Definition
A class of synthetic hydraulic fluid built around phosphate ester chemistry, used in most large transport-category aircraft because of its high fire resistance and stability across a wide temperature range. Commonly identified by trade names such as Skydrol and HyJet, it is purple in color, attacks paint and many common seal materials, and requires specific seals (typically butyl rubber or ethylene-propylene) rather than the nitrile seals used with petroleum-based fluids.
Plain English
A type of hydraulic fluid used in airliners that resists catching fire. It is purple, eats paint, and needs special seals — it cannot be mixed with the red mineral-based hydraulic fluid used in light aircraft.
Context Anchor
Seen in aircraft maintenance manuals, hydraulic system servicing instructions, and fluid compatibility warnings.
Derivation
Phosphate ester refers to the chemical family the fluid is built from — esters formed from phosphoric acid. The phosphorus content is what gives the fluid its fire-resistant behavior, which is why this chemistry was chosen over petroleum oils for large aircraft.
Why Pilots Care
Mixing phosphate ester fluid with petroleum-based hydraulic fluid destroys the seals in the system and can cause hydraulic failure. Maintenance personnel must use the correct fluid type, correct seals, and protect aircraft paint and skin from spills.
Intuition Check
Do not read “base” as meaning the opposite of acid here. In this term, “base” means the main chemical makeup of the fluid.
Example Sentence 1
The Boeing 737 hydraulic system uses a phosphate ester-base fluid, so technicians must wear gloves and avoid spilling it on painted surfaces.
Example Sentence 2
Phosphate ester-base fluids require special seals and careful cleanup because they can damage certain paints and plastics.