Aircraft Airframe part 1

Volume Contents....................................................iii
Preface.....................................................................v
Acknowledgments.................................................vii
Table of Contents.................................................xiii
Chapter 10
Aircraft Instrument Systems.............................10-1
Introduction...................................................................10-1
Classifying Instruments................................................10-3
Flight Instruments.....................................................10-3
Engine Instruments...................................................10-3
Navigation Instruments.............................................10-4
Pressure Measuring Instruments...................................10-5
Types of Pressure......................................................10-7
Pressure Instruments.................................................10-8
Engine Oil Pressure...............................................10-8
Manifold Pressure..................................................10-9
Engine Pressure Ratio (EPR).................................10-9
Fuel Pressure.......................................................10-10
Hydraulic Pressure...............................................10-10
Vacuum Pressure.................................................10-11
Pressure Switches................................................10-11
Pitot-Static Systems ...............................................10-12
Pitot Tubes and Static Vents................................10-12
Air Data Computers (ADC) and Digital Air
Data Computers (DADC)....................................10-14
Pitot-Static Pressure-Sensing Flight Instruments....10-16
Altimeters and Altitude.......................................10-16
Vertical Speed Indicator......................................10-22
Airspeed Indicators..............................................10-24
Remote Sensing and Indication..................................10-26
Synchro-Type Remote-Indicating Instruments ......10-26
DC Selsyn Systems..............................................10-26
AC Synchro Systems...........................................10-28
Remote Indicating Fuel and Oil Pressure
Gauges.................................................................10-28
Mechanical Movement Indicators..............................10-29
Tachometers............................................................10-29
Mechanical Tachometers.....................................10-29
Electric Tachometers...........................................10-30
Synchroscope..........................................................10-32
Accelerometers........................................................10-32
Stall Warning and Angle of Attack (AOA)
Indicators.................................................................10-34
Temperature Measuring Instruments..........................10-35
Non-Electric Temperature Indicators......................10-36
Electrical Temperature Measuring Indication.........10-36
Electrical Resistance Thermometer.....................10-36
Ratiometer Electrical Resistance
Thermometers......................................................10-38
Thermocouple Temperature Indicators...................10-38
Turbine Gas Temperature Indicating Systems....10-39
Total Air Temperature Measurement......................10-42
Direction Indicating Instruments................................10-43
Magnetic Compass..................................................10-43
Vertical Magnetic Compass....................................10-45
Remote Indicating Compass...................................10-45
Remote Indicating Slaved Gyro Compass (Flux
Gate Compass)........................................................10-45
Solid State Magnetometers.....................................10-46
Sources of Power for Gyroscopic Instruments...........10-46
Vacuum Systems.....................................................10-48
Venturi Tube Systems.........................................10-48
Engine-Driven Vacuum Pump............................10-48
Typical Pump-Driven System.............................10-49
Twin-Engine Aircraft Vacuum System
Operation.............................................................10-50
Pressure-Driven Gyroscopic Instrument Systems...10-51
Electrically-Driven Gyroscopic Instrument
Systems...................................................................10-51
Principles of Gyroscopic Instruments.........................10-52
Mechanical Gyros...................................................10-52
Solid State Gyros and Related Systems..................10-54
Ring Laser Gyros (RLG).....................................10-54
Table of Contents
xiv
Microelectromechanical Based Attitude and
Directional Systems.............................................10-55
Other Attitude and Directional Systems..............10-55
Common Gyroscopic Instruments..............................10-56
Vacuum-Driven Attitude Gyros..............................10-56
Electric Attitude Indicators.....................................10-57
Gyroscopic Direction Indicator or Directional
Gyro (DG)...............................................................10-58
Turn Coordinators...................................................10-58
Turn-and-Slip Indicator ..........................................10-58
Autopilot Systems.......................................................10-60
Basis for Autopilot Operation.................................10-60
Autopilot Components................................................10-61
Sensing Elements....................................................10-61
Computer and Amplifier.........................................10-61
Output Elements......................................................10-61
Command Elements................................................10-63
Feedback or Follow-up Element.............................10-63
Autopilot Functions.................................................10-64
Yaw Dampening.....................................................10-65
Automatic Flight Control System (AFCS).................10-65
Flight Director Systems..............................................10-65
Electronic Instruments................................................10-67
Electronic Attitude Director Indicator (EADI).......10-67
Electronic Horizontal Situation
Indicators (EHSI)....................................................10-68
Electronic Flight Information Systems.......................10-68
Electronic Flight Instrument System (EFIS)...........10-70
Electronic Centralized Aircraft Monitor
(ECAM)..................................................................10-71
Engine Indicating and Crew Alerting
System (EICAS)......................................................10-72
Flight Management System (FMS)............................10-73
Warnings and Cautions...............................................10-74
Annunciator Systems..............................................10-74
Aural Warning Systems..........................................10-75
Clocks.........................................................................10-76
Instrument Housings and Handling............................10-77
Instrument Installations and Markings.......................10-78
Instrument Panels....................................................10-78
Instrument Mounting...............................................10-80
Instrument Power Requirements.............................10-81
Instrument Range Markings....................................10-81
Maintenance of Instruments and Instrument
Systems.......................................................................10-81
Altimeter Tests........................................................10-82
Pitot-Static System Maintenance and Tests............10-83
Tachometer Maintenance........................................10-84
Magnetic Compass Maintenance and
Compensation..........................................................10-84
Vacuum System Maintenance.................................10-85
Autopilot System Maintenance...............................10-85
LCD Display Screens..............................................10-86
Chapter 11
Communication and Navigation.......................11-1
Introduction...................................................................11-1
Avionics in Aviation Maintenance............................11-2
History of Avionics...................................................11-2
Fundamentals of Electronics.....................................11-3
Analog Versus Digital Electronics........................11-3
Analog Electronics................................................11-4
Digital Electronics ..............................................11-26
Radio Communication................................................11-30
Radio Waves...........................................................11-31
Types of Radio Waves.........................................11-32
Loading Information onto a Radio Wave................11-32
Amplitude Modulation (AM)..............................11-34
Frequency Modulation (FM)...............................11-35
Single Side Band (SSB) ......................................11-36
Radio Transmitters and Receivers..........................11-37
Transmitters.........................................................11-37
Receivers.............................................................11-37
Transceivers.........................................................11-38
Antennas..................................................................11-38
Length..................................................................11-38
Polarization, Directivity, and Field Pattern.........11-39
Types...................................................................11-40
Transmission Lines..............................................11-41
Radio Navigation........................................................11-41
VOR Navigation System.........................................11-42
Automatic Direction Finder (ADF).........................11-46
Radio Magnetic Indicator (RMI)............................11-48
Instrument Landing Systems (ILS).........................11-49
Localizer..............................................................11-49
Glideslope...........................................................11-49
Compass Locators...............................................11-51
Marker Beacons...................................................11-52
Distance Measuring Equipment (DME)..................11-52
Area Navigation (RNAV).......................................11-54
Radar Beacon Transponder.....................................11-54
Transponder Tests and Inspections.....................11-58
Altitude Encoders................................................11-58
Collision Avoidance Systems.................................11-58
Traffic Collision Avoidance Systems (TCAS) ...11-59
ADS-B.................................................................11-60
Radio Altimeter.......................................................11-62
Weather Radar.........................................................11-64
Emergency Locator Transmitter (ELT)..................11-66
xv
Long Range Aid to Navigation System
(LORAN)................................................................11-69
Global Positioning System (GPS)...........................11-69
Wide Area Augmentation System (WAAS)........11-71
Inertial Navigation System (INS)/Inertial Reference
System (IRS)...............................................................11-71
Installation of Communication and Navigation
Equipment...................................................................11-72
Approval of New Avionics Equipment
Installations.............................................................11-72
Considerations.........................................................11-72
Cooling and Moisture .............................................11-73
Vibration Isolation..................................................11-73
Reducing Radio Interference......................................11-74
Shielding ................................................................11-74
Isolation...................................................................11-74
Bonding ..................................................................11-74
Static Discharge Wicks...........................................11-75
Installation of Aircraft Antenna Systems...................11-75
Transmission Lines.................................................11-76
Maintenance Procedure...........................................11-76
Chapter 12
Hydraulic and Pneumatic Power Systems.......12-1
Aircraft Hydraulic Systems..........................................12-1
Hydraulic Fluid.............................................................12-2
Viscosity....................................................................12-2
Chemical Stability.....................................................12-2
Flash Point.................................................................12-3
Fire Point...................................................................12-3
Types of Hydraulic Fluids............................................12-3
Mineral-Based Fluids................................................12-3
Polyalphaolefin-Based Fluids...................................12-3
Phosphate Ester-Based Fluid (Skydrol®)..................12-3
Intermixing of Fluids................................................12-3
Compatibility with Aircraft Materials.......................12-3
Hydraulic Fluid Contamination................................12-4
Contamination Check ...........................................12-4
Contamination Control..........................................12-5
Hydraulic System Flushing.......................................12-5
Health and Handling.................................................12-5
Basic Hydraulic Systems .............................................12-6
Open Center Hydraulic Systems...............................12-6
Closed-Center Hydraulic Systems............................12-7
Hydraulic Power Systems.............................................12-7
Evolution of Hydraulic Systems...............................12-7
Hydraulic Power Pack System..................................12-7
Hydraulic System Components.................................12-8
Reservoirs..............................................................12-8
Filters......................................................................12-12
Micron-Type Filters.............................................12-14
Maintenance of Filters.........................................12-14
Filter Bypass Valve ............................................12-14
Filter Differential Pressure Indicators ................12-14
Pumps......................................................................12-15
Hand Pumps ...........................................................12-15
Power-Driven Pumps..............................................12-16
Classification of Pumps.......................................12-16
Constant-Displacement Pumps............................12-17
Gear-Type Power Pump......................................12-17
Gerotor Pump......................................................12-17
Piston Pump.........................................................12-18
Vane Pump..........................................................12-19
Variable-Displacement Pump .............................12-20
Valves .....................................................................12-23
Flow Control Valves............................................12-23
Pressure Control Valves......................................12-27
Shuttle Valves......................................................12-29
Accumulators..........................................................12-30
Types of Accumulators........................................12-30
Heat Exchangers......................................................12-32
Actuators.................................................................12-32
Linear Actuators..................................................12-32
Rotary Actuators..................................................12-33
Hydraulic Motor .................................................12-34
Ram Air Turbine (RAT) ........................................12-35
Power Transfer Unit (PTU).....................................12-35
Hydraulic Motor-Driven Generator (HMDG)........12-35
Seals........................................................................12-35
V-Ring Packings..................................................12-35
U-Ring ................................................................12-35
O-Rings................................................................12-35
Backup Rings.......................................................12-37
Gaskets................................................................12-37
Seal Materials......................................................12-37
O-Ring Installation .............................................12-38
Wipers .................................................................12-38
Large Aircraft Hydraulic Systems..............................12-38
Boeing 737 Next Generation Hydraulic System.....12-38
Reservoirs............................................................12-38
Pumps..................................................................12-38
Filter Units...........................................................12-40
Power Transfer Unit (PTU).................................12-40
Landing Gear Transfer Unit................................12-41
Standby Hydraulic System..................................12-42
Indications...........................................................12-42
Boeing 777 Hydraulic System................................12-42
Left and Right System Description.....................12-43
Center Hydraulic System.....................................12-45
xvi
Aircraft Pneumatic Systems.......................................12-47
High-Pressure Systems...........................................12-48
Pneumatic System Components..........................12-48
Emergency Backup Systems...............................12-51
Medium-Pressure Systems......................................12-52
Low-Pressure Systems ...........................................12-52
Pneumatic Power System Maintenance..................12-52
Chapter 13
Aircraft Landing Gear Systems........................13-1
Landing Gear Types.....................................................13-1
Landing Gear Arrangement.......................................13-2
Tail Wheel-Type Landing Gear ............................13-3
