A critical factor in tire life
By Joe Escobar
There are many factors that can shorten the life of an aircraft tire. One of these life shortening factors is operating at improper inflation pressure. Although often overlooked, one of the easiest ways to help ensure longer life of your tires is to ensure they are maintained at recommended pressures.
When inspecting or servicing tires, safety precautions recommended by the tire manufacturer, aircraft manufacturer, and company procedures must be followed. Failure to do so can result in damage to the aircraft and/or injury to personnel. The following general safety precautions should be noted.
Tires should only be serviced with nitrogen. If a tire overheats, hydrocarbon gases are generated within the tire from overheated rubber or hydrocarbon contaminants if present. These gases can mix with high pressure air in the tire and ignite. Since nitrogen is inert and will not support combustion, the possibility of this explosive mixture forming is decreased.
The FAA addressed the danger of oxygen in tires in 1987. It issued AD 87-08-09 that addressed tires mounted on braked wheels of Airbus Industrie, Boeing, British Aerospace, Lockheed, and McDonnel Douglas Aircraft. It said, "To eliminate the possibility of a chemical reaction between atmospheric oxygen and volatile gases from the tire inner liner producing a tire explosion, accomplish the following
. . . On braked wheels, install only tires that have been inflated with dry nitrogen or other gases shown to be inert such that the gas mixture does not exceed 5 percent oxygen by volume."
Advisory Circular 20-97A states that evidence exists that tires inflated with air have a shortened service life because the air diffusing into the carcass degrades its inner-ply adhesion. Nitrogen also helps deter corrosion on the wheel assembly.
Nitrogen bottles should never be used without a regulator. Nitrogen bottles have pressures as high as 3,000 psi. Servicing with this high an unregulated pressure can easily exceed the design limits of the tire and wheel assembly.
To prevent tire burst during servicing, it is good practice to regulate pressure in the supply line so that it doesn't exceed a pressure of 50 percent higher than the rated pressure of the tire.
Heat is a big contributor of shortened tire life. Excessive heat buildup will damage the rubber compounds and fabrics in the tires. It can contribute to tread and carcass separations, bead failures, and destruction of the tire/wheel assembly. Exposure of the tire to temperatures as low as 220 F can shorten the service life of the tire.
Heat buildup occurs when the tire is rolling - taxi, takeoff, and landings. It is a result of the plies in the tire flexing. It is normal for some heat to be generated, however, it is possible to develop an excessive and dangerous amount of heat in the tire. The rate of excessive heat buildup is proportional to tire deflection, speed, and distance. Deflection can be caused by aircraft loading and/or underinflation. So we can see that an underinflated tire will be most susceptible to failure when the aircraft is taking off (high speed, long distance, and maximum loading).
While mechanics cannot control the factors of speed or distance, or even excessive deflection due to overloading, we do have a direct role in ensuring tires are not over deflected due to underinflation.
Factors leading to pressure loss
During normal operation an aircraft tire will lose pressure. As long as this pressure loss is not excessive, it is normal and due mainly to diffusion. If you have ever performed a leak test using a soap solution to find a leak, you probably noticed the effect of this diffusion at the tire's vent holes. The vent holes are small holes placed in the lower sidewall rubber. They are evenly spaced, and are located on both sides of the tire to a controlled depth. The purpose of these vent holes is to prevent internal buildup of pressure in the casing. It allows gas that was trapped in the casing during fabrication to escape as well as allow gas due to innerliner diffusion to escape. Pressure loss from diffusion can be as much as 0.2 percent in a 24-hour period.