Inboard Wheel Half Outboard Wheel Half 13-36 Figure 13-60.  Keys on the inner wheel half of an aircraft wheel  used to engage and rotate the rotors of a disc brake.  Figure 13-61.  Heavy use of the aircraft brakes can cause tire air  temperature and pressure to rise to a level resulting in explosion  of the wheel assembly. To alleviate this, thermal plug(s) mounted  in the inner wheel half of a high performance aircraft wheels are  made with a fusible core that melts and releases the air from the  tire before explosion.  to engage the brake rotor to assist in protecting the wheel and  tire assembly from overheating.  An overinflation safety plug may also be installed in the  inner wheel half. This is designed to rupture and release all  of the air in the tire should it be over inflated. The fill valve  is also often installed in the inner wheel half with the stem  extending through holes in the outer wheel half to permit  access for inflation and deflation.  The outboard wheel half bolts to the inboard wheel half to  make up the wheel assembly upon which the tire is mounted.  The center boss is constructed to receive a bearing cup  and bearing assembly as it does on the inboard wheel half.  The outer bearing and end of the axle is capped to prevent  contaminants from entering this area. Aircraft with anti-skid  brake systems typically mount the wheel-spin transducer  here. It is sealed and may also serve as a hub cap. The 737  outer wheel half illustrated in  Figure 13-59  also has a hub  cap fairing over the entire wheel half. This is to fair it with  the wind since the outer wheel half does not close behind a  gear door on this aircraft. Hub caps may also be found on  fixed gear aircraft.  The outboard wheel half provides a convenient location  of the valve stem used to inflate and deflate tubeless tires.  The inner wheel half of a wheel used on a high-performance  aircraft is likely to have one or more thermal plugs.  [Figure 13-61]  During heavy braking, temperatures can  become so great that tire temperature and pressure rise to a  level resulting in explosion of the wheel and tire assembly.  The thermal plug core is filled with a low melting point  alloy. Before tire and wheel temperatures reach the point of  explosion, the core melts and deflates the tire. The tire must  be removed from service, and the wheel must be inspected in  accordance with the wheel manufacturer’s instructions before  return to service if a thermal plug melts. Adjacent wheel  assemblies should also be inspected for signs of damage. A  heat shield is commonly installed under the inserts designed  Both wheel halves contain a bearing cavity formed into the  center that accepts the polished steel bearing cup, tapered  roller bearing, and grease retainer of a typical wheel  bearing set-up. A groove may also be machined to accept a  retaining clip to hold the bearing assembly in place when the  wheel assembly is removed. The wheel bearings are a very  important part of the wheel assembly and are discussed in a  later section of this chapter.  Wheel halves are not identical. The primary reason for this is  that the inboard wheel half must have a means for accepting  and driving the rotor(s) of the aircraft brakes that are mounted  on both main wheels. Tangs on the rotor are fitted into steel  reinforced keyways on many wheels. Other wheels have  steel keys bolted to the inner wheel halves. These are made  to fit slots in the perimeter of the brake rotor. Some small  aircraft wheels have provisions for bolting the brake rotor  to the inner wheel half. Regardless, the inner wheel half  is distinguishable from the outer wheel half by its brake  mounting feature.  [Figure 13-60] Thermal plug Thermal plug Heat shield Inside wheel half Outside wheel half