Tandem Landing Gear...........................................13-3
Tricycle-Type Landing Gear.................................13-3
Fixed and Retractable Landing Gear.........................13-5
Shock Absorbing and Non-Shock Absorbing
Landing Gear ............................................................13-5
Leaf-Type Spring Gear..........................................13-6
Rigid .....................................................................13-6
Bungee Cord..........................................................13-7
Shock Struts ..........................................................13-7
Shock Strut Operation.............................................13-11
Servicing Shock Struts............................................13-12
Bleeding Shock Struts.............................................13-13
Landing Gear Alignment, Support, and Retraction....13-14
Alignment................................................................13-14
Support....................................................................13-15
Small Aircraft Retraction Systems..........................13-16
Large Aircraft Retraction Systems .........................13-20
Emergency Extension Systems...............................13-22
Landing Gear Safety Devices..................................13-22
Safety Switch.......................................................13-22
Ground Locks......................................................13-23
Landing Gear Position Indicators........................13-24
Nose Wheel Centering.........................................13-24
Landing Gear System Maintenance............................13-25
Landing Gear Rigging and Adjustment..................13-26
Gear Door Clearances..........................................13-28
Drag and Side Brace Adjustment .......................13-28
Landing Gear Retraction Test ............................13-29
Nose Wheel Steering Systems....................................13-30
Small Aircraft .........................................................13-30
Large Aircraft..........................................................13-30
Shimmy Dampers....................................................13-32
Steering Damper..................................................13-33
Piston-Type..........................................................13-33
Vane-Type...........................................................13-33
Non-Hydraulic Shimmy Damper........................13-34
Aircraft Wheels...........................................................13-34
Wheel Construction.................................................13-34
Inboard Wheel Half.............................................13-35
Outboard Wheel Half..........................................13-35
Wheel Inspection.....................................................13-35
On Aircraft Inspection.........................................13-35
Proper Installation...............................................13-35
Off Aircraft Wheel Inspection ............................13-37
Aircraft Brakes............................................................13-42
Types and Construction of Aircraft Brakes ...........13-43
Single Disc Brakes..............................................13-43
Dual-Disc Brakes.................................................13-45
Multiple-Disc Brakes...........................................13-46
Segmented Rotor-Disc Brakes ...........................13-47
Carbon Brakes.....................................................13-50
Expander Tube Brakes........................................13-50
Brake Actuating Systems........................................13-52
Independent Master Cylinders ............................13-52
Boosted Brakes....................................................13-55
Power Brakes ......................................................13-56
Emergency Brake Systems .....................................13-59
Parking Brake......................................................13-61
Brake Deboosters.................................................13-61
Anti-Skid.................................................................13-61
System Operation................................................13-61
Wheel Speed Sensors..........................................13-62
Control Units.......................................................13-63
Anti-Skid Control Valves....................................13-64
Touchdown and Lock Wheel Protection.............13-66
Auto Brakes.........................................................13-66
Anti-Skid System Tests.......................................13-66
Anti-Skid System Maintenance...........................13-68
Brake Inspection and Service..................................13-68
On Aircraft Servicing..........................................13-68
Lining Wear.........................................................13-68
Air in the Brake System......................................13-69
Bleeding Master Cylinder Brake Systems...........13-69
Bleeding Power Brake Systems...........................13-71
Off Aircraft Brake Servicing and Maintenance...13-72
Replacement of Brake Linings............................13-73
Brake Malfunctions and Damage ...........................13-74
Overheating ........................................................13-74
Dragging .............................................................13-75
Chattering or Squealing ......................................13-76
Aircraft Tires and Tubes.............................................13-76
Tire Classification...................................................13-76
Types...................................................................13-76
Ply Rating............................................................13-77
xvii
Tube-Type or Tubeless........................................13-77
Bias Ply or Radial................................................13-78
Tire Construction....................................................13-78
Bead ....................................................................13-78
Carcass Plies........................................................13-79
Tread....................................................................13-79
Sidewall...............................................................13-80
Tire Inspection on the Aircraft................................13-80
Inflation................................................................13-80
Tread Condition...................................................13-82
Sidewall Condition..............................................13-86
Tire Removal...........................................................13-86
Tire Inspection Off of the Aircraft..........................13-88
Tire Repair and Retreading.....................................13-88
Tire Storage ............................................................13-89
Aircraft Tubes.........................................................13-89
Tube Construction and Selection.........................13-89
Tube Storage and Inspection...............................13-90
Tire Inspection ....................................................13-90
Tire Mounting.........................................................13-90
Tubeless Tires......................................................13-90
Tube-Type Tires..................................................13-91
Tire Balancing ........................................................13-93
Operation and Handling Tips......................................13-93
Taxiing ...................................................................13-94
Braking and Pivoting..............................................13-94
Landing Field and Hangar Floor Condition ...........13-94
Takeoffs and Landings............................................13-94
Hydroplaning..........................................................13-94
Chapter 14
Aircraft Fuel System..........................................14-1
Basic Fuel System Requirements.................................14-1
Fuel System Independence........................................14-2
Fuel System Lightning Protection.............................14-2
Fuel Flow..................................................................14-3
Flow Between Interconnected Tanks........................14-3
Unusable Fuel Supply...............................................14-3
Fuel System Hot Weather Operation........................14-3
Fuel Tanks....................................................................14-3
Fuel Tank Tests.........................................................14-3
Fuel Tank Installation...............................................14-4
Fuel Tank Expansion Space......................................14-4
Fuel Tank Sump........................................................14-4
Fuel Tank Filler Connection.....................................14-4
Fuel Tank Vents and Carburetor Vapor Vents..........14-4
Fuel Tank Outlet.......................................................14-5
Pressure Fueling Systems..........................................14-5
Fuel Pumps ...............................................................14-5
Fuel System Lines and Fittings ................................14-5
Fuel System Components..........................................14-5
Fuel Valves and Controls..........................................14-5
Fuel Strainer or Filter................................................14-6
Fuel System Drains...................................................14-6
Fuel Jettisoning System............................................14-6
Types of Aviation Fuel.................................................14-7
Reciprocating Engine Fuel—AVGAS......................14-7
Volatility................................................................14-7
Vapor Lock............................................................14-7
Carburetor Icing.....................................................14-7
Aromatic Fuels......................................................14-8
Detonation.............................................................14-8
Surface Ignition and Preignition............................14-9
Octane and Performance Number Rating..............14-9
Fuel Identification................................................14-10
Purity...................................................................14-10
Turbine Engine Fuels..............................................14-11
Turbine Fuel Volatility........................................14-12
Turbine Engine Fuel Types.................................14-13
Turbine Engine Fuel Issues.................................14-13
Aircraft Fuel Systems.................................................14-13
Small Single-Engine Aircraft Fuel Systems...........14-13
Gravity Feed Systems..........................................14-13
Pump Feed Systems.............................................14-14
High-Wing Aircraft With Fuel Injection
System.................................................................14-14
Small Multiengine (Reciprocating) Aircraft
Fuel Systems...........................................................14-15
Low-Wing Twin..................................................14-15
High-Wing Twin.................................................14-16
Large Reciprocating-Engine Aircraft Fuel
Systems...................................................................14-16
Jet Transport Aircraft Fuel Systems........................14-17
Helicopter Fuel Systems.........................................14-22
Fuel System Components...........................................14-22
Fuel Tanks...............................................................14-22
Rigid Removable Fuel Tanks..............................14-23
Bladder Fuel Tanks..............................................14-25
Integral Fuel Tanks..............................................14-25
Fuel Lines and Fittings............................................14-26
Fuel Valves ............................................................14-26
Hand-Operated Valves........................................14-28
Cone Valves.........................................................14-28
Poppet Valves......................................................14-29
Manually-Operated Gate Valves.........................14-29
Motor-Operated Valves.......................................14-30
Solenoid-Operated Valves...................................14-31
Fuel Pumps..............................................................14-31
Hand-Operated Fuel Pumps................................14-31
Ejector Pumps......................................................14-32
xviii
Pulsating Electric Pumps.....................................14-34
Vane-Type Fuel Pumps.......................................14-35
Fuel Filters..............................................................14-36
Fuel Heaters and Ice Prevention.............................14-39
Fuel System Indicators............................................14-40
Fuel Quantity Indicating Systems.......................14-40
Fuel Flowmeters..................................................14-44
Fuel Temperature Gauges....................................14-46
Fuel Pressure Gauges..........................................14-47
Pressure Warning Signal.....................................14-48
Valve-In-Transit Indicator Lights .......................14-48
Fuel System Repair ....................................................14-49
Troubleshooting the Fuel System...........................14-49
Location of Leaks and Defects............................14-49
Fuel Leak Classification......................................14-50
Replacement of Gaskets, Seals, and Packings.....14-50
Fuel Tank Repair.....................................................14-50
Welded Tanks......................................................14-51
Riveted Tanks......................................................14-51
Soldered Tanks....................................................14-51
Bladder Tanks......................................................14-51
Integral Tanks......................................................14-52
Fire Safety...........................................................14-52
Fuel System Servicing................................................14-54
Checking for Fuel System Contaminants................14-54
Water...................................................................14-54
Solid Particle Contaminants................................14-55
Surfactants...........................................................14-55
Microorganisms...................................................14-55
Foreign Fuel Contamination................................14-56
Detection of Contaminants..................................14-56
Fuel Contamination Control................................14-58
Fueling and Defueling Procedures..............................14-59
Fueling....................................................................14-59
Defueling.................................................................14-61
Fire Hazards When Fueling or Defueling...............14-61
Chapter 15
Ice and Rain Protection.....................................15-1
Ice Control Systems......................................................15-1
Icing Effects..............................................................15-2
Ice Detector System......................................................15-3
Ice Prevention...........................................................15-3
Wing and Horizontal and Vertical Stabilizer
Anti-Icing Systems.......................................................15-4
Thermal Pneumatic Anti-icing..................................15-4
Wing Anti-Ice (WAI) System...............................15-4
Leading Edge Slat Anti-Ice System......................15-6
Thermal Electric Anti-Icing....................................15-10
Chemical Anti-Icing................................................15-10
Wing and Stabilizer Deicing Systems........................15-12
Sources of Operating Air........................................15-12
Turbine Engine Bleed Air...................................15-12
Pneumatic Deice Boot System for GA Aircraft......15-12
GA System Operation..........................................15-13
Deice System for Turboprop Aircraft.....................15-14
Deicing System Components..................................15-14
Wet-Type Engine-Driven Air Pump...................15-14
Dry-Type Engine-Driven Air Pump....................15-16
Oil Separator........................................................15-17
Control Valve......................................................15-17
Deflate Valve.......................................................15-18
Distributor Valve.................................................15-18
Timer/Control Unit..............................................15-18
Regulators and Relief Valves..............................15-18
Manifold Assembly.............................................15-19
Inlet Filter............................................................15-19
Construction and Installation of Deice Boots.........15-19
Inspection, Maintenance, and Troubleshooting
of Rubber Deicer Boot Systems..............................15-19
Operational Checks.............................................15-19
Adjustments.........................................................15-21
Troubleshooting...................................................15-21
Inspection............................................................15-21
Deice Boot Maintenance.........................................15-21
Electric Deice Boots................................................15-22
Propeller Deice System...............................................15-23
Electrothermal Propeller Device System ...............15-23
Chemical Propeller Deice.......................................15-23
Ground Deicing of Aircraft........................................15-23
Frost Removal.........................................................15-25
Deicing and Anti-icing of Transport Type
Aircraft................................................................15-25
Ice and Snow Removal...........................................15-27
Rain Control Systems.................................................15-27
Windshield Wiper Systems ....................................15-27
Chemical Rain Repellant........................................15-27
Windshield Surface Seal Coating........................15-28
Pneumatic Rain Removal Systems.........................15-28
Windshield Frost, Fog, and Ice Control Systems.......15-28
Electric....................................................................15-28
Pneumatic................................................................15-30
