WARNING Do not stand directly in front of the intake duct. HOT BRAKES During a normal or an emergency landing, the landing gear is an item of considerable concern. With the added weight and landing speeds of modern aircraft, and because of the extreme braking required on shorter runways, overheated brakes and wheels are a common occurrence. You, as a fire fighter, must have a thorough understanding of the hazards created by overheated brakes, as well as the techniques and equipment used with this type of emergency. Overheated aircraft wheels and tires present a potential explosion hazard because of built-up air pressure in the tires, which is greatly increased when fire is present. To avoid endangering the crews needlessly, all nonessential personnel should evacuate the area. The recommended procedure for cooling overheated wheel, brake, and tire assemblies is to park the aircraft in an isolated area and allow the assemblies to cool in the surrounding air. Using cooling agents, such as water, is not recommended unless absolutely necessary due to increased hazards to personnel near the overheated assembly. Most aircraft operating manuals for propeller-driven aircraft recommend that flight crews keep the propeller turning fast enough to provide an ample cooling airflow. Most major jet, propeller-driven, and turboprop aircraft now have fusible plugs incorporated in the wheel rims. These fusible plugs are designed to automatically deflate the tires. (Failure of fusible plugs to function properly has occurred.) Releasing the tire pressure reduces the pressure on the wheel, and thus eliminates the possibility of explosion. CAUTION The use of CO₂  for rapid cooling of a hot brake or wheel assembly is extremely dangerous. Explosive fracture may result because of the rapid change in temperature. When responding to a wheel fire or hot brakes as a member of the emergency crew, you should approach the wheel with extreme caution in a fore or aft direction, never from the side in line with the axle. Peak temperatures may not be reached until 15 to 20 minutes after the aircraft has come to a complete stop. See figure 12-15. WHEEL ASSEMBLY FIRES The following types of fires and hazards may occur around an aircraft wheel assembly: 1. The heating of aircraft wheels and tires presents a potential explosion hazard, which is greatly increased when fire is present.  The combination of increased  stress  on  the  brake  wheel  assembly, additional tire pressure, and the deterioration of components by heat may cause an explosion. This explosion is likely to propel pieces of the tire and/or metal through the air at high speeds. 2. Materials that may contribute to wheel assembly fires are grease, hydraulic fluid, bearing lubricants, and tire rubber. a. Grease and bearing lubricant fires. When ignited, wheel grease fires can be identified by long flames around the wheel brake/axle assembly. These fires are usually small and should be extinguished quickly with Halon 1211 or water fog. b. Rubber tires. Rubber from the tires may ignite at temperatures from 500°F (260°C) to 600°F (315°C) and can develop into an extremely hot and destructive fire. Halon 1211 or water fog should be used as early as possible to extinguish the fire. Reignition may occur if the rubber sustains its autoignition temperature or if the rubber is abraded and the fire is deep-seated. c. A broken hydraulic line may result in the misting of petroleum-based fluids onto a damaged or 12-20 Figure 12-15.—Danger zones and attack zones in combating wheel fires. (Attack the fire from fore and aft—do not attack from the side).