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
On 27 October 2019, an under-floor hold fire warning was annunciated in the flight deck of a Boeing 737-900 which had been pushed back at Paris CDG and was about to begin taxiing. Since there were no signs of fire in the passenger cabin or during an emergency services external inspection, a non-emergency disembarkation of all occupants was made. The hold concerned was then opened and fire damage sourced to the overheated lithium battery in a passenger wheelchair was discovered. The Investigation identified a number of weaknesses in both the applicable loading procedures and compliance with the ones in place.
On 31 January 2011, a Singapore Airlines Airbus A380-800 was in the cruise when there was sudden loud noise and signs of associated electrical smoke and potential burning in a toilet compartment with a corresponding ECAM smoke alert. After a fire extinguisher had been discharged into the apparent source, there were no further signs of fire or smoke. Subsequent investigation found signs of burning below the toilet floor and it was concluded that excessive current caused by a short circuit which had resulted from a degraded cable had been the likely cause, with over current protection limiting the damage caused by overheating.
On 29 July 2011 an oxygen-fed fire started in the flight deck of an Egypt Air Boeing 777-200 about to depart from Cairo with most passengers boarded. The fire rapidly took hold despite attempts at extinguishing it but all passengers were safely evacuated via the still-attached air bridge access to doors 1L and 2L. The flight deck and adjacent structure was severely damaged. The Investigation could not conclusively determine the cause of the fire but suspected that wiring damage attributable to inadequately secured cabling may have provided a source of ignition for an oxygen leak from the crew emergency supply
On 7 January 2013, a battery fire on a Japan Air Lines Boeing 787-8 began almost immediately after passengers and crew had left the aircraft after its arrival at Boston on a scheduled passenger flight from Tokyo Narita. The primary structure of the aircraft was undamaged. Investigation found that an internal short circuit within a cell of the APU lithium-ion battery had led to uncontained thermal runaway in the battery leading to the release of smoke and fire. The origin of the malfunction was attributed to system design deficiency and the failure of the type certification process to detect this.
On 25 May 2016, an Embraer ERJ 190 experienced a major electrical system failure soon after reaching its cruise altitude of FL 360. ATC were advised of problems and a descent to enable the APU to be started was made. This action restored most of the lost systems and the crew, not having declared an emergency, elected to complete their planned 400nm flight. The Investigation found that liquid contamination of an underfloor avionics bay had caused the electrical failure which had also involved fire and smoke without crew awareness because the smoke detection and air recirculation systems had been unpowered.
On 1 August 2008, an en-route Embraer 195 despatched with one air conditioning pack inoperative lost all air conditioning and pressurisation when the other pack’s Air Cycle Machine (ACM) failed, releasing smoke and fumes into the aircraft. A MAYDAY diversion was made to the Isle of Man without further event. The Investigation found that the ACM failed due to rotor seizure caused by turbine blade root fatigue, the same failure which had led the other air conditioning system to fail failure four days earlier. It was understood that a modified ACM turbine housing was being developed to address the problem.
On 19 August 1980, a Lockheed L1011 operated by Saudi Arabian Airlines took off from Riyadh, Saudi Arabia - seven minutes later an aural warning indicated a smoke in the aft cargo compartment. Despite the successful landing all 301 persons on board perished due toxic fumes inhalation and uncontrolled fire.
On 26 February 2007, a Boeing 777-222 operated by United Airlines, after pushback from the stand at London Heathrow Airport, experienced internal failure of an electrical component which subsequently led to under-floor fire. The aircraft returned to a stand where was attended by the Airfield Fire Service and the passengers were evacuated.
On 11 May 1996, the crew of a ValuJet DC9-30 were unable to keep control of their aircraft after fire broke out. The origin of the fire was found to have been live chemical oxygen generators loaded contrary to regulations. The Investigation concluded that, whilst the root cause was poor practices at SabreTech (the maintenance contractor which handed over oxygen generators in an unsafe condition), the context for this was oversight failure at successive levels - Valujet over SabreTech and the FAA over Valujet. Failure of the FAA to require fire suppression in Class 'D' cargo holds was also cited.