Chemical.................................................................15-31
Portable Water Tank Ice Prevention...........................15-32
Chapter 16
Cabin Environmental Control Systems............16-1
Physiology of Flight.....................................................16-1
Composition of the Atmosphere...............................16-1
Human Respiration and Circulation..........................16-2
xix
Oxygen and Hypoxia.............................................16-2
Carbon Monoxide Poisoning.................................16-3
Aircraft Oxygen Systems..............................................16-3
Forms of Oxygen and Characteristics.......................16-4
Gaseous Oxygen....................................................16-4
Liquid Oxygen.......................................................16-4
Chemical or Solid Oxygen ...................................16-5
Onboard Oxygen Generating Systems
(OBOGS)...............................................................16-5
Oxygen Systems and Components............................16-6
Gaseous Oxygen Systems......................................16-6
Chemical Oxygen Systems..................................16-16
LOX Systems.......................................................16-16
Oxygen System Servicing.......................................16-16
Servicing Gaseous Oxygen..................................16-16
Filling LOX Systems...........................................16-19
Inspection of Masks and Hoses...........................16-19
Replacing Tubing, Valves, and Fittings..............16-20
Prevention of Oxygen Fires or Explosions.............16-20
Oxygen System Inspection and Maintenance......16-20
Aircraft Pressurization Systems..................................16-21
Pressure of the Atmosphere....................................16-21
Temperature and Altitude.......................................16-22
Pressurization Terms...............................................16-23
Pressurization Issues...............................................16-23
Sources of Pressurized Air......................................16-24
Reciprocating Engine Aircraft ............................16-25
Turbine Engine Aircraft .....................................16-26
Control of Cabin Pressure ......................................16-27
Pressurization Modes...........................................16-27
Cabin Pressure Controller....................................16-28
Cabin Air Pressure Regulator and Outflow
Valve....................................................................16-30
Cabin Air Pressure Safety Valve Operation........16-31
Pressurization Gauges.........................................16-32
Pressurization Operation ....................................16-32
Air Distribution...................................................16-34
Cabin Pressurization Troubleshooting................16-34
Air Conditioning Systems...........................................16-34
Air Cycle Air Conditioning.....................................16-35
System Operation................................................16-36
Pneumatic System Supply...................................16-36
Component Operation.........................................16-36
Water Separator...................................................16-39
Cabin Temperature Control System....................16-39
Vapor Cycle Air Conditioning................................16-40
Theory of Refrigeration.......................................16-41
Vapor Cycle Air Conditioning System
Components.........................................................16-46
Vapor Cycle Air Conditioning Servicing
Equipment............................................................16-51
System Servicing.................................................16-55
Technician Certification......................................16-58
Aircraft Heaters..........................................................16-58
Bleed Air Systems...................................................16-58
Electric Heating Systems........................................16-58
Exhaust Shroud Heaters..........................................16-58
Combustion Heaters................................................16-60
Combustion Air System......................................16-60
Ventilating Air System .......................................16-60
Fuel System.........................................................16-61
Ignition System....................................................16-61
Controls...............................................................16-61
Safety Features ...................................................16-62
Maintenance and Inspection................................16-62
Chapter 17
Fire Protection Systems....................................17-1
Introduction...................................................................17-1
Classes of Fires.........................................................17-2
Requirements for Overheat and Fire Protection
Systems.....................................................................17-2
Fire Detection/Overheat Systems.................................17-2
Thermal Switch System............................................17-2
Thermocouple System...............................................17-3
Continuous-Loop Systems........................................17-4
Fenwal System.......................................................17-4
Kidde System.........................................................17-4
Pressure Type Sensor Responder Systems................17-6
Pneumatic Continuous-Loop Systems...................17-6
Fire Zones.................................................................17-8
Smoke, Flame, and Carbon Monoxide Detection
Systems.........................................................................17-8
Smoke Detectors.......................................................17-8
Light Refraction Type...........................................17-8
Ionization Type......................................................17-8
Flame Detectors........................................................17-9
Carbon Monoxide Detectors.....................................17-9
Extinguishing Agents and Portable Fire
Extinguishers..............................................................17-10
Halogenated Hydrocarbons.....................................17-10
Inert Cold Gases......................................................17-10
Dry Powders............................................................17-10
Water.......................................................................17-11
Cockpit and Cabin Interiors....................................17-11
Extinguisher Types..............................................17-11
Installed Fire Extinguishing Systems.........................17-11
CO2 Fire Extinguishing Systems.............................17-11
xx
Halogenated Hydrocarbons Fire Extinguishing
Systems...................................................................17-11
Containers ..............................................................17-12
Discharge Valves....................................................17-13
Pressure Indication..................................................17-13
Two-Way Check Valve...........................................17-13
Discharge Indicators................................................17-13
Thermal Discharge Indicator (Red Disk)............17-13
Yellow Disk Discharge Indicator........................17-14
Fire Switch..............................................................17-14
Cargo Fire Detection...................................................17-14
Cargo Compartment Classification.........................17-15
Class A.................................................................17-15
Class B.................................................................17-15
Class C.................................................................17-15
Class E.................................................................17-15
Cargo and Baggage Compartment Fire
Detection and Extinguisher System........................17-15
Smoke Detector System......................................17-16
Cargo Compartment Extinguishing System........17-16
Lavatory Smoke Detectors.........................................17-17
Lavatory Smoke Detector System ..........................17-17
Lavatory Fire Extinguisher System.........................17-18
Fire Detection System Maintenance...........................17-18
Fire Detection System Troubleshooting.....................17-19
Fire Extinguisher System Maintenance......................17-20
Container Pressure Check.......................................17-20
Discharge Cartridges...............................................17-20
Agent Containers.....................................................17-20
Fire Prevention............................................................17-21
Glossary...............................................................G-1
Index.......................................................................I-1
10-1
Introduction
Since the beginning of manned flight, it has been recognized
that supplying the pilot with information about the aircraft
and its operation could be useful and lead to safer flight.
The Wright Brothers had very few instruments on their
Wright Flyer, but they did have an engine tachometer, an
anemometer (wind meter), and a stop watch. They were
obviously concerned about the aircraft’s engine and the
progress of their flight. From that simple beginning, a wide
variety of instruments have been developed to inform flight
crews of different parameters. Instrument systems now
exist to provide information on the condition of the aircraft,
engine, components, the aircraft’s attitude in the sky,
weather, cabin environment, navigation, and communication.
Figure 10-1 shows various instrument panels from the Wright
Flyer to a modern jet airliner.
Aircraft Instrument
Systems
Chapter 10
10-2
Figure 10-2. A conventional instrument panel of the C-5A Galaxy
(top) and the glass cockpit of the C-5B Galaxy (bottom).
Figure 10-1. From top to bottom: instruments of the Wright Flyer,
instruments on a World War I era aircraft, a late 1950s/early 1960s
Boeing 707 airliner cockpit, and an Airbus A380 glass cockpit.
The ability to capture and convey all of the information a
pilot may want, in an accurate, easily understood manner,
has been a challenge throughout the history of aviation. As
the range of desired information has grown, so too have the
size and complexity of modern aircraft, thus expanding even
further the need to inform the flight crew without sensory
overload or overcluttering the cockpit. As a result, the old
flat panel in the front of the cockpit with various individual
instruments attached to it has evolved into a sophisticated
computer-controlled digital interface with flat-panel display
screens and prioritized messaging. A visual comparison
between a conventional cockpit and a glass cockpit is shown
in Figure 10-2.
There are usually two parts to any instrument or instrument
system. One part senses the situation and the other part
displays it. In analog instruments, both of these functions
often take place in a single unit or instrument (case). These are
called direct-sensing instruments. Remote-sensing requires
the information to be sensed, or captured, and then sent to a
separate display unit in the cockpit. Both analog and digital
instruments make use of this method. [Figure 10-3]
10-3
Figure 10-3. There are two parts to any instrument system—the
sensing mechanism and the display mechanism.
Sensor + Indication Sensor Indication
Direct-sensing
instrument system
Remote-sensing
instrument system
Figure 10-4. The basic T arrangement of analog flight instruments.
At the bottom of the T is a heading indicator that functions as
a compass but is driven by a gyroscope and not subject to the
oscillations common to magnetic direction indicators.
30.0
29.8
300.0
29.9
2929299.2 8 2929.29.8
The relaying of important bits of information can be done
in various ways. Electricity is often used by way of wires
that carry sensor information into the cockpit. Sometimes
pneumatic lines are used. In complex, modern aircraft,
this can lead to an enormous amount of tubing and wiring
terminating behind the instrument display panel. More
efficient information transfer has been accomplished via the
use of digital data buses. Essentially, these are wires that share
message carrying for many instruments by digitally encoding
the signal for each. This reduces the number of wires and
weight required to transfer remotely sensed information for
the pilot’s use. Flat-panel computer display screens that can
be controlled to show only the information desired are also
lighter in weight than the numerous individual gauges it
would take to display the same information simultaneously.
An added bonus is the increased reliability inherent in these
solid-state systems.
It is the job of the aircraft technician to understand and
maintain all aircraft, including these various instrument
systems. Accordingly, in this chapter, discussions begin
with analog instruments and refer to modern digital
instrumentation when appropriate.
Classifying Instruments
There are three basic kinds of instruments classified by the
job they perform: flight instruments, engine instruments,
and navigation instruments. There are also miscellaneous
gauges and indicators that provide information that do not
fall into these classifications, especially on large complex
aircraft. Flight control position, cabin environmental systems,
electrical power, and auxiliary power units (APUs), for
example, are all monitored and controlled from the cockpit
via the use of instruments systems. All may be regarded as
position/condition instruments since they usually report the
position of a certain moveable component on the aircraft, or
the condition of various aircraft components or systems not
included in the first three groups.
Flight Instruments
The instruments used in controlling the aircraft’s flight
attitude are known as the flight instruments. There are basic
flight instruments, such as the altimeter that displays aircraft
altitude; the airspeed indicator; and the magnetic direction
indicator, a form of compass. Additionally, an artificial
horizon, turn coordinator, and vertical speed indicator are
flight instruments present in most aircraft. Much variation
exists for these instruments, which is explained throughout
this chapter. Over the years, flight instruments have come to
be situated similarly on the instrument panels in most aircraft.
This basic T arrangement for flight instruments is shown in
Figure 10-4. The top center position directly in front of the
pilot and copilot is the basic display position for the artificial
horizon even in modern glass cockpits (those with solid-state,
flat-panel screen indicating systems).
Original analog flight instruments are operated by air
pressure and the use of gyroscopes. This avoids the use of
electricity, which could put the pilot in a dangerous situation
if the aircraft lost electrical power. Development of sensing
and display techniques, combined with advanced aircraft
electrical systems, has made it possible for reliable primary
and secondary instrument systems that are electrically
operated. Nonetheless, often a pneumatic altimeter, a gyro
artificial horizon, and a magnetic direction indicator are
retained somewhere in the instrument panel for redundancy.
[Figure 10-5]
Engine Instruments
Engine instruments are those designed to measure operating
parameters of the aircraft’s engine(s). These are usually
quantity, pressure, and temperature indications. They also
include measuring engine speed(s). The most common engine
instruments are the fuel and oil quantity and pressure gauges,
tachometers, and temperature gauges. Figure 10-6 contains
various engine instruments found on reciprocating and
turbine-powered aircraft.
10-4
Figure 10-7. An engine instrumentation located in the middle of the instrument panel is shared by the pilot and co-pilot.
Figure 10-5. This electrically operated flat screen display instrument
panel, or glass cockpit, retains an analog airspeed indicator, a
gyroscope-driven artificial horizon, and an analog altimeter as a
backup should electric power be lost, or a display unit fails.
XPDR 5537 IDNT LCL23:00:34
VOR 1
270°
2
1
1
2
4300
4200
4100
3900
3900
3800
4300
20
80
4000
4000
130
120
110
90
80
70
1
100
9
TAS 100KT
OAT 7°C
ALERTS
NAV1 117.60 117.90
NAV2 117.90 117.60
132.675 120.000 COM1
118.525 132.900 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 360°
N-S E-W
VOLTS
27.3
2090
NAV1 117.60 117.90
NAV2 117.90 117.60
132.675 120.000 COM1
118.525 132.900 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 360°
MAP - NAVIGATION MAP
Figure 10-6. Common engine instruments. Note: For example purposes only. Some aircraft may not have these instruments or may be
equipped with others.
Reciprocating engines
Oil pressure Oil pressure
Oil temperature Exhaust gas temperature (EGT)
Cylinder head temperature (CHT) Turbine inlet temperature (TIT) or turbine gas temperature (TGT)
Manifold pressure Engine pressure ratio (EPR)
Fuel quantity Fuel quantity
Fuel pressure Fuel pressure
Fuel flow
Tachometer Tachometer (percent calibrated)
N1 and N2 compressor speeds
Carburetor temperature Torquemeter (on turboprop and turboshaft engines)
Turbine engines
Engine instrumentation is often displayed in the center of
the cockpit where it is easily visible to the pilot and copilot.
[Figure 10-7] On light aircraft requiring only one flight
crewmember, this may not be the case. Multiengine aircraft
often use a single gauge for a particular engine parameter,
but it displays information for all engines through the use of
multiple pointers on the same dial face.
Navigation Instruments
Navigation instruments are those that contribute information
used by the pilot to guide the aircraft along a definite course.
This group includes compasses of various kinds, some of
which incorporate the use of radio signals to define a specific
course while flying the aircraft en route from one airport
to another. Other navigational instruments are designed
specifically to direct the pilot’s approach to landing at an
airport. Traditional navigation instruments include a clock
and a magnetic compass. Along with the airspeed indicator
and wind information, these can be used to calculate
navigational progress. Radios and instruments sending
locating information via radio waves have replaced these
manual efforts in modern aircraft. Global position systems
(GPS) use satellites to pinpoint the location of the aircraft
via geometric triangulation. This technology is built into
some aircraft instrument packages for navigational purposes.
