
gear actuation
Brakes
Left
ACMP
ADP
C2
ADP
C1
RAT Right
RCMP
Right
CDP
Flap/slat
priority VLV
Autoblat
priority VLV
ALTN EXT
power pack Heat exchanger
Heat
exchanger
RSVR fill bel MLV
Center
system
return
Hand pump
Left system Right system
Pressure
Return
Supply
Bleed air
Center
system
return
Left system
return
Right system return
Right
system
return
Left
system
return Center system
Figure 12-6. Large commercial aircraft hydraulic system.
there is no need for a centralized hydraulic power supply
system and long stretches of hydraulic lines, which reduces
weight. Power packs could be driven by either an engine
gearbox or electric motor. Integration of essential valves,
filters, sensors, and transducers reduces system weight,
virtually eliminates any opportunity for external leakage, and
simplifies troubleshooting. Some power pack systems have
an integrated actuator. These systems are used to control the
stabilizer trim, landing gear, or flight control surfaces directly,
thus eliminating the need for a centralized hydraulic system.
Hydraulic System Components
Figure 12-6 is a typical example of a hydraulic system in a
large commercial aircraft. The following sections discuss the
components of such system in more detail.
Reservoirs
The reservoir is a tank in which an adequate supply of fluid
for the system is stored. Fluid flows from the reservoir to the
pump, where it is forced through the system and eventually
returned to the reservoir. The reservoir not only supplies the
operating needs of the system, but it also replenishes fluid
lost through leakage. Furthermore, the reservoir serves as an
overflow basin for excess fluid forced out of the system by
thermal expansion (the increase of fluid volume caused by
temperature changes), the accumulators, and by piston and
rod displacement.
The reservoir also furnishes a place for the fluid to purge
itself of air bubbles that may enter the system. Foreign matter
picked up in the system may also be separated from the fluid
in the reservoir or as it flows through line filters. Reservoirs
are either pressurized or nonpressurized.
Baffles and/or fins are incorporated in most reservoirs to keep
the fluid within the reservoir from having random movement,
such as vortexing (swirling) and surging. These conditions
can cause fluid to foam and air to enter the pump along with
the fluid. Many reservoirs incorporate strainers in the filler
neck to prevent the entry of foreign matter during servicing.
These strainers are made of fine mesh screening and are
usually referred to as finger strainers because of their shape.
Finger strainers should never be removed or punctured as a
means of speeding up the pouring of fluid into the reservoir.
12-9
Brakes PFCS
(R SYS) T/R
S
Sample valve
From RSVR servicing
Heat exchanger
ACMP
filter
module
GND
SVC
RTN
disk
SYS pressurized XDCR
Temp XDCR
ACMP
EDP
Pressurized XDCR
GND SVC
pressurized disk
RLF valve
RSVR
pressurized
SW
Return filter module
RSVR pressurized module
EDP filter
module
EDP supply shutoff valve
RSVR pressurized shutoff valve
Drain valve
RSVR pressurized relief valve
RSVR
temp
XDCR
Pressurized air
Supply
Pressure
Return
Depress solenoid valve
TEMP
XDCR
Pressurized
XDCR
Figure 12-7. Hydraulic reservoir standpipe for emergency operations.
metal. Filter elements are normally installed within the
reservoir to clean returning system hydraulic fluid.
In some of the older aircraft, a filter bypass valve is
incorporated to allow fluid to bypass the filter in the event the
filter becomes clogged. Reservoirs can be serviced by pouring
fluid directly into the reservoir through a filler strainer (finger
strainer) assembly incorporated within the filler well to strain
out impurities as the fluid enters the reservoir. Generally,
nonpressurized reservoirs use a visual gauge to indicate the
fluid quantity. Gauges incorporated on or in the reservoir may
be a direct reading glass tube-type or a float-type rod that is
visible through a transparent dome. In some cases, the fluid
quantity may also be read in the cockpit through the use of
quantity transmitters. A typical nonpressurized reservoir is
shown in Figure 12-8. This reservoir consists of a welded
body and cover assembly clamped together. Gaskets are
incorporated to seal against leakage between assemblies.
