Skydrol is a brand name of fire-resistant hydraulic fluid[1] used in aviation and aerospace applications. It is a phosphate ester-based fluid that is known for its excellent fire resistance and ability to withstand extreme temperature and pressure conditions. It is manufactured by Solutia (now part of Eastman Chemical Company), and formerly manufactured by Monsanto. There are various lines of Skydrol including Skydrol 500B-4, Skydrol LD-4, and Skydrol 5.
Skydrol is made of a fire-resistant phosphate ester base stock, with a number of oil additives dissolved into it to inhibit corrosion and prevent erosion damage to servo valves. It also includes a purple or green dye to ease identification. It has been approved by most airframe manufacturers including Airbus, Boeing and BAE Systems and has been used in their products for over 50 years.