Pressure: A critical factor in tire life

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...


Tire growth
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.

Pressure checks
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.

Tandem configuration
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.

Overinflation
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.

Additional ReSource
Advisory Circular 20-97A - High-speed tire maintenance and operational practices.

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