5. Inflation Pressure Control

It is recommended that inflation pressure of each aircraft tire be checked daily, and that the pressure gauge be calibrated regularly. Maintaining correct tire inflation pressure is the most important factor in any preventive maintenance program.

A. Pressure Readjustment After Installing Tire on Aircraft

PNZ (loaded service pressure) shall be used instead of the PNO, if the aircraft is on the ground.
PNZ is specified as add 4% to the PNO, to adjust the target pressure considering increase by small shrink of tire gas chamber due to vertical loading.

1.04 X PNO (Unloaded)=PNZ (Loaded)

B. Pressure Control In Service

The pressure check should be done exclusively with "cold tire"; tire temperature is within a rage of ambient temperature.
If the tire is not operated for successive 3 hours or longer, the tire is granted as the "cold tire", unless otherwise exposed to direct sun light for a significant time period.

C. Normal Pressure Loss During Service

Slight pressure loss occurs with aircraft tires due to natural leakage of small amount of gas. Permissible range of the loss is 5% or less within 24 hours.
A small amount of gas diffusion through the "vent holes"; artificial holes situated at the lower sidewall, is a normal mechanism to bleed off trapped air, preventing internal separation or blistering. However, such air leakage should not be detectable by hand. If pressure is found to be less than the minimum pressure, refer to Table 3.

Table 3: Tire Pressure Verification

D. Identification of Axle Mate

When a tire is removed for any of the following reasons, the axle mate tire must be identified and the serial number must be provided to the appropriate retreader (Refer to Table 3).

E. Adverse Effects of Underinflation

An example of Casing Break Up (CBU) in a bias tire at the lower sidewall caused by running the tire at pressures below those recommended.

Underinflation will cause high tire deflection and heat build-up, which in turn may lead to ply separation.
Low inflation pressure may also cause uneven wearing of the tread and rapid wearing of the shoulder. It may also increase the tire footprint, possibly leading to damage of the tire sidewall during landing.

F. Tire Deflection and Durability

Figure 1 shows the relationship between the changes in tire deflection at various inflation pressures and the durability of the tire carcass. With aircraft tires, an increase in deflection during operation greatly weakens the carcass.

G. Adverse Effects of Overinflation on Tires

Excessive inflation pressure may cause uneven tread wear (Wearing of the tread center), less braking efficiency, abnormal tire growth, and may make the tread more susceptible to cutting by foreign objects.

H. Compensation for Climate Change

When a flight is done from a warmer city to a colder city, inflation pressure becomes lower at the destination due to change in ambient temperature. If the temperature fall of 25 deg C or more is anticipated, the pressure shall be altered by using a rule shown below, prior to the take off at the origin airport. The minimum required inflation pressure must be maintained always.

I. Pressure Control for "Hot Tires"

A "Hot Tire" is defined to be a tire heated by aircraft operation (Braking) or by exposure to sunlight and having a surface temperature higher than ambient temperature by at least 30℃. A "Hot Tire" may also result from operating with an abnormally large tire deflection.

If a "Hot Tire" is identified, the following actions should be taken:

Reference: Tire cooling curve

The graph above shows a natural cooling curve (as a typical example) from a temperature of 95℃ for each the bead and crown section of the tire at a room temperature of 25℃. As compared to the crown section of the tire, the bead section cools slowly. These curves are based on typical examples obtained through bench testing. When other factors such as heat build-up from braking, etc., are taken into consideration, the cooling rate will become slower. Accordingly, the tires must be artificially cooled or else left at rest for a longer period of time.

J. Extraordinary Hot Tire

If the tire temperature is significantly higher than what experienced at normal daily operation, tire pressure investigation shall be done.

Make sure to cool down the tire/rim sufficiently before do this check.

Tires are not recommended for further use when the tire surface temperature exceeds 225℉ (107℃), or when the brake heat creates temperatures that exceed 300℉ (149℃) at points where the tire is in contact with the wheel surface.

Category Link:
Tire Care and Maintenance

Instructions specific to radial tires