What types of hydraulic fluids are used in airplanes?
The hydraulic fluids used in aircraft are relatively thin compared to industrial fluids. They also need to be fire-resistant, which is especially important when the nearest fire station is five miles away, not a few blocks away.
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Hydraulic fluids used in aircraft also differ from industrial applications in that they must remain at -65° and up to 275° F. At these temperatures, water and vegetable-based lubricants will freeze or boil.
The main hydraulic fluids used in military and commercial aircraft will depend on the specific application and environment:
The MIL-H-5606 has been used in many aircraft for more than fifty years. It is still used on some United States Air Force aircraft and business aircraft. However, it is highly flammable and may have resulted in the loss of military aircraft in the past and is therefore no longer used as often as before.
The use of MIL-H-83282 hydraulic fluid in aircraft has been used by the Air Force since 1982 and as a primary fluid in naval aircraft since the late 1990s. Its popularity is largely due to its much more flammable nature than 5606.
MIL-H-87257 is a newer formulation and is used on C135, E3, and U2 aircraft. It is also less flammable than 5606 and can remain liquid to -65°F. It is the most popular hydraulic fluid in new aircraft.
Skydrol™ is a phosphate ester technology manufactured by Eastman's Aviation Solutions which became popular in the industry as the first refractory hydraulic fluid.