10-5
Pointer linkage
attaches here
Spur gear
Pressure
entrance
Pressure in
Sector gear
Free end
Fixed end Bourdon tube
Figure 10-9. The Bourdon tube is one of the basic mechanisms for
sensing pressure.
XPDR 5537
IDNT LCL23:00:34
270°
2
2
4300
4200
4100
3900
3900
3800
4300
20
80
4000
4000
130
110
100
9
TAS 100KT
OAT 7°C
ALERTS
NAV1 117.60 117.90
NAV2 117.90 117.60
132.675 120.000
COM1
118.525 132.900
COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK _ _ _° TRK 360°
N-S E-W
NAV1 117.60 117.90
NAV2 117.90 117.60
132.675 120.000 COM1
118.525 132.900 COM2
WPT _ _ _ _ _ _ DIS _ _ ._ NM DTK
_ _ _° TRK 360°
MAP - NAVIGATION MAP
7 1
1
V1 V2 NAVIGATIO
VOR 1
DME TUNING
DME MODE NAV1
120
1101
1
NAV controls
COM frequency window
0 117
0 117
1
1
7.90
7.60
NAV frequency window
NAV
NAV
COM controls
Com section
audio panel
NAV section
audio panel DME tuning window
70
90
80
V Glideslope indicator
K ON Moving map
Figure 10-8. Navigation instruments.
Many of these aircraft navigational systems are discussed in
chapter 11 of this handbook. [Figure 10-8]
To understand how various instruments work and can be
repaired and maintained, they can be classified according to
the principle upon which they operate. Some use mechanical
methods to measure pressure and temperature. Some utilize
magnetism and electricity to sense and display a parameter.
Others depend on the use of gyroscopes in their primary
workings. Still others utilize solid state sensors and computers
to process and display important information. In the following
sections, the different operating principles for sensing
parameters are explained. Then, an overview of many of the
engine, flight, and navigation instruments is given.
Pressure Measuring Instruments
A number of instruments inform the pilot of the aircraft’s
condition and flight situations through the measurement of
pressure. Pressure-sensing instruments can be found in the
flight group and the engine group. They can be either direct
reading or remote sensing. These are some of the most critical
instruments on the aircraft and must accurately inform the
pilot to maintain safe operations. Pressure measurement
involves some sort of mechanism that can sense changes
in pressure. A technique for calibration and displaying the
information is then added to inform the pilot. The type of
pressure needed to be measured often makes one sensing
mechanism more suited for use in a particular instance.
The three fundamental pressure-sensing mechanisms used
in aircraft instrument systems are the Bourdon tube, the
diaphragm or bellows, and the solid-state sensing device.
A Bourdon tube is illustrated in Figure 10-9. The open end
of this coiled tube is fixed in place and the other end is sealed
and free to move. When a fluid that needs to be measured is
directed into the open end of the tube, the unfixed portion
of the coiled tube tends to straighten out. The higher the
pressure of the fluid, the more the tube straightens. When the
pressure is reduced, the tube recoils. A pointer is attached
to this moving end of the tube, usually through a linkage
of small shafts and gears. By calibrating this motion of the
straightening tube, a face or dial of the instrument can be
created. Thus, by observing the pointer movement along the
scale of the instrument face positioned behind it, pressure
increases and decreases are communicated to the pilot.
The Bourdon tube is the internal mechanism for many
pressure gauges used on aircraft. When high pressures need to
be measured, the tube is designed to be stiff. Gauges used to
indicate lower pressures use a more flexible tube that uncoils
and coils more readily. Most Bourdon tubes are made from
brass, bronze, or copper. Alloys of these metals can be made
to coil and uncoil the tube consistently numerous times.
Bourdon tube gauges are simple and reliable. Some of the
instruments that use a Bourdon tube mechanism include
the engine oil pressure gauge, hydraulic pressure gauge,
oxygen tank pressure gauge, and deice boot pressure gauge.
Since the pressure of the vapor produced by a heated liquid
10-6
Sector
Set screw
Pinion
Spring stop screw
Socket assembly
Bourdon tube
Figure 10-10. The Bourdon tube mechanism can be used to measure
pressure or temperature by recalibrating the pointer’s connecting
linkage and scaling instrument face to read in degrees Celsius or
Fahrenheit.
Pressure diaphragm
Spring action
Aneroid
Figure 10-11. A diaphragm used for measuring pressure. An
evacuated sealed diaphragm is called an aneroid.
or gas increases as temperature increases, Bourdon tube
mechanisms can also be used to measure temperature. This
is done by calibrating the pointer connecting linkage and
relabeling the face of the gauge with a temperature scale. Oil
temperature gauges often employ Bourdon tube mechanisms.
[Figure 10-10]
Since the sensing and display of pressure or temperature
information using a Bourdon tube mechanism usually occurs
in a single instrument housing, they are most often direct
reading gauges. But the Bourdon tube sensing device can
also be used remotely. Regardless, it is necessary to direct the
fluid to be measured into the Bourdon tube. For example, a
common direct-reading gauge measuring engine oil pressure
and indicating it to the pilot in the cockpit is mounted in
the instrument panel. A small length of tubing connects a
pressurized oil port on the engine, runs though the firewall,
and into the back of the gauge. This setup is especially
functional on light, single-engine aircraft in which the engine
is mounted just forward of the instrument panel in the forward
end of the fuselage. However, a remote sensing unit can be
more practical on twin-engine aircraft where the engines are
a long distance from the cockpit pressure display. Here, the
Bourdon tube’s motion is converted to an electrical signal and
carried to the cockpit display via a wire. This is lighter and
more efficient, eliminating the possibility of leaking fluids
into the passenger compartment of the aircraft.
The diaphragm and bellows are two other basic sensing
mechanisms employed in aircraft instruments for pressure
measurement. The diaphragm is a hollow, thin-walled metal
disk, usually corrugated. When pressure is introduced through
an opening on one side of the disk, the entire disk expands.
By placing linkage in contact against the other side of the
disk, the movement of the pressurized diaphragm can be
transferred to a pointer that registers the movement against
the scale on the instrument face. [Figure 10-11]
Diaphragms can also be sealed. The diaphragm can be
evacuated before sealing, retaining absolutely nothing
inside. When this is done, the diaphragm is called an aneroid.
Aneroids are used in many flight instruments. A diaphragm
can also be filled with a gas to standard atmospheric pressure
and then sealed. Each of these diaphragms has their uses,
which are described in the next section. The common factor
in all is that the expansion and contraction of the side wall of
the diaphragm is the movement that correlates to increasing
and decreasing pressure.
When a number of diaphragm chambers are connected
together, the device is called a bellows. This accordionlike
assembly of diaphragms can be very useful when
measuring the difference in pressure between two gases,
called differential pressure. Just as with a single diaphragm,
it is the movement of the side walls of the bellows assembly
that correlates with changes in pressure and to which a
pointer linkage and gearing is attached to inform the pilot.
[Figure 10-12]
10-7
Pressure entrance Pressure entrance
Bellows Bellows
Figure 10-12. A bellows unit in a differential pressure gauge
compares two different pressure values. End movement of the
bellows away from the side with the highest pressure input occurs
when the pressures in the bellows are not equal. The indicator
linkage is calibrated to display the difference.
Diaphragms, aneroids, and bellows pressure sensing devices
are often located inside the single instrument housing that
contains the pointer and instrument dial read by the pilot on
the instrument panel. Thus, many instruments that make use
of these sensitive and reliable mechanisms are direct reading
gauges. But, many remote sensing instrument systems also
make use of the diaphragm and bellows. In this case, the
sensing device containing the pressure sensitive diaphragm
or bellows is located remotely on the engine or airframe.
It is part of a transducer that converts the pressure into an
electrical signal. The transducer, or transmitter, sends the
signal to the gauge in the cockpit, or to a computer, for
processing and subsequent display of the sensed condition.
Examples of instruments that use a diaphragm or bellows in
a direct reading or remote sensing gauge are the altimeter,
vertical speed indicator, cabin differential pressure gauge (in
pressurized aircraft), and manifold pressure gauge.
Solid-state microtechnology pressure sensors are used in
modern aircraft to determine the critical pressures needed
for safe operation. Many of these have digital output ready
for processing by electronic flight instrument computers and
other onboard computers. Some sensors send microelectric
signals that are converted to digital format for use by
computers. As with the analog sensors described above, the
key to the function of solid-state sensors is their consistent
property changes as pressure changes.
The solid-state sensors used in most aviation applications
exhibit varying electrical output or resistance changes
when pressure changes occur. Crystalline piezoelectric,
piezoresistor, and semiconductor chip sensors are most
common. In the typical sensor, tiny wires are embedded in
the crystal or pressure-sensitive semiconductor chip. When
pressure deflects the crystal(s), a small amount of electricity
is created or, in the case of a semiconductor chip and some
crystals, the resistance changes. Since the current and
resistance changes vary directly with the amount of deflection,
outputs can be calibrated and used to display pressure values.
Nearly all of the pressure information needed for engine,
airframe, and flight instruments can be captured and/or
calculated through the use of solid-state pressure sensors in
combination with temperature sensors. But continued use of
aneroid devices for comparisons involving absolute pressure
is notable. Solid-state pressure-sensing systems are remote
sensing systems. The sensors are mounted on the aircraft at
convenient and effective locations.
Types of Pressure
Pressure is a comparison between two forces. Absolute
pressure exists when a force is compared to a total vacuum,
or absolutely no pressure. It is necessary to define absolute
pressure, because the air in the atmosphere is always exerting
pressure on everything. Even when it seems there is no
pressure being applied, like when a balloon is deflated, there
is still atmospheric pressure inside and outside of the balloon.
To measure that atmospheric pressure, it is necessary to
compare it to a total absence of pressure, such as in a vacuum.
Many aircraft instruments make use of absolute pressure
values, such as the altimeter, the rate-of-climb indicator,
and the manifold pressure gauge. As stated, this is usually
done with an aneroid.
The most common type of pressure measurement is gauge
pressure. This is the difference between the pressure to be
measured and the atmospheric pressure. The gauge pressure
inside the deflated balloon mentioned above is therefore
0 pounds per square inch (psi). Gauge pressure is easily
measured and is obtained by ignoring the fact that the
atmosphere is always exerting its pressure on everything. For
example, a tire is filled with air to 32 psi at a sea level location
and checked with a gauge to read 32 psi, which is the gauge
pressure. The approximately 14.7 psi of air pressing on the
outside of the tire is ignored. The absolute pressure in the tire
is 32 psi plus the 14.7 psi that is needed to balance the 14.7
psi on the outside of the tire. So, the tire’s absolute pressure
is approximately 46.7 psi. If the same tire is inflated to 32
10-8
Figure 10-13. An analog oil pressure gauge is driven by a Bourdon
tube. Oil pressure is vital to engine health and must be monitored
by the pilot.
psi at a location 10,000 feet above sea level, the air pressure
on the outside of the tire would only be approximately 10
psi, due to the thinner atmosphere. The pressure inside the
tire required to balance this would be 32 psi plus 10 psi,
making the absolute pressure of the tire 42 psi. So, the same
tire with the same amount of inflation and performance
characteristics has different absolute pressure values. Gauge
pressure, however, remains the same, indicating the tires are
inflated identically. It this case, gauge pressure is more useful
in informing us of the condition of the tire.
Gauge pressure measurements are simple and widely useful.
They eliminate the need to measure varying atmospheric
pressure to indicate or monitor a particular pressure situation.
Gauge pressure should be assumed, unless otherwise
indicated, or unless the pressure measurement is of a type
known to require absolute pressure.
In many instances in aviation, it is desirable to compare
the pressures of two different elements to arrive at useful
information for operating the aircraft. When two pressures
are compared in a gauge, the measurement is known as
differential pressure and the gauge is a differential pressure
gauge. An aircraft’s airspeed indicator is a differential
pressure gauge. It compares ambient air pressure with ram air
pressure to determine how fast the aircraft is moving through
the air. A turbine’s engine pressure ratio (EPR) gauge is also
a differential pressure gauge. It compares the pressure at the
inlet of the engine with that at the outlet to indicate the thrust
developed by the engine. Both of these differential pressure
gauges and others are discussed further in this chapter and
throughout this handbook.
