Definition
A phosphate-ester-based fire-resistant hydraulic fluid used in most large transport-category aircraft. Skydrol is purple, has a high flash point, and is chemically aggressive toward many paints, sealants, and skin, so it requires specific seals (typically butyl rubber or ethylene-propylene) and careful handling.
Plain English
A special purple hydraulic fluid that resists catching fire, used in airliners and other large aircraft instead of regular oil-based hydraulic fluid.
Context Anchor
Seen in aircraft maintenance manuals, hydraulic system servicing instructions, and maintenance discussions about leaks, reservoirs, seals, or fluid compatibility.
Derivation
Skydrol is a brand name originally developed by Monsanto (now produced by Eastman) in the 1940s to give aviation a hydraulic fluid that wouldn't fuel a fire after a hydraulic line ruptured. The 'sky' part points to its aviation use; the rest is trade naming.
Why Pilots Care
Using the correct fluid prevents hydraulic fires and maintains reliable operation of flight controls, brakes, and landing gear.
Intuition Check
Do not assume Skydrol means any hydraulic fluid used in an airplane. It means a specific fire-resistant fluid type, and mixing it with the wrong fluid can damage the system.
Example Sentence 1
Before opening the hydraulic reservoir, the technician put on gloves and goggles because Skydrol can irritate skin and eyes.
Example Sentence 2
Only Skydrol hydraulic fluid is approved for this aircraft to meet the fire-resistance requirement.