Figure 13-75.  Inspection for cracks in the wheel disc drive key area  is performed with dye penetrant on many wheels. Figure 13-76.  Visually inspect the core of a thermal or fusible plug  for deformation associated with heat exposure. Replace all of the  plugs if any appear to have begun to deform. Figure 13-77.  Two piece aircraft wheels are statically balanced  when manufactured and may include weights attached to each wheel  half that must stay with the wheel during its entire serviceable life. 13-42 Balance Weights Fusible Plug Inspection Aircraft Brakes Key and Key Screw Inspection Wheel Tie Bolt Inspection Very early aircraft have no brake system to slow and stop the  aircraft while it is on the ground. Instead, they rely on slow  speeds, soft airfield surfaces, and the friction developed by  the tail skid to reduce speed during ground operation. Brake  systems designed for aircraft became common after World  War I as the speed and complexity of aircraft increased,  and the use of smooth, paved runway surfaces proliferated.  All modern aircraft are equipped with brakes. Their proper  functioning is relied upon for safe operation of the aircraft  on the ground. The brakes slow the aircraft and stop it in a  reasonable amount of time. They hold the aircraft stationary  Fusible plugs or thermal plugs must be inspected visually.  These threaded plugs have a core that melts at a lower  temperature than the outer part of the plug. This is to release  air from the tire should the temperature rise to a dangerous  level. A close inspection should reveal whether any core  has experienced deformation that might be due to high  temperature. If detected, all thermal plugs in the wheel should  be replaced with new plugs.  [Figure 13-76] The balance of an aircraft wheel assembly is important. When  manufactured, each wheel set is statically balanced. Weights  are added to accomplish this if needed. They are a permanent  part of the wheel assembly and must be installed to use the  wheel. The balance weights are bolted to the wheel halves  and can be removed when cleaning and inspecting the wheel.  They must be re-fastened in their original position. When a  tire is mounted to a wheel, balancing of the wheel and tire  assembly may require that additional weights be added. These  are usually installed around the circumference of the outside  On most aircraft inner wheel halves, keys are screwed or  bolted to the wheel to drive the brake disc(s). The drive keys  are subject to extreme forces when the brakes are applied.  As mentioned, there should be no movement between the  wheel and the keys. The bolts should be checked for security,  and the area around the keys should be inspected for cracks.  There is also a limitation on how worn the keys can be since  too much wear allows excessive movement. The wheel  manufacturer’s maintenance instructions should be used to  perform a complete inspection of this critical area. of the wheel and should not be taken as substitutes for the  factory wheel set balance weights.  [Figure 13-77] Wheel half tie bolts are under great stress while in service and  require inspection. The tie bolts stretch and change dimension  usually at the threads and under the bolt head. These are  areas where cracks are most common. Magnetic particle  inspection can reveal these cracks. Follow the maintenance  manual procedures for inspecting tie bolts.  Balance weights Outboard wheel half Inboard wheel half