Hydraulic Fluid: A Potential Fire Source
A hydraulic system uses pressurized fluid to drive machinery. A standard system consists of the hydraulic fluid and three mechanical components: the pressure generator, plumbing, and motor. The pressure generator is a hydraulic pump driven by an electric motor, engine, or manual pump. The plumbing refers to the valves, filters, and pipes, and the motor is either a hydraulic motor, hydraulic cylinder, or hydraulic actuator.
Hydraulic systems of some configuration are found on nearly all types of aircraft. For example, in small aircraft, hydraulic components are used to control wheel brakes. In larger aircraft, hydraulic systems can provide the power for systems such as nose wheel steering, landing gear extensions/retraction, flight controls, and more. However, as most hydraulic systems are combustible, a compromised hydraulic system combined with an ignition source can start a fire. Hydraulic fluid fires can be catastrophic and ultimately lead to the loss of an aircraft, and in the event of a post-crash fire, hydraulic fire can add yet another fuel source.
Aircraft have two main defenses against hydraulic fires. The first is to use hydraulic fluids with special fire-resistant properties that have been developed for aviation use. These fluids contain phosphate esters which, unlike hydraulic fluids based in mineral oil, are very difficult to ignite at normal operating temperatures. However, should its temperature exceed 180 degrees Celsius, it can combust. For most aviation hydraulic fluids, the auto-ignition temperature is approximately 475 degrees Celsius. The second defense is cockpit brake temperature indicators, which give pilots warning of a potential wheel well fire.
There are many factors that can contribute to a hydraulic fluid fire. First, a leak from a pressurized system can cause misting of the hydraulic fluid, making the fluid more susceptible to ignition. Second, aircraft brakes can reach temperatures higher than 500 degrees Celsius, high above hydraulic fluid’s auto-ignition temperature. Finally, in the event of a post-crash fire, temperatures will also far exceed the auto-ignition temperature of aviation hydraulic fluid. However, there are steps you can take to prevent hydraulic fluid fire. For one, stringent maintenance practices and inspections will help minimize these risks. Furthermore, as manufacturers continue to develop more fire resistant fluids, the amount of such fires will reduce.
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