Hydraulic Fluid as a Fire Source
Hydraulic Fluid as a Fire Source
A hydraulic system uses pressurised fluid to drive machinery. Most hydraulic fluids are combustible and a compromised hydraulic system, in combination with an ignition source, can lead to a fire.
A hydraulic drive system consists of the hydraulic fluid plus three major mechanical components. These components are the “pressure generator” or hydraulic pump which can be driven by an electric motor, an engine or a manual pump, the system “plumbing” consisting of valves, filters and pipes and the “motor” which could be a hydraulic motor, hydraulic cylinder or hydraulic actuator.
Virtually all aircraft have hydraulic systems of some description. In small, general aviation aircraft, the hydraulic components may be limited to wheel brakes. In larger aircraft, hydraulic systems can also provide the motive power for many other systems including nose wheel steering, landing gear retraction/extension, flap and slat retraction/extension, flight control actuation, emergency electrical generation and other ancillaries such as airstairs and cargo doors.
- Hydraulic fluid fires have the potential to lead to the loss of the aircraft.
- In the event of a post crash fire, the hydraulic fluids are an additional fuel source.
- Special hydraulic fluids with fire resistant properties have been developed for aviation use. These fluids are phosphate esters and, unlike mineral oil based hydraulic fluids, are very difficult to ignite at room temperature. However, if the fluid is heated to temperatures in excess of 180 degrees C, it will sustain combustion. The auto-ignition temperature of most aviation hydraulic fluids is in the range of 475 degrees C.
- Fitment of cockpit brake temperature indicators will give the pilots warning of a wheel well fire.
EASA Certification Specifications
Present EASA certification specifications address the issue of hydraulic fluid fires, notably:
- CS 25.735 (brakes and braking system) (b) (2) (Fluid loss);
- CS 25.1435 (Hydraulic systems) (b) (3) (minimize harmful concentrations of hydraulic fluids or vapors in crew and passenger compartments) and (4) (meet power-plant fire protection specifications if flammable fluid is used)
- CS 25.1707 (System separation Electrical Wiring Interconnection Systems (EWIS)) (f) (separation from hydraulic systems)
AMC 25-735 and 1189 provide further information.
- After a high speed rejected takeoff, brake temperatures on a large, heavy aircraft exceed 700 degrees C. Hydraulic fluid from a crack in a brake line leaks onto the hot brake and ignites. The fire is extinguished by the RFFS that had responded to the rejected takeoff.
- An undetected “dragging” brake during the takeoff roll results in very hot brake temperatures once the landing gear has been retracted. Hydraulic fluid from a leak in the landing gear actuator assembly ignites. The ensuing wheel well fire results in the structural failure of the wing before the aircraft is able to land.
- A leak from a pressurized system can lead to misting of the hydraulic fluid. This makes the fluid more susceptible to fire should it come in contact with an ignition source.
- Aircraft brakes can easily reach temperatures in excess of 500 degrees C.
- Temperatures in a post crash fuel-fed fire will exceed the auto-ignition temperature of aviation hydraulic fluid.
- Robust maintenance practices and flight crew vigilance during pre-flight inspection will help minimize the risk of fire due to hydraulic fluid leakage.
- Manufacturers continue to develop more fire resistant fluids and more robust hydraulic system components.
- Aircraft designers reduce the presence of hydraulically powered aircraft systems.
Accidents and Incidents
On 4 October 2014, the fracture of a hydraulic hose during an A330-200 pushback at night at Karachi was followed by dense fumes in the form of hydraulic fluid mist filling the aircraft cabin and flight deck. After some delay, during which a delay in isolating the APU air bleed exacerbated the ingress of fumes, the aircraft was towed back onto stand and an emergency evacuation completed. During the return to stand, a PBE unit malfunctioned and caught fire when one of the cabin crew attempted to use it which prevented use of the exit adjacent to it for evacuation.
On July 30 2008, a Boeing 777-200 being operated by Vietnam Airlines on a scheduled passenger flight landed at Narita in daylight and normal visibility and shortly afterwards experienced a right engine fire warning with the appropriate crew response following. Subsequently, after the aircraft had arrived at the parking stand and all passengers and crewmembers had left the aircraft, the right engine caught fire again and this fire was extinguished by the Airport RFFS who were already in attendance. There were no injuries and the aircraft sustained only minor damage.
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.
- Fire in the Air
- In-Flight Fire: Guidance for Flight Crews
- In-Flight Fire: Guidance for Controllers
- Hot Brakes
- Wing Fire
- FAA Advisory Circular 120-80A “In Flight Fires”
- Boeing article: Reducing Smoke and Burning Odor Events
- Society of Automotive Engineers, SAE ARP4752A (R) Aerospace - "Design and Installation of Commercial Transport Aircraft Hydraulic Systems"
- An analysis of fumes and smoke events in Australian aviation ATSB (Australia), 2014
- Study of Reported Occurrences in Conjunction with Cabin Air Quality in Transport Aircraft BFU (Germany), 2014