Skydrol is made up of a group of chemical additives dissolved into a fire-resistant phosphate ester base stock which inhibits corrosion and prevents erosion damage to servo valves and 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 40 years.
Skydrol fluids are irritating to human tissue. Gloves and goggles are recommended safety equipment when servicing Skydrol systems. If the fluid gets on the skin it creates an itchy, red rash with a burning sensation which feels similar to a sunburn. The effects subside within a few hours and castor oil can be applied to the affected area to neutralize the burning. Animal studies have shown that repeated exposure to tributylphosphate, one of the phosphate esters used in Skydrol fluids, may cause urinary bladder damage.
Skydrol fluids are incompatible with many plastics and paints, which can be softened and eventually destroyed by exposure to Skydrol. Some materials (for example, rayon acetate) and rubber-soled shoes may also be damaged by Skydrol.
The Skydrol series of phosphate ester hydraulic fluids were originally developed by the Douglas Aircraft Company in the late 1940s to reduce the obvious fire risk from leaking high pressure mineral oil based hydraulic fluids impinging on potential ignition sources. A secondary use for the original Skydrol 7000, now obsolete, was as a fire resistant lubricant in Douglas designed cabin pressure superchargers used in the DC-6 and 7 series piston-engined aircraft, and first flight tested by United Airlines in 1949.
With the arrival of jet powered aircraft operating at higher altitudes, lower external temperatures, and with a number of other changing operational requirements it became necessary over the years to introduce successive improved phosphate ester fluids, a process encouraged by competition in the market. The story of the introduction of Skydrol type fluids into the civil aviation industry is covered in a book published on Kindle entitled "The Skydrol Story"
In 1949 Douglas first licensed the chemical manufacturer Monsanto to produce a range of Skydrol materials under their patents, and when by the 1990s Monsanto had become primarily a biotechnology company, an independent chemical producer, Solutia, was created in 1997 to handle its chemical interests including Skydrol. In 2012, Solutia was acquired by Eastman Chemical.