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.
Another factor that leads to pressure loss is tire growth. After a new tire is installed and inflated to operating pressure, it grows. This growth from initial inflation to when it stabilizes can be from 6 to 10 percent for bias tires and 6 to 7 percent for radial tires. In addition, the tire can grow another 2 to 3 percent after rolling from the centrifugal forces acting on it.
As the volume of the tire grows, its pressure drops. That is why it is important to keep a close eye on pressure values of a new tire after installation. It is recommended to check the tire pressure after 12 hours and reinflate it before placing aircraft in service.
Damage patterns of underinflation
Tires with chronic underinflation will exhibit excessive shoulder wear. This increases the chance of damaging shoulders and sidewalls which shortens tire life because of excessive flex heating.
The best defense against underinflation is regular tire pressure checks. The tires should be checked at least once per day. This pressure check should be on a cold tire. A cold tire is one that is approximately the same temperature as the surrounding air. If pressure is checked on a hot tire, it will give a higher pressure indication than that of a cold one. It should be noted that this indicated high pressure should not lead to a mechanic removing pressure from the tire as this will lead to an underinflated condition.
Pressure should be checked using an approved, calibrated gauge. Always refer to the manufacturer's instructions and avoid overinflating the tire, as severe damage can occur.
It can be beneficial to keep a log of tire pressure checks. Information such as date, time, temperature, pressure, and action taken (re-inflated for example) should be included. This can help find trends such as a leaking tire that may otherwise go unnoticed.
Changes in temperature
Tire pressure changes with temperature. For every 5 F change in temperature, the pressure changes about 1 percent. This can lead to an underinflated condition during extreme temperature drops.
It is good to know how varying temperatures affect inflation and adjust tire pressures appropriately.
So far, the discussion on underinflation in this article has focused on the damage experienced by an underinflated tire. But in tandem configurations, this changes. In a tandem configuration, it is the properly inflated tire that suffers the most damage. That is because it is subjected to a significant higher load as it takes up the work of the underinflated bogey. This increased load significantly increases the heat that is generated. If a tire pressure is less than 80 percent of operating pressure, it is recommended to remove the underinflated tire/wheel assembly from service as well as the axlemate tire/wheel assembly.
Besides the perils of underinflation, overinflation poses a significant danger. Although tires are engineered to withstand several times their rated pressure for a few seconds under test conditions, excessive inflation pressure can cause the tire/wheel assembly to explode with catastrophic force.
Other effects of overinflation are:
- Excessive strain on the cord body of the tire
- Excessive stress on the wheels
- Accelerated center-tread wear
- Reduced tire traction
- Significantly increased tire/wheel assembly susceptibility to cutting by foreign objects, bruises, and impact breaks.
Regular inflation checks are an integral part of an effective tire maintenance program. With all of the hidden damage that can be caused by inproper inflation, ensuring that the tires are not subjected to this damage can help ensure we get the most life out of our tires.
Advisory Circular 20-97A - High-speed tire maintenance and operational practices.