Brake and Undercarriage Fires
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A 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.
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.
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 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.
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.
Accidents and Incidents
- B772, Manchester UK, 2005 (On 1 March 2005, a Boeing 777-200 being operated by Pakistan International Airlines on a scheduled passenger flight from Lahore to Manchester experienced a landing gear fire during taxi in at destination after an apparently routine landing in normal day visibility. There were no flight deck indications of a significant fire but an emergency evacuation was recommended by attending Fire Crew and carried out. Thirty one of the 344 occupants sustained minor injuries during this evacuation and the rest were uninjured. Five firefighters also sustained minor injuries as they assisted passengers from the slides. Damage to the aircraft was minor)
- SW4, Mirabel Montreal Canada, 1998 (On 18 June 1998, the crew of a Swearingen SA226 did not associate directional control difficulty and an extended take off ground run at Montreal with a malfunctioning brake unit. Subsequent evidence of hydraulic problems prompted a decision to return but when evidence of control difficulties and fire in the left engine followed, a single engine diversion to Mirabel was flown where, just before touchdown, the left wing failed upwards. All occupants were killed when the aircraft crashed inverted. The Investigation found that overheated brakes had caused an engine nacelle fire which spread and eventually caused the wing failure)
- DC86, Jeddah Saudi Arabia, 1991 (Nigeria Airways Flight 2120 was a chartered passenger flight from Jeddah, Saudi Arabia, to Sokoto, Nigeria on 11 July 1991 which crashed shortly after takeoff from King Abdulaziz International Airport, killing all 247 passengers and 14 crew members on board. An under-inflated tire which overheated, lead to a fire shortly after takeoff. The aircraft was a Douglas DC-8 operated by Nationair for Nigeria Airways. Flight 2120 is the deadliest accident involving a DC-8 and remains the deadliest aviation disaster involving a Canadian airline)
- Fire in the Air
- In-Flight Fire: Guidance for Flight Crews
- In-Flight Fire: Guidance for Controllers
- Hydraulic Fluid as a Fire Source
- Fusible Plug
- "Tire Failure on Takeoff Sets Stage for Fatal Inflight Fire and Crash," Accident Prevention, Flight Safety Foundation, Sept. 1993.