In aviation, there is also a commonly used pressure known as
standard pressure. Standard pressure refers to an established
or standard value that has been created for atmospheric
pressure. This standard pressure value is 29.92 inches of
mercury ("Hg), 1,013.2 hectopascal (hPa), or 14.7 psi. It is
part of a standard day that has been established that includes
a standard temperature of 15 °C at sea level. Specific standard
day values have also been established for air density, volume,
and viscosity. All of these values are developed averages
since the atmosphere is continuously fluctuating. They are
used by engineers when designing instrument systems and
are sometimes used by technicians and pilots. Often, using
a standard value for atmospheric pressure is more desirable
than using the actual value. For example, at 18,000 feet and
above, all aircraft use 29.92 "Hg as a reference pressure for
their instruments to indicate altitude. This results in altitude
indications in all cockpits being identical. Therefore, an
accurate means is established for maintaining vertical
separation of aircraft flying at these high altitudes.
Pressure Instruments
Engine Oil Pressure
The most important instrument used by the pilot to perceive
the health of an engine is the engine oil pressure gauge.
[Figure 10-13] Oil pressure is usually indicated in psi. The
normal operating range is typically represented by a green arc
on the circular gauge. For exact acceptable operating range,
consult the manufacturer’s operating and maintenance data.
In reciprocating and turbine engines, oil is used to lubricate
and cool bearing surfaces where parts are rotating or sliding
past each other at high speeds. A loss of pressurized oil
to these areas would rapidly cause excessive friction and
over temperature conditions, leading to catastrophic engine
failure. As mentioned, aircraft using analog instruments
often use direct reading Bourdon tube oil pressure gauges.
Figure 10-13 shows the instrument face of a typical oil
pressure gauge of this type. Digital instrument systems use
an analog or digital remote oil pressure sensing unit that
sends output to the computer, driving the display of oil
pressure value(s) on the aircraft’s cockpit display screens.
Oil pressure may be displayed in a circular or linear gauge
fashion and may even include a numerical value on screen.
Often, oil pressure is grouped with other engine parameter
displays on the same page or portion of a page on the display.
Figure 10-14 shows this grouping on a Garmin G1000 digital
instrument display system for general aviation aircraft.
10-9
Figure 10-14. Oil pressure indication with other engine-related parameters shown in a column on the left side of this digital cockpit
display panel.
the Garmin G1000 multifunctional display in Figure 10-14.
The aircraft’s operating manual contains data on managing
manifold pressure in relation to fuel flow and propeller
pitch and for achieving various performance profiles during
different phases of run-up and flight.
Engine Pressure Ratio (EPR)
Turbine engines have their own pressure indication that
relates the power being developed by the engine. It is called
the engine pressure ratio (EPR) indicator (EPR gauge). This
gauge compares the total exhaust pressure to the pressure
of the ram air at the inlet of the engine. With adjustments
for temperature, altitude, and other factors, the EPR gauge
presents an indication of the thrust being developed by
the engine. Since the EPR gauge compares two pressures,
it is a differential pressure gauge. It is a remote-sensing
instrument that receives its input from an engine pressure
ratio transmitter or, in digital instrument systems displays,
from a computer. The pressure ratio transmitter contains the
bellows arrangement that compares the two pressures and
converts the ratio into an electric signal used by the gauge
for indication. [Figure 10-16]
Manifold Pressure
In reciprocating engine aircraft, the manifold pressure gauge
indicates the pressure of the air in the engine’s induction
manifold. This is an indication of power being developed by
the engine. The higher the pressure of the fuel air mixture
going into the engine, the more power it can produce. For
normally aspirated engines, this means that an indication
near atmospheric pressure is the maximum. Turbocharged or
supercharged engines pressurize the air being mixed with the
fuel, so full power indications are above atmospheric pressure.
Most manifold pressure gauges are calibrated in inches of
mercury, although digital displays may have the option to
display in a different scale. A typical analog gauge makes
use of an aneroid described above. When atmospheric
pressure acts on the aneroid inside the gauge, the connected
pointer indicates the current air pressure. A line running
from the intake manifold into the gauge presents intake
manifold air pressure to the aneroid, so the gauge indicates
the absolute pressure in the intake manifold. An analog
manifold pressure gauge, along with its internal workings, is
shown in Figure 10-15. The digital presentation of manifold
pressure is at the top of the engine instruments displayed on
10-10
IN
Hg
ALg.
MANIFOLD
PRESS
35
10 50
15
25
40
30
20
45
LD
SS
35
IN Hg ALg.
50
40
30
N
45
Lg.
L
Capillary assembly Manifold pressure sensing aneroid
Frame Rocking shaft
Altitude compensating aneroid
Figure 10-16. An analog manifold pressure indicator instrument dial calibrated in inches of mercury (left). The internal workings of an
analog manifold pressure gauge are shown on the right. Air from the intake manifold surrounds the aneroid causing it to deflect and
indicate pressure on the dial through the use of linkage to the pointer (right).
PRESSURE
RATIO
3.4
1.2
1.5
2.5
3.0
2.0
0
8
6
4
A. An analog EPR gauge
from a turbine engine
C. Engine pressure
ratio transducer
B. A digital EPR indication and other turbine engine
parameters on a cockpit digital display screen
1 1.02
1.4
1.8
24.0
6 10
330
1
1.4
1.8
24.0
6 10
EPR
EGT
°C
FF KG/H
540
N3 %
17.3
FF KG/H
800
N3 %
55.3
FOB: 28000 KG
N1
%
Figure 10-15. Engine pressure ratio gauges.
mechanism that is used be part of a transmitter device that
uses electricity to send a signal to the indicator in the cockpit.
Sometimes, indications monitoring the fuel flow rate are
used instead of fuel pressure gauges. Fuel flow indications
are discussed in the fuel system chapter of this handbook.
Hydraulic Pressure
Numerous other pressure monitoring gauges are used on
complex aircraft to indicate the condition of various support
systems not found on simple light aircraft. Hydraulic systems
are commonly used to raise and lower landing gear, operate
flight controls, apply brakes, and more. Sufficient pressure in
the hydraulic system developed by the hydraulic pump(s) is
Fuel Pressure
Fuel pressure gauges also provide critical information to
the pilot. [Figure 10-17] Typically, fuel is pumped out of
various fuel tanks on the aircraft for use by the engines.
A malfunctioning fuel pump, or a tank that has been
emptied beyond the point at which there is sufficient fuel
entering the pump to maintain desired output pressure, is a
condition that requires the pilot’s immediate attention. While
direct-sensing fuel pressure gauges using Bourdon tubes,
diaphragms, and bellows sensing arrangements exist, it is
particularly undesirable to run a fuel line into the cockpit,
due to the potential for fire should a leak develop. Therefore,
the preferred arrangement is to have whichever sensing
10-11
Figure 10-19. Vacuum suction gauge.
Figure 10-17. A typical analog fuel pressure gauge.
Transmitter
Figure 10-18. A hydraulic pressure transmitter senses and converts
pressure into an electrical output for indication by the cockpit gauge
or for use by a computer that analyzes and displays the pressure in
the cockpit when requested or required.
required for normal operation of hydraulic devices. Hydraulic
pressure gauges are often located in the cockpit and at or
near the hydraulic system servicing point on the airframe.
Remotely located indicators used by maintenance personnel
are almost always direct reading Bourdon tube type gauges.
Cockpit gauges usually have system pressure transmitted
from sensors or computers electrically for indication.
Figure 10-18 shows a hydraulic pressure transmitter in place
in a high-pressure aircraft hydraulic system.
Vacuum Pressure
Gyro pressure gauge, vacuum gauge, or suction gauge are
all terms for the same gauge used to monitor the vacuum
developed in the system that actuates the air driven
gyroscopic flight instruments. Air is pulled through the
instruments, causing the gyroscopes to spin. The speed at
which the gyros spin needs to be within a certain range for
correct operation. This speed is directly related to the suction
pressure that is developed in the system. The suction gauge
is extremely important in aircraft relying solely on vacuumoperated
gyroscopic flight instruments.
Vacuum is a differential pressure indication, meaning the
pressure to be measured is compared to atmospheric pressure
through the use of a sealed diaphragm or capsule. The gauge
is calibrated in inches of mercury. It shows how much
less pressure exists in the system than in the atmosphere.
Figure 10-19 shows a suction gauge calibrated in inches
of mercury.
Pressure Switches
In aviation, it is often sufficient to simply monitor whether
the pressure developed by a certain operating system is
too high or too low, so that an action can take place should
one of these conditions occur. This is often accomplished
through the use of a pressure switch. A pressure switch is
a simple device usually made to open or close an electric
circuit when a certain pressure is reached in a system. It can
be manufactured so that the electric circuit is normally open
and can then close when a certain pressure is sensed, or the
circuit can be closed and then opened when the activation
pressure is reached. [Figure 10-20]
Pressure switches contain a diaphragm to which the pressure
being sensed is applied on one side. The opposite side of
the diaphragm is connected to a mechanical switching
mechanism for an electric circuit. Small fluctuations or
a buildup of pressure against the diaphragm move the
diaphragm, but not enough to throw the switch. Only when
10-12
Pressure inlet Atmospheric pressure
Microswitch
ssure in mosphe
Figure 10-21. A normally open pressure switch positioned in an
electrical circuit causes the circuit to be open as well. The switch
closes, allowing electricity to flow when pressure is applied beyond
the switch’s preset activation point. Normally, closed pressure
switches allow electricity to flow through the switch in a circuit but
open when pressure reaches a preset activation point, thus opening
the electrical circuit.
Figure 10-20. A pressure switch can be used in addition to, or
instead of, a pressure gauge.
pressure meets or exceeds a preset level designed into the
structure of the switch does the diaphragm move far enough
for the mechanical device on the opposite side to close the
switch contacts and complete the circuit. [Figure 10-21]
Each switch is rated to close (or open) at a certain pressure,
and must only be installed in the proper location.
A low oil pressure indication switch is a common example
of how pressure switches are employed. It is installed in
an engine so pressurized oil can be applied to the switch’s
diaphragm. Upon starting the engine, oil pressure increases
and the pressure against the diaphragm is sufficient to hold
the contacts in the switch open. As such, current does not
flow through the circuit and no indication of low oil pressure
is given in the cockpit. Should a loss of oil pressure occur,
the pressure against the diaphragm becomes insufficient to
hold the switched contacts open. When the contacts close,
they close the circuit to the low oil pressure indicator, usually
a light, to warn the pilot of the situation.
Pressure gauges for various components or systems work
similarly to those mentioned above. Some sort of sensing
device, appropriate for the pressure being measured or
monitored, is matched with an indicating display system. If
appropriate, a properly rated pressure switch is installed in the
system and wired into an indicating circuit. Further discussion
of specific instruments occurs throughout this handbook as the
operation of various systems and components are discussed.
Pitot-Static Systems
Some of the most important flight instruments derive their
indications from measuring air pressure. Gathering and
distributing various air pressures for flight instrumentation
is the function of the pitot-static system.
Pitot Tubes and Static Vents
On simple aircraft, this may consist of a pitot-static system
head or pitot tube with impact and static air pressure ports
and leak-free tubing connecting these air pressure pickup
points to the instruments that require the air for their
indications. The altimeter, airspeed indicator, and vertical
speed indicator are the three most common pitot-static
instruments. Figure 10-22 illustrates a simple pitot-static
system connected to these three instruments.
A pitot tube is shown in Figure 10-23. It is open and faces
into the airstream to receive the full force of the impact
air pressure as the aircraft moves forward. This air passes
through a baffled plate designed to protect the system from
moisture and dirt entering the tube. Below the baffle, a drain
hole is provided, allowing moisture to escape. The ram air
is directed aft to a chamber in the shark fin of the assembly.
An upright tube, or riser, leads this pressurized air out of the
pitot assemble to the airspeed indicator.
10-13
Pitot line Pitot-static tube
Static line
Figure 10-22. A simple pitot-static system is connected to the
primary flight instruments.
30.0
29.8
300.0 3
29.9
2
29299.8 29299.89.2
Altimeter
Heater (35 watts)
Static port
Airspeed indicator (ASI) Vertical speed indicator (VSI)
Pitot heater switch
Drain hole
Pressure chamber
Alternate static source
Static hole
Heater (100 watts)
Pitot tube
Baffle plate
Static chamber
Ram air
Figure 10-23. A typical pitot-static system head, or pitot tube, collects ram air and static pressure for use by the flight instruments.
The aft section of the pitot tube is equipped with small holes
on the top and bottom surfaces that are designed to collect
air pressure that is at atmospheric pressure in a static, or still,
condition. [Figure 10-23] The static section also contains a
riser tube and the air is run out the pitot assembly through
tubes and is connected to the altimeter, the airspeed indicator,
and the vertical speed indicator.
Many pitot-static tube heads contain heating elements to
prevent icing during flight. The pilot can send electric current
to the element with a switch in the cockpit when ice-forming
conditions exist. Often, this switch is wired through the
ignition switch so that when the aircraft is shut down, a pitot
tube heater inadvertently left on does not continue to draw
current and drain the battery. Caution should be exercised
when near the pitot tube, as these heating elements make the
tube too hot to be touched without receiving a burn.