Nonpressurized reservoirs are slightly pressurized due to
thermal expansion of fluid and the return of fluid to the
reservoir from the main system. This pressure ensures that
there is a positive flow of fluids to the inlet ports of the
hydraulic pumps. Most reservoirs of this type are vented
directly to the atmosphere or cabin with only a check valve
and filter to control the outside air source. The reservoir system
Reservoirs could have an internal trap to make sure fluid goes
to the pumps during negative-G conditions.
Most aircraft have emergency hydraulic systems that take
over if main systems fail. In many such systems, the pumps of
both systems obtain fluid from a single reservoir. Under such
circumstances, a supply of fluid for the emergency pump is
ensured by drawing the hydraulic fluid from the bottom of the
reservoir. The main system draws its fluid through a standpipe
located at a higher level. With this arrangement, should the
main system’s fluid supply become depleted, adequate fluid
is left for operation of the emergency system. Figure 12-7
illustrates that the engine-driven pump (EDP) is not able
to draw fluid any more if the reservoir gets depleted below
the standpipe. The alternating current motor-driven pump
(ACMP) still has a supply of fluid for emergency operations.
Nonpressurized Reservoirs
Nonpressurized reservoirs are used in aircraft that are not
designed for violent maneuvers, do not fly at high altitudes, or
in which the reservoir is located in the pressurized area of the
aircraft. High altitude in this situation means an altitude where
atmospheric pressure is inadequate to maintain sufficient
flow of fluid to the hydraulic pumps. Most nonpressurized
reservoirs are constructed in a cylindrical shape. The outer
housing is manufactured from a strong corrosion-resistant
12-10
Glass sight gauge
Filler neck, cap, and fastener
Connection for emergency system pump
Connection for return line
Connection for main system pump
Connection for vent line or pressurizing line
Normal fluid level
Finger strainer
Baffle
Standpipe
Fin Fin
Fin Fin
Figure 12-8. Nonpressurized reservoir. Figure 12-9. Air-pressurized reservoir.
From reservoir
pressurization
module
Quantity
indicator/
transmitter
Return and fill line
Drain/sampling valve
EDP supply line
ACMP supply line
Reservoir pressure relief valve
Vent line
Figure 12-10. Components of an air-pressurized reservoir.
is not enough atmospheric pressure to move the fluid to
the pump inlet. Engine bleed air is used to pressurize the
reservoir. The reservoirs are typically cylindrical in shape.
The following components are installed on a typical reservoir:
• Reservoir pressure relief valve—prevents over
pressurization of the reservoir. Valve opens at a preset
value.
includes a pressure and vacuum relief valve. The purpose of
the valve is to maintain a differential pressure range between
the reservoir and cabin. A manual air bleed valve is installed
on top of the reservoir to vent the reservoir. The valve is
connected to the reservoir vent line to allow depressurization
of the reservoir. The valve is actuated prior to servicing the
reservoir to prevent fluid from being blown out of the filler as
the cap is being removed. The manual bleed valve also needs
to be actuated if hydraulic components need to be replaced.
Pressurized Reservoirs
Reservoirs on aircraft designed for high-altitude flight are
usually pressurized. Pressurizing assures a positive flow of
fluid to the pump at high altitudes when low atmospheric
pressures are encountered. On some aircraft, the reservoir is
pressurized by bleed air taken from the compressor section
of the engine. On others, the reservoir may be pressurized
by hydraulic system pressure.
Air-Pressurized Reservoirs
Air-pressurized reservoirs are used in many commercial
transport-type aircraft. [Figures 12-9 and 12-10]
Pressurization of the reservoir is required because the
reservoirs are often located in wheel wells or other nonpressurized
areas of the aircraft and at high altitude there
12-11
Figure 12-11. Temperature and quantity sensors.
