must be used to ascertain the correct inflation pressure for  a tire on a particular aircraft. Do not inflate to a pressure  displayed on the sidewall of the tire or by how the tire looks.  Tire pressure is checked while under load and is measured  with the weight of the aircraft on the wheels. Loaded versus  unloaded pressure readings can vary as much as 4 percent.  Tire pressure measured with the aircraft on jacks or when  the tire is not installed is lower due to the larger volume of  the inflation gas space inside of the tire. On a tire designed  to be inflated to 160 psi, this can result in a 6.4 psi error. A  calibrated pressure gauge should always be used to measure  inflation pressure. Digital and dial-type pressure gauges are  more consistently accurate and preferred.  [Figure 13-133]  Aircraft tires disperse the energy from landing, rollout, taxi,  and takeoff in the form of heat. As the tire flexes, heat builds  and is transferred to the atmosphere, as well as to the wheel  rim through the tire bead. Heat from braking also heats the  tire externally. A limited amount of heat is able to be handled  by any tire beyond which structural damage occurs. An improperly inflated aircraft tire can sustain internal  damage that is not readily visible and that can lead to tire  failure. Tire failure upon landing is always dangerous.  An aircraft tire is designed to flex and absorb the shock  of landing. Temperature rises as a result. However, an  underinflated tire may flex beyond design limits of the tire.  This causes excessive heat build-up that weakens the carcass  construction. To ensure tire temperature is maintained within  limits, tire pressure must be checked and maintained within  the proper range on a daily basis or before each flight if the  aircraft is only flown periodically.  Tire pressure should be measured at ambient temperature.  Fluctuations of ambient temperature greatly affect tire  pressure and complicate maintenance of pressure within the  allowable range for safe operation. Tire pressure typically  changes 1 percent for every 5 °F of temperature change. When  aircraft are flown from one environment to another, ambient  temperature differences can be vast. Maintenance personnel  must ensure that tire pressure is adjusted accordingly. For  example, an aircraft with the correct tire pressure departing  Phoenix, Arizona where the ambient temperature is 100 °F  arrives in Vail, Colorado where the temperature is 50 °F.  The 50° difference in ambient temperature results in a 10  percent reduction in tire pressure. Therefore, the aircraft  13-80 S 7 0 2 6 x 1 0 . 0 - 1 1 M A D E I$_{N}$  $_{U}$ .$_{S}$ . A . 100 120 140 160 180 200 80 60 40 20 0 psi ZERO ON UNITS P/V Figure 13-133.  A calibrated bourdon tube dial-type pressure  gauge or a digital pressure gauge is recommended for checking  tire pressure.  Figure 13-132.  A sidewall vent marked by a colored dot must be  kept free from obstruction to allow trapped air or nitrogen to escape  from the carcass plies of the tire. 