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Hydraulic Fluid as a Fire Source

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Category: Fire Smoke and Fumes Fire Smoke and Fumes
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Definition

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.

Hydraulic System

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.

Effects

  • 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.

Defences

  • 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 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.

Typical Scenarios

  • 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.

Contributing Factors

  • 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.

Solutions

  • 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.

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Further Reading