Manual bleed valve
Test port
To HYD RSWR
Filters Check valves
RSVR pressurized shutoff valve
Gauge port Reservoir pressure switch
Figure 12-12. Reservoir pressurization module.
• Sight glasses (low and overfull)—provides visual
indication for flight crews and maintenance personnel
that the reservoir needs to be serviced.
• Reservoir sample valve—used to draw a sample of
hydraulic fluid for testing.
• Reservoir drain valve—used to drain the fluids out of
the reservoir for maintenance operation.
• Reservoir temperature transducer—provides hydraulic
fluid temperature information for the flight deck.
[Figure 12-11]
• Reservoir quantity transmitter—transmits fluid
quantity to the flight deck so that the flight crew can
monitor fluid quantity during flight. [Figure 12-11]
A reservoir pressurization module is installed close to the
reservoir. [Figure 12-12] The reservoir pressurization
module supplies airplane bleed air to the reservoirs. The
module consists of the following parts:
• Filters (2)
• Check valves (2)
• Test port
• Manual bleed valve
• Gauge port
A manual bleeder valve is incorporated into the module.
During hydraulic system maintenance, it is necessary to
relieve reservoir air pressure to assist in the installation and
removal of components, lines, etc. This type of valve is small
in size and has a push button installed in the outer case. When
the bleeder valve push button is pushed, pressurized air from
the reservoir flows through the valve to an overboard vent
until the air pressure is depleted or the button is released.
When the button is released, the internal spring causes the
poppet to return to its seat. Some hydraulic fluid can escape
from the manual bleed valve when the button is depressed.
Caution: Put a rag around the air bleed valve on the reservoir
pressurization module to catch hydraulic fluid spray.
Hydraulic fluid spray can cause injuries to persons.
Fluid-Pressurized Reservoirs
Some aircraft hydraulic system reservoirs are pressurized
by hydraulic system pressure. Regulated hydraulic pump
output pressure is applied to a movable piston inside the
cylindrical reservoir. This small piston is attached to and
moves a larger piston against the reservoir fluid. The reduced
force of the small piston when applied by the larger piston is
adequate to provide head pressure for high altitude operation.
The small piston protrudes out of the body of the reservoir.
The amount exposed is used as a reservoir fluid quantity
indicator. Figure 12-13 illustrates the concept behind the
fluid-pressurized hydraulic reservoir.
The reservoir has five ports: pump suction, return,
pressurizing, overboard drain, and bleed port. Fluid is
supplied to the pump through the pump suction port. Fluid
returns to the reservoir from the system through the return
port. Pressure from the pump enters the pressurizing cylinder
in the top of the reservoir through the pressurizing port. The
12-12
Hydraulic system pressure (3,000 PSI)
Hydraulic 30–50 psi
fluid
Fluid
pressurized
reservoir
Pressurized
fluid to pump inlet
Amount piston
protrudes indicates
reservoir fluid
quantity
Large piston
Vent
Small piston
Figure 12-13. Operating principle behind a fluid-pressurized
hydraulic reservoir.
overboard drain port drains the reservoir, when necessary,
while performing maintenance. The bleed port is used as
an aid in servicing the reservoir. When servicing a system
equipped with this type of reservoir, place a container under
the bleed drain port. The fluid should then be pumped into the
reservoir until air-free fluid flows through the bleed drain port.
The reservoir fluid level is indicated by the markings on
the part of the pressurizing cylinder that moves through
the reservoir dust cover assembly. There are three fluid
level markings indicated on the cover: full at zero system
pressure (FULL ZERO PRESS), full when system is
pressurized (FULL SYS PRESS), and REFILL. When the
system is unpressurized and the pointer on the reservoir lies
between the two full marks, a marginal reservoir fluid level
is indicated. When the system is pressurized and the pointer
lies between REFILL and FULL SYS PRESS, a marginal
reservoir fluid level is also indicated.
