In the event an aircraft has been subjected to excessive braking, the wheels may be heated to the point where there is danger of a blowout or fire.

NOTE: Excessive brake heating weakens tire and wheel structures, increases tire pressure, and creates the possibility of fire in the magnesium wheels. When the brakes on an aircraft have been used excessively, the fire department should be notified immediately, and all unnecessary personnel should be advised to leave the immediate area.

If blowout screens, such as the one shown in figure 12-41, are available, they should be placed around both main wheels. These screens help to eliminate the possibility of damage or injury in the event of a blowout. Sudden cooling may cause an overheated wheel to fracture or fly apart, which could hurl bolts or fragments through the air with sufficient speed to injure personnel. Required personnel should approach overheated wheels with extreme caution in the fore or aft directions-never in line with the axle.

NOTE: The area on both sides of the tire and wheel, in line with the axle, is where the fragments would be hurled if the tire were to explode; therefore, it is called the danger area. See figure 12-41.

Figure 1241.-Use of blowout screen on overheated brakes.

Heat transfer to the wheel will continue for some period of time until the brake is cooled. The danger of explosive failure may exist after the aircraft is secured if action is not taken to cool the overheated brake.

The recommended procedure for cooling overheated wheel, brake, and tire assemblies is to park the aircraft in an isolated location. Allow the assembly to cool in ambient air for a period of 45 to 60 minutes. The use of cooling agents to accelerate cooling is not recommended unless operational necessity dictates their use. The application of the agents exposes personnel to danger by requiring their presence near the overheated assembly. However, if it is necessary to accelerate coding, use an intermittent stream of water or fog. When using water, direct a stream to the brake. The water should be applied in 10- to 15-second periodic bursts, not in a continuous discharge. Each application should be separated by a waiting period of at least 30 to 60 seconds. A minimum of three to five applications is usually necessary.

When fog is used, the fog is deflected to the brake side of the wheel for a period of 5 to 10 seconds. Each application should be separated by a waiting period of at least 20 seconds. This method is applied as long as it is necessary to control the temperature of the affected assembly.

Once the brake has been properly cooled, permit the wheel to cool in ambient air. A crosswind or forced air from a blower or fan will assist in cooling the wheel. The aircraft should not be moved for at least 15 minutes after cooling operations.

Learning Objective: Recognize the various componen ts of a representative brake system, such as valves, reservoirs, and swivels. Identify the operation of a brake master cylinder.

Components of brake systems are not peculiar to any one system. A given component will vary in shape, size, capacity, and manner of operation (depending upon the manufacturer), but the function remains the same. In this section, we will discuss some of the more common brake system component maintenance practices.