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.

* Vent holes
Bridgestone Aircraft Tires have small vent holes in the lower sidewall on both sides to enable any gas or air entrapped in the tire casing structure to be vented at the initial inflation of the tire. The location of each vent hole is indicated by a series of small circular rings molded in relief on the lower sidewall near the lower part appearing just above the wheel flange area. A green dot can be found in the middle of the indentation of the vent hole position. The quantity of vent holes varies by tire size & design.

All vent holes are inspected during new tire manufacturing for form & function. A minor deviation in the appearance of a vent hole does not have any adverse effect on the functionality of the tire. If in doubt please contact your local Bridgestone Aircraft Tire Technical Services team for further information.

Note:
It is normal in some cases for tires to exhibit bubbles at vent holes after inflation if in contact with a soapy liquid such as leak detection fluid. Only the Air Retention test following the recommended inflation procedure shown in section 4A (p41) should be used to judge if a tire is leaking or not.

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. 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 a significant temperature drop is anticipated, the pressure shall be adjusted prior to take off at the original airport, by using a rule shown below or by following the AMM recommendation. The minimum required inflation pressure must be maintained.

Note: All pressure measurements shall be made on cold or stabilised tire temperatures.

H. Pressure Control for "Hot Tires"

A "Hot Tire" is a tire which hasn’t had sufficient time to cool down to a stabilised temperature after arrival of the aircraft at the gate.
Typically a hot tire pressure measurement will occur if tires pressures are controlled before the minimum elapsed cooling time defined in the AMM for servicing of tire.

Pressure measurements made on hot tires will always be higher than stabilised tire temperatures and may lead to incorrect servicing of tire pressure.

If an abnormal tire pressure is suspected on a hot tire, 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.

I. 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