The pitot-static tube is mounted on the outside of the aircraft
at a point where the air is least likely to be turbulent. It is
pointed in a forward direction parallel to the aircraft’s line
of flight. The location may vary. Some are on the nose of
the fuselage and others may be located on a wing. A few
may even be found on the empennage. Various designs exist
but the function remains the same, to capture impact air
pressure and static air pressure and direct them to the proper
instruments. [Figure 10-24]
Most aircraft equipped with a pitot-static tube have an
alternate source of static air pressure provided for emergency
use. The pilot may select the alternate with a switch in
the cockpit should it appear the flight instruments are not
providing accurate indications. On low-flying unpressurized
aircraft, the alternate static source may simply be air from
10-14
Pitot-static tube
Alternate static source (cockpit air)
Pitot line
Static line
Figure 10-25. On unpressurized aircraft, an alternate source of
static air is cabin air.
Figure 10-26. Heated primary and alternate static vents located on
the sides of the fuselage.
Figure 10-24. Pitot-static system heads, or pitot tubes, can be of
various designs and locations on airframes.
the cabin. [Figure 10-25] On pressurized aircraft, cabin
air pressure may be significantly different than the outside
ambient air pressure. If used as an alternate source for static
air, instrument indications would be grossly inaccurate. In
this case, multiple static vent pickup points are employed. All
are located on the outside of the aircraft and plumbed so the
pilot can select which source directs air into the instruments.
On electronic flight displays, the choice is made for which
source is used by the computer or by the flight crew.
Another type of pitot-static system provides for the location
of the pitot and static sources at separate positions on the
aircraft. The pitot tube in this arrangement is used only to
gather ram air pressure. Separate static vents are used to
collect static air pressure information. Usually, these are
located flush on the side of the fuselage. [Figure 10-26] There
may be two or more vents. A primary and alternate source
vent is typical, as well as separate dedicated vents for the pilot
and first officer’s instruments. Also, two primary vents may
be located on opposite sides of the fuselage and connected
with Y tubing for input to the instruments. This is done to
compensate for any variations in static air pressure on the
vents due to the aircraft’s attitude. Regardless of the number
and location of separate static vents, they may be heated as
well as the separate ram air pitot tube to prevent icing.
The pitot-static systems of complex, multiengine, and
pressurized aircraft can be elaborate. Additional instruments,
gauges, the autopilot system, and computers may need pitot
and static air information. Figure 10-27 shows a pitot-static
system for a pressurized multiengine aircraft with dual
analog instrument panels in the cockpit. The additional set
of flight instruments for the copilot alters and complicates
the pitot-static system plumbing. Additionally, the autopilot
system requires static pressure information, as does the cabin
pressurization unit. Separate heated sources for static air
pressure are taken from both sides of the airframe to feed
independent static air pressure manifolds; one each for the
pilot’s flight instruments and the copilot’s flight instruments.
This is designed to ensure that there is always one set of flight
instruments operable in case of a malfunction.
Air Data Computers (ADC) and Digital Air Data
Computers (DADC)
High performance and jet transport category aircraft
pitot-static systems may be more complicated. These
10-15
Autopilot amplifier
Static manifold Static manifold
1
3
2 3 1
2
4
5
1
2
3
4
5
Altimeter indicator
Airspeed indicator
Rate-of-climb indicator
Cabin pressure controller
Cabin differential pressure gauge
Drain valves
Static selector valve
Cabin pressure control panel
Static drain tee
Static selector valve
Sumps and drains
Copilot’s flight
instrument panel
Pilot’s flight
instrument panel
Flush-mounted
heated static tubes
Flush-mounted heated static tubes
Autopilot
static
drain
valve
Pilot’s equalizer manifold
Copilot’s equalizer manifold
Static vent drain valve in nose gear well Static vent drain valve in nose gear well
Flush-mounted unheated static tube Flush-mounted unheated static tube
Pilot’s instrument pitot tube Copilot’s instrument pitot tube
Static system
Pitot pressure system
Figure 10-27. Schematic of a typical pitot-static system on a pressurized multiengine aircraft.
aircraft frequently operate at high altitude where the
ambient temperature can exceed 50 °F below zero. The
compressibility of air is also altered at high speeds and at
high altitudes. Airflow around the fuselage changes, making
it difficult to pick up consistent static pressure inputs. The
pilot must compensate for all factors of air temperature and
density to obtain accurate indications from instruments.
While many analog instruments have compensating devices
built into them, the use of an air data computer (ADC) is
common for these purposes on high-performance aircraft.
Moreover, modern aircraft utilize digital air data computers
(DADC). The conversion of sensed air pressures into digital
values makes them more easily manipulated by the computer
to output accurate information that has compensated for the
many variables encountered. [Figure 10-28]
10-16
Figure 10-28. Teledyne’s 90004 TAS/Plus air data computer (ADC)
computes air data information from the pitot-static pneumatic
system, aircraft temperature probe, and barometric correction
device to help create a clear indication of flight conditions.
Essentially, all pressures and temperatures captured by
sensors are fed into the ADC. Analog units utilize transducers
to convert these to electrical values and manipulate them in
various modules containing circuits designed to make the
proper compensations for use by different instruments and
systems. A DADC usually receives its data in digital format.
Systems that do not have digital sensor outputs will first
convert inputs into digital signals via an analog-to-digital
converter. Conversion can take place inside the computer
or in a separate unit designed for this function. Then, all
calculation and compensations are performed digitally by
the computer. Outputs from the ADC are electric to drive
servo motors or for use as inputs in pressurization systems,
flight control units, and other systems. DADC outputs are
distributed to these same systems and the cockpit display
using a digital data bus.
There are numerous benefits of using ADCs. Simplification
of pitot-static plumbing lines creates a lighter, simpler,
system with fewer connections, so it is less prone to leaks and
easier to maintain. One-time compensation calculations can
be done inside the computer, eliminating the need to build
compensating devices into numerous individual instruments
or units of the systems using the air data. DADCs can run a
number of checks to verify the plausibility of data received
from any source on the aircraft. Thus, the crew can be alerted
automatically of a parameter that is out of the ordinary.
Change to an alternate data source can also be automatic so
accurate flight deck and systems operations are continuously
maintained. In general, solid-state technology is more reliable
and modern units are small and lightweight. Figure 10-29
shows a schematic of how a DADC is connected into the
aircraft’s pitot-static and other systems.
Pitot-Static Pressure-Sensing Flight Instruments
The basic flight instruments are directly connected to
the pitot-static system on many aircraft. Analog flight
instruments primarily use mechanical means to measure and
indicate various flight parameters. Digital flight instrument
systems use electricity and electronics to do the same.
Discussion of the basic pitot-static flight instruments begins
with analog instruments to which further information about
modern digital instrumentation is added.
Altimeters and Altitude
An altimeter is an instrument that is used to indicate the height
of the aircraft above a predetermined level, such as sea level
or the terrain beneath the aircraft. The most common way
to measure this distance is rooted in discoveries made by
scientists centuries ago. Seventeenth century work proving
that the air in the atmosphere exerted pressure on the things
around us led Evangelista Torricelli to the invention of the
barometer. Also in that century, using the concept of this first
atmospheric air pressure measuring instrument, Blaise Pascal
was able to show that a relationship exists between altitude
and air pressure. As altitude increases, air pressure decreases.
The amount that it decreases is measurable and consistent
for any given altitude change. Therefore, by measuring air
pressure, altitude can be determined. [Figure 10-30]
Altimeters that measure the aircraft’s altitude by measuring
the pressure of the atmospheric air are known as pressure
altimeters. A pressure altimeter is made to measure the
ambient air pressure at any given location and altitude. In
aircraft, it is connected to the static vent(s) via tubing in the
pitot-static system. The relationship between the measured
pressure and the altitude is indicated on the instrument face,
which is calibrated in feet. These devises are direct-reading
instruments that measure absolute pressure. An aneroid or
aneroid bellows is at the core of the pressure altimeter’s
inner workings. Attached to this sealed diaphragm are the
linkages and gears that connect it to the indicating pointer.
Static air pressure enters the airtight instrument case and
surrounds the aneroid. At sea level, the altimeter indicates
zero when this pressure is exerted by the ambient air on the
aneroid. As air pressure is reduced by moving the altimeter
higher in the atmosphere, the aneroid expands and displays
altitude on the instrument by rotating the pointer. As the
altimeter is lowered in the atmosphere, the air pressure around
the aneroid increases and the pointer moves in the opposite
direction. [Figure 10-31]
The face, or dial, of an analog altimeter is read similarly to
a clock. As the longest pointer moves around the dial, it is
registering the altitude in hundreds of feet. One complete
revolution of this pointer indicates 1,000 feet of altitude.
10-17
Flight director system 1 & 2
Fuel temperature indicator
Autopilot system
Flight recorder/locator
Transponder
FLT management computer units 1 & 2
Inertial reference units 1, 2, & 3
FLT control augmentation computer
Auto transformer AOA probe Total air temperature probe
Pilot’s
mach/airspeed
Digital air data
computer
Pilot’s
altimeter
Auto
throttle
Copilot’s
altimeter
Static air temp
gauge
True airspeed
indicator
TAT indicator
Copilot’s
mach/airspeed
1
2
2
3
Computed
airspeed
DADC data
bus
DADC data
bus
Altitude
encoding
Altitude error
airspeed
Altitude
Altitude
error
Computed
airspeed altitude
TAS
Airspeed mach
altitude
Altitude rate TAT
Static
pressure
Static
pressure
Mach
Copilot’s
pitot pressure
Pilot’s pitot
pressure
Pitot pressure
Static pressure
Electrical connection
Direction of data flow
Pilot’s altimeter provides altitude signal to
flight recorder/locator if in reset mode
Servo-corrected altitude
In reset mode, copilot uses pilot’s static source
1
2
3
Figure 10-29. ADCs receive input from the pitot-static sensing devices and process them for use by numerous aircraft systems.
The second-longest point moves more slowly. Each time it
reaches a numeral, it indicates 1,000 feet of altitude. Once
around the dial for this pointer is equal to 10,000 feet. When
the longest pointer travels completely around the dial one
time, the second-longest point moves only the distance
between two numerals—indicating 1,000 feet of altitude
has been attained. If so equipped, a third, shortest or thinnest
pointer registers altitude in 10,000 foot increments. When
this pointer reaches a numeral, 10,000 feet of altitude has
been attained. Sometimes a black-and-white or red-and-white
cross-hatched area is shown on the face on the instrument
until the 10,000 foot level has been reached. [Figure 10-32]
10-18
Atmosphere pressure
Altitude (ft) Pressure (psi)
Sea level
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
22,000
24,000
26,000
28,000
30,000
32,000
34,000
36,000
38,000
40,000
42,000
44,000
46,000
48,000
50,000
14.69
13.66
12.69
11.77
10.91
10.10
9.34
8.63
7.96
7.34
6.75
6.20
5.69
5.22
4.77
4.36
3.98
3.62
3.29
2.99
2.72
2.47
2.24
2.04
1.85
1.68
Figure 10-30. Air pressure is inversely related to altitude. This
consistent relationship is used to calibrate the pressure altimeter.
0
2
3
1
4
29.9
30.0
29.8
Barometric dial
100 feet scale
1,000 feet scale
10,000 feet scale
Aneroids
Balance assembly
Adjustment knob
Out: sets barometrical dial
In: sets dial hands
Figure 10-31. The internal arrangement of a sealed diaphragm pressure altimeter. At sea level and standard atmospheric conditions, the
linkage attached to the expandable diaphragm produces an indication of zero. When altitude increases, static pressure on the outside
of the diaphragm decreases and the aneroid expands, producing a positive indication of altitude. When altitude decreases, atmospheric
pressure increases. The static air pressure on the outside of the diaphragm increases and the pointer moves in the opposite direction,
indicating a decrease in altitude.
30.0
29.9
29.8 9.8.8.8
I 00
FEET
I
6 5
4
7
9
2
0
8
3
30
292
29292929
66 CALIBRATED
TO
20,000 FEET
ALT
A
10,000-foot increments
1,000-foot increments
100-foot increments
Displayed at altitudes
below 10,000 feet
Figure 10-32. A sensitive altimeter with three pointers and a crosshatched
area displayed during operation below 10,000 feet.