Reservoir Servicing
Nonpressurized reservoirs can be serviced by pouring fluid
directly into the reservoir through a filler strainer (finger
strainer) assembly incorporated within the filler well to
strain out impurities as the fluid enters the reservoir. Many
reservoirs also have a quick disconnect service port at
the bottom of the reservoir. A hydraulic filler unit can be
connected to the service port to add fluid to the reservoir. This
method reduces the chances of contamination of the reservoir.
Aircraft that use pressurized reservoirs often have a central
filling station in the ground service bay to service all
reservoirs from a single point. [Figure 12-14]
A built-in hand pump is available to draw fluid from
a container through a suction line and pump it into the
reservoirs. Additionally, a pressure fill port is available for
attachment of a hydraulic mule or serving cart, which uses an
external pump to push fluid into the aircraft hydraulic system.
A check valve keeps the hand pump output from exiting the
pressure fill port. A single filter is located downstream of
both the pressure fill port and the hand pump to prevent the
introduction of contaminants during fluid servicing.
It is very important to follow the maintenance instructions
when servicing the reservoir. To get the correct results when
the hydraulic fluid quantities are checked or the reservoirs are
to be filled, the airplane should be in the correct configuration.
Failure to do so could result in overservicing of the reservoir.
This configuration could be different for each aircraft. The
following service instructions are an example of a large
transport-type aircraft.
Before servicing always make sure that the:
• Spoilers are retracted,
• Landing gear is down,
• Landing gear doors are closed,
• Thrust reversers are retracted, and
• Parking brake accumulator pressure reads at least
2,500 psi.
Filters
A filter is a screening or straining device used to clean
the hydraulic fluid, preventing foreign particles and
contaminating substances from remaining in the system.
[Figure 12-15] If such objectionable material were not
removed, the entire hydraulic system of the aircraft could
fail through the breakdown or malfunctioning of a single
unit of the system.
The hydraulic fluid holds in suspension tiny particles of
metal that are deposited during the normal wear of selector
valves, pumps, and other system components. Such minute
particles of metal may damage the units and parts through
which they pass if they are not removed by a filter. Since
tolerances within the hydraulic system components are quite
small, it is apparent that the reliability and efficiency of the
entire system depends upon adequate filtering.
12-13
System B reservoir
System A reservoir
Check valve
Hand pump
To reservoir pressurization system
Filter
Balance lines
Standby reservoir
Pressure fill fitting for
hydraulic service cart
Suction line to clean
container of fluid
Fluid quantity indicator
Figure 12-14. The hydraulic ground serive station on a Boeing 737 provides for hydraulic fluid servicing with a hand pump or via an
external pressure fluid source. All three reservoirs are serviced from the same location.
System pressure gauge
Filter head assembly with bypass valve
Differential pressure indicator
Filters (bowl outside, filter inside)
Figure 12-16. A transport category filter module with two filters.
Differential pressure indicators
Pressure filter
Temperature
transducer
Pressure
transducer
Ground service disconnect Case drain filter
Figure 12-15. Filter module components.
Filters may be located within the reservoir, in the pressure
line, in the return line, or in any other location the designer
of the system decides that they are needed to safeguard the
hydraulic system against impurities. Modern design often
uses a filter module that contains several filters and other
components. [Figure 12-16] There are many models and
styles of filters. Their position in the aircraft and design
requirements determine their shape and size. Most filters
used in modern aircraft are of the inline type. The inline filter
assembly is comprised of three basic units: head assembly,
bowl, and element. The head assembly is secured to the
aircraft structure and connecting lines. Within the head,
there is a bypass valve that routes the hydraulic fluid directly
from the inlet to the outlet port if the filter element becomes
clogged with foreign matter. The bowl is the housing that
holds the element to the filter head and is removed when
element removal is required.
The element may be a micron, porous metal, or magnetic
type. The micron element is made of a specially treated paper
and is normally thrown away when removed. The porous
metal and magnetic filter elements are designed to be cleaned
by various methods and replaced in the system.
12-14
Detail A-B thermal lockout
Cold condition Hot condition
A
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