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Skydrol is an advanced fire-resistant aviation hydraulic fluid[1] manufactured by Solutia Inc. There are various lines of Skydrol including Skydrol 500B-4, Skydrol LD-4, and Skydrol 5.

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 contains a small amount of a PFOS, which is not listed on the MSDS.[2]

Solutia Inc. built a new facility to produce Skydrol and SkyKleen aviation cleaning solutions in Anniston, Alabama in 2005.

Acid and contamination must be monitored while using Skydrol, and generally hydraulic systems should be sampled every C check.[3] Skydrol has a 5-year shelf life from the date of manufacture.

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 persistent burning sensation. The effects subside within a few hours; egg white can be applied to the affected area to neutralize the burning.[4] Animal studies have shown that repeated exposure to tributylphosphate, one of the phosphate esters used in Skydrol fluids, may cause urinary bladder damage.[5]

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.[6]

The Skydrol series of phosphate ester hydraulic fluids were originally developed by the Douglas Aircraft Company in the late 1940s to reduce the 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 introduction of jet aircraft operating at higher altitudes, and lower external temperatures there was a need for improved phosphate ester fluids. The story of the introduction of Skydrol type fluids in civil aviation is covered in a Kindle book entitled "The Skydrol Story"

In 1949 Douglas first licensed Monsanto to produce a range of Skydrol materials under their patents. In the 1990s Monsanto became primarily a biotechnology company, and an independent chemical producer, Solutia, was created in 1997 to handle its chemical interests, including Skydrol. In 2012, Solutia was acquired by Eastman Chemical.

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