Many altimeters also contain linkages that rotate a numerical
counter in addition to moving pointers around the dial. This
quick reference window allows the pilot to simply read the
numerical altitude in feet. The motion of the rotating digits
or drum-type counter during rapid climb or descent makes
it difficult or impossible to read the numbers. Reference
can then be directed to the classic clock-style indication.
Figure 10-33 illustrates the inner workings behind this type
of mechanical digital display of pressure altitude.
10-19
7
2
0
8
BARO
IN Hg
1 0 1 3
2 9 9 2
27
26
25
24
I00
F E E T
I
5 4
6
9
3
MB
2 992
1 881
077 0
3003
2992
1881
077 0
3003
2727
30
26
25
29
28
24
22
23
21
BARO
Pointer
Dial
Baro set knob
Spiral gear
1,000 feet drum
Barometric counters
Mechanism body
Calibration arm Static port
Aneroid capsules
Bimetal compensator
Figure 10-33. A drum-type counter can be driven by the altimeter’s aneroid for numerical display of altitude. Drums can also be used
for the altimeter’s setting indications.
True digital instrument displays can show altitude in
numerous ways. Use of a numerical display rather than a
reproduction of the clock-type dial is most common. Often a
digital numeric display of altitude is given on the electronic
primary flight display near the artificial horizon depiction.
A linear vertical scale may also be presented to put this hard
numerical value in perspective. An example of this type of
display of altitude information is shown in Figure 10-34.
Accurate measurement of altitude is important for numerous
reasons. The importance is magnified in instrument
flight rules (IFR) conditions. For example, avoidance of
tall obstacles and rising terrain relies on precise altitude
indication, as does flying at a prescribed altitude assigned by
air traffic control (ATC) to avoid colliding with other aircraft.
Measuring altitude with a pressure measuring device is
fraught with complications. Steps are taken to refine pressure
altitude indication to compensate for factors that may cause
an inaccurate display.
A major factor that affects pressure altitude measurements
is the naturally occurring pressure variations throughout the
atmosphere due to weather conditions. Different air masses
develop and move over the earth’s surface, each with inherent
pressure characteristics. These air masses cause the weather
we experience, especially at the boundary areas between air
masses known as fronts. Accordingly, at sea level, even if
the temperature remains constant, air pressure rises and falls
as weather system air masses come and go. The values in
Figure 10-30, therefore, are averages for theoretical purposes.
To maintain altimeter accuracy despite varying atmospheric
pressure, a means for setting the altimeter was devised. An
adjustable pressure scale visible on the face of an analog
altimeter known as a barometric or Kollsman window is set to
read the existing atmospheric pressure when the pilot rotates
the knob on the front of the instrument. This adjustment is
linked through gears inside the altimeter to move the altitude
indicating pointers on the dial as well. By putting the current
known air pressure (also known as the altimeter setting) in
the window, the instrument indicates the actual altitude. This
altitude, adjusted for atmospheric pressure changes due to
weather and air mass pressure inconsistency, is known as
the indicated altitude.
It must be noted that in flight the altimeter setting is changed
to match that of the closest available weather reporting
station or airport. This keeps the altimeter accurate as the
flight progresses.
While there was little need for exact altitude measurement in
early fixed wing aviation, knowing one’s altitude provided
the pilot with useful references while navigating in the
three dimensions of the atmosphere. As air traffic grew
10-20
XPDR 5537 IDNT LCL23:00:34
GPS TERM
197°
HDG 357° CRS 152°
2
1
1
2
2300
2200
2100
4000
1900
1800
1700
60
20
2000
2000
150
140
130
110
100
90
1
120
9
TAS 120KT
OAT 7°C
ALERTS
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°
TRAFFIC
Figure 4-35. HSI Trend Indicator elongates proportionate to the rate of turn.
Altimeter indication
Altitude scale
Vertical speed scale
Airspeed scale
Figure 10-34. This primary flight display unit of a Garmin 1000 series glass cockpit instrumentation package for light aircraft indicates
altitude using a vertical linear scale and a numerical counter. As the aircraft climbs or descends, the scale behind the black numerical
altitude readout changes.
and the desire to fly in any weather conditions increased,
exact altitude measurement became more important and
the altimeter was refined. In 1928, Paul Kollsman invented
the means for adjusting an altimeter to reflect variations
in air pressure from standard atmospheric pressure. The
very next year, Jimmy Doolittle made his successful flight
demonstrating the feasibility of instrument flight with no
visual references outside of the cockpit using a Kollsman
sensitive altimeter.
The term pressure altitude is used to describe the indication
an altimeter gives when 29.92 is set in the Kollsman window.
When flying in U.S. airspace above 18,000 feet mean sea
level (MSL), pilots are required to set their altimeters to
29.92. With all aircraft referencing this standard pressure
level, vertical separation between aircraft assigned to
different altitudes by ATC should be assured. This is the
case if all altimeters are functioning properly and pilots
hold their assigned altitudes. Note that the true altitude or
actual height of an aircraft above sea level is only the same
as the pressure altitude when standard day conditions exist.
Otherwise, all aircraft with altimeters set to 29.92 "Hg could
have true altitudes higher or lower than the pressure altitude
indicated. This is due to the pressure within the air mass in
which they are flying being above or below standard day
pressure (29.92). The actual or true altitude is less important
than keeping aircraft from colliding, which is accomplished
by all aircraft above 18,000 feet referencing the same pressure
level (29.92 "Hg). [Figure 10-35]
Temperature also affects the accuracy of an altimeter. The
aneroid diaphragms used in altimeters are usually made of
metal. Their elasticity changes as their temperature changes.
This can lead to a false indication, especially at high altitudes
when the ambient air is very cold. A bimetallic compensating
device is built into many sensitive altimeters to correct for
varying temperature. Figure 10-33 shows one such device
on a drum-type altimeter.
Temperature also affects air density, which has great impact
on the performance of an aircraft. Although this does not
cause the altimeter to produce an errant reading, flight crews
must be aware that performance changes with temperature
variations in the atmosphere. The term density altitude
describes altitude corrected for nonstandard temperature.
That is, the density altitude is the standard day altitude
(pressure altitude) at which an aircraft would experience
similar performance as it would on the non-standard day
10-21
30.3
30.1
300.0.3 3
30.2
3
0..1 3030303030.130
30.0
29.8
300.0.0 3
29.9
2
9.8 2929292929.829.8
18,000 feet 18,000 feet
High pressure Low pressure
Figure 10-35. Above 18,000 feet MSL, all aircraft are required to set 29.92 as the reference pressure in the Kollsman window. The
altimeter then reads pressure altitude. Depending on the atmospheric pressure that day, the true or actual altitude of the aircraft may
be above or below what is indicated (pressure altitude).
currently being experienced. For example, on a very cold
day, the air is denser than on a standard day, so an aircraft
performs as though it is at a lower altitude. The density
altitude is lower that day. On a very hot day, the reverse is
true, and an aircraft performs as though it were at a higher
elevation where the air is less dense. The density altitude is
higher that day.
Conversion factors and charts have been produced so pilots
can calculate the density altitude on any particular day.
Inclusion of nonstandard air pressure due to weather systems
and humidity can also be factored. So, while the effects of
temperature on aircraft performance do not cause an altimeter
to indicate falsely, an altimeter indication can be misleading
in terms of aircraft performance if these effects are not
considered. [Figure 10-36]
Other factors can cause an inaccurate altimeter indication.
Scale error is a mechanical error whereby the scale of the
instrument is not aligned so the altimeter pointers indicate
correctly. Periodic testing and adjustment by trained
technicians using calibrated equipment ensures scale error
is kept to a minimum.
The pressure altimeter is connected to the pitot-static system
and must receive an accurate sample of ambient air pressure
to indicate the correct altitude. Position error, or installation
error, is that inaccuracy caused by the location of the static
vent that supplies the altimeter. While every effort is made
to place static vents in undisturbed air, airflow over the
airframe changes with the speed and attitude of the aircraft.
The amount of this air pressure collection error is measured in
test flights, and a correction table showing the variances can
be included with the altimeter for the pilot’s use. Normally,
location of the static vents is adjusted during these test flights
so that the position error is minimal. [Figure 10-37] Position
error can be removed by the ADC in modern aircraft, so the
pilot need not be concerned about this inaccuracy.
Static system leaks can affect the static air input to the
altimeter or ADC resulting in inaccurate altimeter indications.
It is for this reason that static system maintenance includes
leak checks every 24 months, regardless of whether any
discrepancy has been noticed. See the instrument maintenance
section toward the end of this chapter for further information
on this mandatory check. It should also be understood
that analog mechanical altimeters are mechanical devices
that often reside in a hostile environment. The significant
vibration and temperature range swings encountered by
the instruments and the pitot static system (i.e., the tubing
connections and fittings) can sometime create damage or a
leak, leading to instrument malfunction. Proper care upon
installation is the best preventive action. Periodic inspection
and testing can also insure integrity.
The mechanical nature of the analog altimeter’s diaphragm
pressure measuring apparatus has limitations. The diaphragm
itself is only so elastic when responding to static air pressure
changes. Hysteresis is the term for when the material from
which the diaphragm is made takes a set during long periods
of level flight. If followed by an abrupt altitude change,
the indication lags or responds slowly while expanding or
contracting during a rapid altitude change. While temporary,
this limitation does cause an inaccurate altitude indication.
It should be noted that many modern altimeters are
constructed to integrate into flight control systems, autopilots,
and altitude monitoring systems, such as those used by ATC.
The basic pressure-sensing operation of these altimeters is the
same, but a means for transmitting the information is added.
10-22
Static ports Pitot tube probes
Figure 10-37. The location of the static vent is selected to keep
altimeter position error to a minimum.
3
4
I
2
I 2
3
VERTICAL SPEED
THOUSAND FT PER MIN
UP
.5
.5
0
DOWN
5
0
Figure 3-10. Vertical speed indicator shows the rate of climb or
descent in thousands of feet per minute.
Zeroing adjustment screw
Figure 10-38. A typical vertical speed indicator.
Density Altitude Chart
Outside air temperature
Approximate density altitude (thousand feet)
15
14
13
12
11
10
9
8
7
6
5
4
3
2
1
S.L.
14,000
13,000
12,000
11,000
10,000
9,000 Pressure altitude (feet)
8,000
7,000
6,000
5,000
4,000
3,000
2,000
1,000
–1,000
Sea level
–18 –12° –7° –1° 4° 10° 16° 21° 27° 32° 38°
0° 10° 20° 30° 40° 50° 60° 70° 80° 90° 100°
Standard temperature
F
C
28.0 1,824
28.1 1,727
28.2 1,630
28.3 1,533
28.4 1,436
28.5 1,340
28.6 1,244
28.7 1,148
28.8 1,053
28.9 957
29.0 863
29.1 768
29.2 673
29.3 579
29.4 485
29.5 392
29.6 298
29.7 205
29.8 112
29.9 20
29.92 0
30.0 −73
30.1 −165
30.2 −257
30.3 −348
30.4 −440
30.5 −531
30.6 −622
30.7 −712
30.8 −803
Altimeter setting
("Hg)
Pressure altitude
conversion factor
Figure 10-36. The effect of air temperature on aircraft performance
is expressed as density altitude.
Vertical Speed Indicator
An analog vertical speed indicator (VSI) may also be referred
to as a vertical velocity indicator (VVI), or rate-of-climb
indicator. It is a direct reading, differential pressure gauge
that compares static pressure from the aircraft’s static system
directed into a diaphragm with static pressure surrounding
the diaphragm in the instrument case. Air is free to flow
unrestricted in and out of the diaphragm but is made to flow
in and out of the case through a calibrated orifice. A pointer
attached to the diaphragm indicates zero vertical speed when
the pressure inside and outside the diaphragm are the same.
The dial is usually graduated in 100s of feet per minute.
A zeroing adjustment screw, or knob, on the face of the
instrument is used to center the pointer exactly on zero while
the aircraft is on the ground. [Figure 10-38]
As the aircraft climbs, the unrestricted air pressure in
the diaphragm lowers as the air becomes less dense. The
case air pressure surrounding the diaphragm lowers more
slowly, having to pass through the restriction created by the
orifice. This causes unequal pressure inside and outside the
diaphragm, which in turn causes the diaphragm to contract
10-23
3
4
I
2
I 2
THOUSAND FT PER MIN
UP
DOWN
.5
0
3
VERTICAL SPEED
.5
TH 3
4
2
2
3
VERTICAL SPEED
THOUSAND FT PER MIN
OWN
Free flow out Restricted passage
Diaphragm
Restricted flow
Climb
Descent
Static port connection
Figure 10-39. The VSI is a differential pressure gauge that compares free-flowing static air pressure in the diaphragm with restricted
static air pressure around the diaphragm in the instrument case.
