Airplane Wheel and Tire Servicing

A small number of commercial airplane maintenance personnel have been seriously injured by wheel/tire assembly explosions during servicing. In each case, an unregulated supply of nitrogen or air was responsible for the explosion. Though many wheels include overinflation pressure relief valves to prevent the buildup of excessive pressure in the wheel/tire assembly, a regulator should always be used when inflating 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 (table 1).

These types of injuries may be prevented by understanding

  1. Causes of wheel/tire assembly explosions.

  2. Preventive measures.

Causes of Wheel/Tire Assembly Explosions
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 shown in figure 1. 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.

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

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:

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.

In the past 20 years, a few accidents have occurred during tire servicing in which 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 ensure the safety of maintenance personnel during tire servicing. In addition, it is essential that tire-servicing equipment is equipped with a regulator to prevent tires from being subjected to excessive pressures that can result in an explosion.

Table 1
Airplane Description Injury/damage
737 After replacing a nose wheel/tire assembly, the mechanic connected the hose to a tire-servicing cart with nitrogen bottles pressurized to 3,000 psi. A regulator was not used. The valve on the cart was fully opened. The wheel exploded, striking the mechanic who was crouched near the wheel, severing both of his arms above the elbow and severely injuring his leg.
737 Two mechanics were inflating a nose landing gear tire using an unregulated pressure source. The wheel exploded, causing one mechanic to lose both arms and a second mechanic to lose a leg.
737 While servicing a nose wheel/tire assembly, both inboard and outboard wheel halves
fractured and the tie bolt nuts sheared
and separated.
The mechanic was killed when he was struck by the outboard half of the wheel and fragments.
DC-9-32 After replacing the left nose landing gear tire, the wheel exploded as the tire was being inflated using a high-pressure gas bottle. The ground service person was killed.
DC-9 While servicing a nose wheel/tire assembly using a high-pressure dry air bottle, the wheel exploded. A regulator was not being used. The explosion severed the mechanic's arm.

Table 2: Information On Wheel/Tire Assembly Accidents
Title Number Date Source
All-Operator Message:
"Injury as a Result of Wheel Over-Pressurization"
M-7200-98-01293 Apr. 9, 1998 Boeing Commercial Airplanes
All-Operator Letter:
"Nose Landing Gear Tire Failure"
AOL 9-2274 Jul. 21, 1992 McDonnell Douglas
Service Information Letter:
"Safety Precautions for Inflation of Wheel and Tire Assemblies"
SIL 639 May 31, 1998 Allied Signal Aerospace (Bendix)
Maintenance Tip:
"Wheel/Tire Assembly Servicing Precautions"
707 MT-32-001
727 MT-32-001
737 MT-32-005
747 MT-32-043
747-400 MT-32-020
757 MT-32-014
767 MT-32-024
777 MT-32-018
Oct. 20, 1998 Boeing Commercial Airplanes


Figure 2:

Brian Chelius
Principal Engineer
Service Engineering

Boeing Commercial

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