You might know that we have a wide variety of hydraulic fluids created for different applications. We have discussed various applications in the article applications of hydraulics and pneumatics. Here you will find the applications of hydraulics in the aerospace industry and aircraft hydraulic fluid types.
When you consider the aerospace industry, you can find many applications of hydraulics in it. Not only hydraulics but also the pneumatic and electric power transmission methods are using in this industry. Hydraulics are widely used, because of its features like control accuracy, safe & quieter operations, and ease of control.
Aircraft, rockets, and spaceships use applications of hydraulics. Flight controls, spoilers, thrust reversers, landing gears, flaps, and brakes are some applications of hydraulics. The ability of a hydraulic system to transfer high pressure with a small volume of fluid makes hydraulics suitable for these applications.
Hydraulic fluids are considered as the lifeblood for every hydraulic system. Not all the hydraulic system uses the same fluids. Hydraulic fluids are selected depending on certain properties required for those applications. Which means, a wrong selection of hydraulic fluid can damage the system. The important aircraft hydraulic fluid types are discussed below.
The hydraulic fluids used in aircraft will range from basic mineral oils to specialized synthetic oils. The most desired properties for these fluids are low-temperature fluidity, better lubrication, and thermal and oxidation stability.
During the 1950s, most of the commercial aircraft used phosphate ester fluids. But due to the temperature limitations of these fluids experts introduced the silicate ester fluids. Silicate ester fluids had high thermal stability and good fire resistance properties. This fluid contained alkyl silicate esters with ethylmethyl siloxane for improving viscosity. Poor hydrolytic stability was the drawback for this fluid.
Other hydraulic fluid types used in aircraft and their applications are listed below.
Mineral oils: Mineral oils had applications in civil and military aircraft for gun mechanisms, aiming devices, gyrocompass and general lubrication at moderate temperatures. Also, used in hydraulically operated rockets and steering gears. The nato codes for mineral oil based aircraft hydraulic fluids are H-515, H-572, H-573, H-575, H-576, etc..
Chlorophenyl silicone oil: These fluids are used for lubrication of gyrocompass and rocket systems over a wide temperature range. The nato code for chlorophenyl silicone oil is H-536.
Synthetic hydrocarbon: Aircraft, weapons and other hydraulic systems with synthetic seals use this fluid. Also used as a hydraulic fluid for compact vehicles. The nato codes for synthetic hydrocarbon are H-537 and H-544.
Water/glycol: These fluids are used for applications like catapult systems onboard aircraft carriers. The nato code is H-579.
Phosphate ester: Most of the catapult systems and deck elevators on aircraft carriers uses this fluid. The nato code is H-580.
Silicon oils can be considered as a good hydraulic fluid because of the high viscosity indices, low volatility, good hydrolytic stability and high oxidation stability. But, due to the poor fire resistance capacity, other additives need to be added with the silicon oil for obtaining all desired characteristics for an aircraft hydraulic fluid.

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