contact area of the tire, causing it to wear faster at the tread center. Failure due to carcass ruptures and breaks in the tire cords that result from contact with foreign objects are usually caused by overinflation. Underinflation increases the contact area and causes the tire to wear rapidly and unevenly at the outer edges of the tread. An underinflated tire flexes excessively and develops high temperatures that weaken tire cords. An underinflated tire also may slip on the wheel during landing and shear off the valve stem. To determine the proper inflation pressure, you should check an inflation chart like the one shown in figure 5-6. If the gross weight of an aircraft is 20,000 pounds, the correct tire pressure for that aircraft when shore based is 310 psi. If the aircraft is carrier based, the pressure is maintained at 350 psi, regardless of the gross weight. When aircraft tire pressure is low, you should add air from a regulated source. CAUTION An unregulated, high-pressure air source for tire inflation is a hazard. Tire inflation source pressure should be carefully moni- tored. If high-pressure cylinders (such as the portable air bottle or the air or nitrogen serv- icing trailer) are used, a regulator must be used to prevent inadvertent overinflation. Maintenance personnel must always use a remote inflator unit when inflating tires. The operator of this unit should always stand at right angles to the landing gear axle, directly in front or in the rear of the tire. The operator should also stand at the full length of the inflator unit hose. This will prevent the operator from being struck by pieces of the wheel if it were to fail CAUTION When an aircraft wheel is to be removed from the aircraft, maintenance personnel must deflate the tire before removing the wheel assembly from the aircraft. This precaution is necessary because of the possibility that the bolts in split-type wheels might have been sheared during landing, causing the wheel halves to separate when the axle nut is re- moved. Personnel have been killed because they failed to remove the air from the tire before removing the axle nut. Oxygen Servicing Plane captains are responsible for making sure that the liquid oxygen systems of the aircraft are serviced. Personnel in the Aviation Structural Mechanic, Safety Equipment, (AME) rating refill these converters. A typical liquid oxygen converter is shown in figure 5-7. Liquid oxygen is dangerous to handle and requires special handling procedures. These special handling procedures are discussed later in this chapter. SERVICING EQUIPMENT The following text discusses the equipment used to service aircraft, such as the high pressure air valve, servicing trailers, preoiler hand pumps, and fluid service unit HSU-1. High-Pressure Air Valve The high-pressure air valve is used to service struts, accumulators, air storage bottles, and other components serviced with high-pressure air. The high-pressure air valve, shown in figure 5-8, is used on most naval aircraft. It is referred to by its military specification (MS) number MS 28889-1. Air valve MS 28889-1 does not have a valve core. When servicing a system equipped with a high-pressure air valve, you should exercise extreme Figure 5-7.—Typical liquid oxygen converter. 5-12