Why Nitrogen is used in Aircraft Tires

Thousands of airplane tires are inflated during routine maintenance each day around the world. On occasion, a mechanic or other ground service employee has been severely or fatally injured in an explosion caused by use of unregulated pressure from an air or nitrogen tank. The latest reported incident occurred in 1998 when a mechanic was inflating a nose wheel/tire assembly on a 737 airplane. A total of five similar incidents have been reported as the cause of severe injury or death to maintenance personnel

These types of injuries may be prevented by understanding:

1. Causes of wheel/tire assembly explosions.
2. Preventive measures.

Airplane wheel/tire assemblies are inflated to high pressures (many in excess of 200 psi). In addition, the pressure in the bottle or tire-servicing cart can be as high as 3,000 psi. As a result, when the high-pressure bottle is connected directly to the wheel without a regulator, the wheel is suddenly subjected to the high pressure, which can exceed the design limits for the wheel and the wheel tie bolts. Consequently, the wheel, the wheel tie bolts, or both experience an explosive fracture and become projectiles.

In most of the reported cases of related injuries, the wheel/tire assembly that exploded was a nose wheel on a smaller-configuration airplane such as the 737 or DC-9. Reportedly, the wheels were being inflated from a high-pressure bottle or cart without a regulator when the explosion occurred.

In addition, the wheels were not equipped with an overinflation pressure relief (OPR) valve. An OPR valve is a device similar to that. It is included in many wheel assemblies to limit the pressure in the wheel/tire assembly. However, certain older wheels do not include this valve. If the pressure in the wheel exceeds a predetermined value, a disk in the OPR valve will rupture, allowing the gas to escape, thus reducing the pressure in the wheel before it can fracture. After the disk ruptures, the gas in the wheel will exit through the OPR valve. The valve is designed so that when the disk ruptures, the gas will exit from the wheel faster than it can be supplied from the pressure source.

In addition, Boeing has received reports of three confirmed cases and other suspected cases in which a wheel/tire assembly exploded when the oxygen in air-filled tires combined with volatile gases given off by a severely overheated tire. In one case, the tire became overheated as a result of a dragging brake, and the wheel/tire assembly exploded when it reached the auto-ignition temperature. In another case, a wheel/tire assembly explosion in the wheel well during flight was suspected in the catastrophic loss of one airplane. A similar explosion caused severe damage to two others.

As a result, the U.S. Federal Aviation Administration issued Airworthiness Directive 87-08-09 requiring that only nitrogen be used to inflate airplane tires on braked wheels. However, tires may be topped off with air in remote locations where nitrogen may not be available if the oxygen content in the tire does not exceed 5 percent by volume.

Several precautions can be taken to prevent these types of accidents, including:

• Using a regulator.
• Using inflation cages.
• Following established maintenance manual procedures.
• Outfitting all servicing equipment with regulators.
• Inflating wheel/tire assemblies only with nitrogen.

Using a regulator.
It is essential for maintenance personnel to always use a regulator when inflating any wheel/tire assembly. All of the reported accidents involved nose wheels, as shown in table 1. This may be related to the fact that the wheels did not include an OPR valve and that nose wheels generally are small, which means they contain a smaller volume of air or nitrogen. Inflation without a regulator will rapidly produce pressures in the wheel that are significantly beyond the capabilities of the wheel. As a result, the wheel or wheel tie bolts fracture into pieces that can severely injure the person servicing the tire or damage adjacent equipment.

Using inflation cages.
Most airline or repair-station tire shops are equipped with inflation cages. An inflation cage consists of a strong steel structure that surrounds the wheel/tire assembly during tire inflation. Accordingly, when wheel/tire assemblies are initially inflated with bottled nitrogen in the tire shop, the wheel/tire assembly is enclosed in a cage to protect against injury and damage in case of an explosion. However, it is not always practical to use inflation cages if the wheel/tire assembly is installed on the airplane.

Following established maintenance manual procedures.
To prevent accidents, it is critical for maintenance personnel to use the following procedures designed to reduce the risk of explosion during tire servicing:

• Procedures for inflating wheel/tire assemblies during build-up in the shop are provided in the appropriate supplier Component Maintenance Manual (CMM).
• Procedures for inflating the wheel/tire assembly when it is installed on the airplane are typically located in chapter 12 of the Aircraft Maintenance Manual (AMM). These procedures typically include the following or equivalent warning:
  WARNING: USE A REGULATED PRESSURE SOURCE TO SERVICE THE TIRES. AN UNREGULATED PRESSURE SOURCE CAN CAUSE INJURIES AND DAMAGE TO EQUIPMENT.
• Maintenance personnel should never attempt to inflate a wheel/tire assembly without a regulator between the pressure source, such as a tire-servicing bottle or cart, and the inflation valve on the wheels. The regulator is required, even though the wheel might include an OPR valve. The regulator may be an integral part of the tire-servicing cart.
• Maintenance personnel should always follow procedures in the AMM for removal and installation of the wheel/ tire assembly. Removal and installation procedures can typically be found in chapter 32 of the AMM. (Other information on the subject is listed in .)

Outfitting all servicing equipment with regulators.
Operators should ensure that tire-servicing equipment is properly equipped with regulators to preclude an over-pressure condition.

Inflating wheel/tire assemblies only with nitrogen.
Tires must be initially inflated only with nitrogen. However, air can be used to top off a low-pressure tire if the airplane is in a location where nitrogen is not readily available, provided that the oxygen content does not exceed 5 percent by volume. Optional procedures for ensuring that the oxygen content in the tire will not exceed 5 percent are typically found in chapter 12 of the AMM. These procedures include a table that lists the maximum refill pressure versus the initial tire inflation pressure. The sum of all air pressures added to a given tire cannot exceed the pressure shown in the table for the corresponding initial inflation pressure.

SummaryIn the past 20 years, a few accidents have occurred during tire servicing inwhich the wheel exploded because of overpressurization or high oxygen content,causing serious injury or death to service personnel or damage to equipment.Strict adherence to established procedures in the AMM and CMM will help ensurethe safety of maintenance personnel during tire servicing. In addition, it isessential that tire-servicing equipment is equipped with a regulator to preventtires from being subjected to excessive pressures that can result in anexplosion.

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