Definition
A synthetic, fire-resistant hydraulic fluid based on phosphate-ester chemistry, used in the hydraulic systems of most large transport-category aircraft. It is identified by Skydrol and Hyjet trade names, is typically purple in color, and is chemically incompatible with petroleum-based hydraulic fluids and with many common paints, sealants, and rubber compounds.
Plain English
A man-made hydraulic fluid that resists catching fire, used in airliners and other large aircraft. It cannot be mixed with regular oil-based hydraulic fluid, and it eats through certain paints and rubber, so it must be handled carefully.
Context Anchor
You may see this term in aircraft maintenance manuals, hydraulic system servicing instructions, fluid container labels, and placards near hydraulic reservoirs.
Derivation
Phosphate-ester names the chemical family the fluid is built from: an ester (a compound formed from an acid and an alcohol) where the acid is phosphoric acid. This chemistry is what gives the fluid its high resistance to ignition, which is why it replaced flammable petroleum fluids in large aircraft.
Why Pilots Care
Using the correct fluid prevents fires from hydraulic leaks near engines or brakes and avoids corrosion or seal damage that could lead to loss of control systems.
Intuition Check
Do not assume all hydraulic fluids are interchangeable. “Phosphate-ester” identifies a specific chemical type, not just a brand or color.
Example Sentence 1
The mechanic checked the reservoir label to confirm the aircraft used phosphate-ester hydraulic fluid before servicing the system.
Example Sentence 2
A small leak of phosphate-ester hydraulic fluid was noted during the preflight walk-around and cleaned per the aircraft manual.