3
4
I
0
2
2 I
3
UP
DOWN
.5
.5
Diaphragm
Static connection
Restricted passage
Bypass restriction
Dashpot acceleration pump
Dashpot piston
Figure 10-40. The small dashpot in this IVSI reacts abruptly to a
climb or descent pumping air into or out of the diaphragm causing
an instantaneously vertical speed indication.
a bit and the pointer indicates a climb. The process works in
reverse for an aircraft in a descent. If a steady climb or descent
is maintained, a steady pressure differential is established
between the diaphragm and case pressure surrounding it,
resulting in an accurate indication of the rate of climb via
graduations on the instrument face. [Figure 10-39]
A shortcoming of the rate-of-climb mechanism as described
is that there is a lag of six to nine seconds before a stable
differential pressure can be established that indicates the
actual climb or descent rate of the aircraft. An instantaneous
vertical speed indicator (IVSI) has a built-in mechanism to
reduce this lag. A small, lightly sprung dashpot, or piston,
reacts to the direction change of an abrupt climb or descent.
As this small accelerometer does so, it pumps air into or
out of the diaphragm, hastening the establishment of the
pressure differential that causes the appropriate indication.
[Figure 10-40]
Gliders and lighter-than-air aircraft often make use of a
variometer. This is a differential VSI that compares static
pressure with a known pressure. It is very sensitive and gives
an instantaneous indication. It uses a rotating vane with a
pointer attached to it. The vane separates two chambers.
One is connected to the aircraft’s static vent or is open to
the atmosphere. The other is connected to a small reservoir
inside the instrument that is filled to a known pressure. As
static air pressure increases, the pressure in the static air
chamber increases and pushes against the vane. This rotates
the vane and pointer, indicating a descent since the static
pressure is now greater than the set amount in the chamber
with reservoir pressure. During a climb, the reservoir pressure
is greater than the static pressure; the vane is pushed in the
opposite direction, causing the pointer to rotate and indicate
a climb. [Figure 10-41]
The rate-of-climb indication in a digitally displayed
instrument system is computed from static air input to the
ADC. An aneroid, or solid-state pressure sensor, continuously
reacts to changes in static pressure. The digital clock within
the computer replaces the calibrated orifice found on an
analog instrument. As the static pressure changes, the
computer’s clock can be used to develop a rate for the change.
Using the known lapse rate conversion for air pressure as
altitude increases or decreases, a figure for climb or descent
in fpm can be calculated and sent to the cockpit. The vertical
10-24
100
200
50
150
20200 2
50500
Diaphragm
Handstaff
Static connection
Rocking shaft
Hairspring
Sector
Long lever
Pitot connection
Restraining spring
Figure 10-42. An airspeed indicator is a differential pressure gauge
that compares ram air pressure with static pressure.
0
A
B
Hairspring
Vane
Pivot
Pointer and scale to fixed
pressure
reservoir
to static
source
Figure 10-41. A variometer uses differential pressure to indicate
vertical speed. A rotating vane separating two chambers (one with
static pressure, the other with a fixed pressure reservoir), moves
the pointer as static pressure changes.
speed is often displayed near the altimeter information on
the primary flight display. [Figure 10-34]
Airspeed Indicators
The airspeed indicator is another primary flight instrument
that is also a differential pressure gauge. Ram air pressure
from the aircraft’s pitot tube is directed into a diaphragm in
an analog airspeed instrument case. Static air pressure from
the aircraft static vent(s) is directed into the case surrounding
the diaphragm. As the speed of the aircraft varies, the ram
air pressure varies, expanding or contracting the diaphragm.
Linkage attached to the diaphragm causes a pointer to move
over the instrument face, which is calibrated in knots or miles
per hour (mph). [Figure 10-42]
The relationship between the ram air pressure and static air
pressure produces the indication known as indicated airspeed.
As with the altimeter, there are other factors that must be
considered in measuring airspeed throughout all phases of
flight. These can cause inaccurate readings or indications
that are not useful to the pilot in a particular situation. In
analog airspeed indicators, the factors are often compensated
for with ingenious mechanisms inside the case and on the
instrument dial face. Digital flight instruments can have
calculations performed in the ADC so the desired accurate
indication is displayed.
While the relationship between ram air pressure and static
air pressure is the basis for most airspeed indications, it can
be more accurate. Calibrated airspeed takes into account
errors due to position error of the pitot static pickups. It also
corrects for the nonlinear nature of the pitot static pressure
differential when it is displayed on a linear scale. Analog
airspeed indicators come with a correction chart that allows
cross-referencing of indicated airspeed to calibrated airspeed
for various flight conditions. These differences are typically
very small and often are ignored. Digital instruments have
these corrections performed in the ADC.
More importantly, indicated airspeed does not take into
account temperature and air pressure differences needed to
indicate true airspeed. These factors greatly affect airspeed
indication. True airspeed, therefore, is the same as indicated
airspeed when standard day conditions exist. But when
atmospheric temperature or pressure varies, the relationship
between the ram air pressure and static pressure alters. Analog
airspeed instruments often include bimetallic temperature
compensating devices that can alter the linkage movement
between the diaphragm and the pointer movement. There can
also be an aneroid inside the airspeed indicator case that can
compensate for non-standard pressures. Alternatively, true
airspeed indicators exist that allow the pilot to set temperature
and pressure variables manually with external knobs on the
instrument dial. The knobs rotate the dial face and internal
linkages to present an indication that compensates for nonstandard
temperature and pressure, resulting in a true airspeed
indication. [Figure 10-43]
Digital flight instrument systems perform all of the
calculations for true airspeed in the ADC. Ram air from the
pitot tube and static air from the static vent(s) are run into the
sensing portion of the computer. Temperature information
is also input. This information can be manipulated and
calculations performed so a true airspeed value can be
digitally sent to the cockpit for display. Refer to Figure 10-34
for the display of airspeed information on the primary flight
display on a light aircraft. Note that similar to its position in
the standard T configuration of an analog cockpit, the airspeed
indication is just left of the artificial horizon display.
10-25
120
130
140 150
2 4 6
MPH
TEMP
30 + 0 - 30
AIRSPEED
TRUE SPEED
KNOTS
I80 40
60
I60
80
I00
I20
I40
40
I40
I20
60
80
100
PE
E S
NO
20
4
1
Figure 3-12. A true airspeed indicator allows the pilot to
correct indicated airspeed for nonstandard temperature and
pressure.
Figure 10-43. An analog true airspeed indicator. The pilot manually
aligns the outside air temperature with the pressure altitude scale,
resulting in an indication of true airspeed.
Standard Altitude, Temperature, and the Speed of Sound
Altitude (feet) Temperature (°F) Speed of sound (knots)
Sea level
2,000
4,000
6,000
8,000
10,000
12,000
14,000
16,000
18,000
20,000
22,000
24,000
26,000
28,000
30,000
32,000
34,000
36,000
38,000
40,000
42,000
44,000
46,000
48,000
50,000
59
52
48
38
30
23
16
9
2
–5
–12
–19
–27
–34
–41
–48
–55
–62
–69
–70
–70
–70
–70
–70
–70
–70
661
657
652
648
643
638
633
629
624
619
614
609
604
599
594
589
584
579
574
574
574
574
574
574
574
574
Figure 10-44. As temperatures fall at higher altitudes, the speed
of sound is reduced.
Figure 3-13. A Machmeter shows the ratio of the speed of
sound to the true airspeed the aircraft is flying.
Figure 10-45. A Machmeter indicates aircraft speed relative to the
speed of sound.
Complications continue when considering airspeed
indications and operating limitations. It is very important to
keep high-speed aircraft from traveling faster than the speed
of sound if they are not designed to do so. Even as an aircraft
approaches the speed of sound, certain parts on the airframe
may experience airflows that exceed it. The problem with
this is that near the speed of sound, shock waves can develop
that can affect flight controls and, in some cases, can literally
tear the aircraft apart if not designed for supersonic airflow.
A further complication is that the speed of sound changes
with altitude and temperature. So a safe true airspeed at sea
level could put the aircraft in danger at altitude due to the
lower speed of sound. [Figure 10-44]
In order to safeguard against these dangers, pilots monitor
airspeed closely. A maximum allowable speed is established
for the aircraft during certification flight testing. This speed is
known the critical Mach number or Mcrit. Mach is a term for
the speed of sound. The critical Mach number is expressed
as a decimal of Mach such as 0.8 Mach. This means 8⁄10 of
the speed of sound, regardless of what the actual speed of
sound is at any particular altitude.
Many high performance aircraft are equipped with a
Machmeter for monitoring Mcrit. The Machmeter is
essentially an airspeed instrument that is calibrated in relation
to Mach on the dial. Various scales exist for subsonic and
supersonic aircraft. [Figure 10-45] In addition to the ram air/
static air diaphragm arrangement, Machmeters also contain
an altitude sensing diaphragm. It adjusts the input to the
pointer so changes in the speed of sound due to altitude are
incorporated into the indication. Some aircraft use a Mach/
airspeed indicator as shown in Figure 10-46. This two-inone
instrument contains separate mechanisms to display the
airspeed and Mach number. A standard white pointer is used
to indicate airspeed in knots against one scale. A red and white
10-26
40
35 .5
.4
.3
6
0
10
12
14
30
25 20
16
18
M
A
C
H
Figure 10-46. A combination Mach/airspeed indicator shows
airspeed with a white pointer and Mach number with a red and
white striped pointer. Each pointer is driven by separate internal
mechanisms.
0°
Transmitter
A
N
S
N
S
N
S
N
S
C B
D
25°
50°
Indicator
Rotor shaft
Brushes
Contact arm
Resistance
winding
Pointer
Instrument
scale
Permanent
magnet
Figure 10-47. A schematic of a DC selsyn synchro remote indicating
system.
striped pointer is driven independently and is read against the
Mach number scale to monitor maximum allowable speed.
Remote Sensing and Indication
It is often impractical or impossible to utilize direct reading
gauges for information needed to be conveyed in the cockpit.
Placing sensors at the most suitable location on the airframe
or engine and transmitting the collected data electrically
through wires to the displays in the cockpit is a widely
used method of remote-sensing and indicating on aircraft.
Many remote sensing instrument systems consist simply of
the sensing and transmitter unit and the cockpit indicator
unit connected to each other by wires. For pressure flight
instruments, the ADC and pickup devices (pitot tubes, static
vents, etc.) comprise the sensing and transmitter unit. Many
aircraft collect sensed data in dedicated engine and airframe
computers. There, the information can be processed. An
output section of the computer then transmits it electrically
or digitally to the cockpit for display. Remote-sensing
instrument systems operate with high reliability and accuracy.
They are powered by the aircraft’s electrical system.
Small electric motors inside the instrument housings are used
to position the pointers, instead of direct-operating mechanical
linkages. They receive electric current from the output section
of the ADC or other computers. They also receive input from
sensing transmitters or transducers that are remotely located on
the aircraft. By varying the electric signal, the motors are turned
to the precise location needed to reflect the correct indication.
Direct electric transmission of information from different types
of sensors is accomplished with a few reliable and relatively
simple techniques. Note that digital cockpit displays receive all
of their input from a DADC and other computers, via a digital
data bus and do not use electric motors. The data packages
transmitted via the bus contain the instructions on how to
illuminate the display screen.
Synchro-Type Remote-Indicating Instruments
A synchro system is an electric system used for transmitting
information from one point to another. The word “synchro”
is a shortened form of the word “synchronous,” and refers to
any one of a number of similarly operating two-unit electrical
systems capable of measuring, transmitting, and indicating
a certain parameter on the aircraft. Most position-indicating
instruments are designed around a synchro system, such as
the flap position indicator. Fluid pressure indicators also
commonly use synchro systems. Synchro systems are used as
remote position indicators for landing gear, autopilot systems,
radar, and many other remote-indicating applications. The
most common types of synchro system are the autosyn,
selsyn, and magnesyn synchro systems.
These systems are similar in construction, and all operate
by exploiting the consistent relationship between electricity
and magnetism. The fact that electricity can be used to create
magnetic fields that have definite direction, and that magnetic
fields can interact with magnets and other electromagnetic
fields, is the basis of their operation.
DC Selsyn Systems
On aircraft with direct current (DC) electrical systems, the
DC selsyn system is widely used. As mentioned, the selsyn
system consists of a transmitter, an indicator, and connecting
wires. The transmitter consists of a circular resistance
winding and a rotatable contact arm. The rotatable contact
arm turns on a shaft in the center of the resistance winding.
The two ends of the arm are brushes and always touch the
winding on opposite sides. [Figure 10-47] On position

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