Hydraulics

By Chris Grosenick Top: A Boeing 757 No. 1 engine pylon hydraulic pressure and thrust reverser modules. Middle and bottom: Boeing 757 left system hydraulic service center. Hydraulic/pneumatic...


Hydraulic Fluids

There are two general categories of hydraulic fluids: petroleum and synthetic. Petroleum-based fluids were developed first, and are used primarily in military aircraft, light aircraft, and almost universally in landing gear shock struts. Synthetic oils are used mainly in the commercial fleet and military aircraft derived from civilian designs. The main reason for developing synthetic fluids was the increase of system operating temperatures and fire retardancy. The latest revision MIL-H-83282 synthetic/petroleum blended fluid has an operating range of -40 degrees F to 275 degrees F, with a flashpoint of 445 degrees F. Phosphate ester based (synthetic) fluids like BMS 3-11, Skydrol® and Hyjet® have ranges of up to 275 degrees F and a flashpoint of 320 degrees F. Phosphate ester based fluids and petroleum fluids are not interchangeable, and require O-rings made for that particular fluid type. There are synthetic O-rings that work with synthetic oils, and have universal application in hydraulic, engine oil and fuel systems. Check the parts book and maintenance manual for the correct packing application.

"Petroleum-based fluids are used primarily in military aircraft, light aircraft, and almost universally in landing gear shock struts. Synthetic oils are used mainly in the commercial fleet and military aircraft."




Chris Grosenick is Quality Assurance Specialist at NASA Langley Research Center, Hampton, VA. He holds an A&P certificate and private pilot's license.

Photos by Chris Grosenick

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