Brake and Undercarriage Fires

Description

braking system works by converting the kinetic energy of a moving aircraft into heat. This heat is generated by friction between the rotating and the stationary components of the brake assemblies and between the rotating tyres and the runway or taxiway. If the amount of heat generated becomes excessive, or if flammable contaminates such as hydraulic fluid or grease are introduced, a brake or undercarriage fire may occur.

Aircraft Design

Brake systems must be designed with sufficient capacity to absorb the amount of kinetic energy that the aircraft type can generate in the worst case situations. The design must consider and test the following scenarios throughout the defined wear range of the brakes:

  • Design landing stop. The design landing stop is an operational landing stop at maximum landing weight. The design landing stop brake kinetic energy absorption requirement of each wheel, brake, and tire assembly must be determined and substantiated by dynamometer testing
  • Maximum kinetic energy accelerate-stop. The maximum kinetic energy accelerate-stop is a rejected takeoff for the most critical combination of airplane takeoff weight and speed. The accelerate-stop brake kinetic energy absorption requirement of each wheel, brake, and tire assembly must be determined and substantiated by dynamometer testing
  • Most severe landing stop. The most severe landing stop is a stop at the most critical combination of airplane landing weight and speed. The most severe landing stop brake kinetic energy absorption requirement of each wheel, brake, and tire assembly must be determined and substantiated by dynamometer testing. The most severe landing stop need not be considered for extremely improbable failure conditions or if the maximum kinetic energy accelerate-stop energy is more severe

Following these high kinetic energy stop test scenarios, with the parking brake promptly and fully applied for at least 3 minutes, it must be demonstrated that for at least 5 minutes from application of the parking brake, no condition occurs, including fire associated with the tire or wheel and brake assembly, that could prejudice the safe and complete evacuation of the airplane. Means must also be provided in each braked wheel to prevent a wheel failure, a tire burst, or both, that may result from elevated brake temperatures. This requirement is normally met by installing a fusible plug in the wheel.

Causes

As per the Fire Triangle, a fire requires three items; a heat source, fuel and oxygen. Brake and undercarriage fires are no exception. Oxygen is always available when the undercarriage is extended and will also be available in the more confined area of undercarriage bay when retracted.

The most common source of heat is from the brakes and events such as a rejected takeoff, sequential stop and go landing events during circuit training, or overuse of, or a dragging brake during taxi can all result in extreme brake temperatures. Underinflated tyres can cause additional load and strain on the other tyre on the axle and can also lead to elevated temperatures in the tyre itself and in the undercarriage bay when the wheels are retracted.

The most common fuel source on the undercarriage is the grease used to lubricate the wheel bearings, brake assemblies and retraction mechanisms. If undercarriage components are over-lubricated or if old grease is not removed during wheel changes, an excessive amount of grease can accumulate and result in a fire if heated to its flashpoint. Aircraft tyres, leaking hydraulic fluid, and other flammables such as residual cleaning solvent from maintenance procedures or fuel leaking from a compromised tank or pipe are also potential fuel sources.

Prevention

Prevention of undercarriage fire is rooted in adherence to appropriate procedures and aircraft limitations. Pilots should avoid "riding" the brakes during taxi, make appropriate use of brake temperature indicators and brake fans when installed and adhere to the limitations of the brake energy charts and cooling time requirements when required. Appropriate cooling techniques following a rejected takeoff or leaving the gear extended between multiple stop and go circuit events will help to ensure that critical brake temperatures are not exceeded. If high brake or tyre temperatures are indicated or suspected, the undercarriage should not be retracted after takeoff until an adequate period of time to allow cooling has lapsed.

Maintenance procedure and protocols are equally important. Tyre pressure should be checked regularly and corrected as necessary. Consumable brake parts, such as pads and rotors, should be replaced when their wear limits are reached. Old grease should be removed from the axles when wheels are changed and new assemblies should be lubricated in accordance with manufacturer guidelines. Fluid leaks on the undercarriage assembly or in the bay should be repaired without delay.

Fire Fighting

Whilst the usual strategies for cooling "hot" brakes include use of a remote location, parking into wind, chocking the nose wheel, releasing the parking brake, and use of brake fans when available, should a fire develop, more direct intervention is required. Declaration of an emergency, shut down of the aircraft and evacuation of passengers and crew will normally take place. Responders must exercise caution when approaching burning or overheated wheel assemblies as, so long as the wheels remain inflated, there is a risk of explosive failure of the wheel assembly both laterally and in a fore and aft direction. Any approach to the wheel should therefore be conducted obliquely on a 45 degree angle to the tyre sidewall. Most manufacturers recommend water misting to cool an overheated wheel assembly but, in the event of a fire, more aggressive intervention such as water cannon, foam or halon might be used.

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