SkydrolTM is a phosphate ester-based hydraulic fluid manufactured in the United States by Eastman Chemical Company based in Kingsport, Tennessee. It contains several oil additives dissolved into a phosphate ester base to make it highly corrosion resistant and fire resistant, then dyed purple mainly to help become identifiable. These excellent properties have made it among the favourite recommendation for aircraft manufacturers such as Boeing and Airbus.
A phosphate ester, also called an OPE, is an organophosphorus compound formed when the hydrogen in phosphoric acid is replaced with either alkyl or aryl group. Chemically, phosphate esters can be called organic salts of orthophosphoric acid O=P(OH)3. However, only one relatively small group has found significant use as base-stocks for synthetic fluids. These are the trisubstituted, or tertiary (t), phosphate esters. Triaryl phosphate esters are composed of three aromatic (aryl) groups bonded to a phosphate group. These fluids offer superior thermal stability and chemical resistance, making them ideal for demanding applications like for use as aerospace hydraulic fluid.
Skydrol Hydraulic Fluid is available in four principle brands today:
Skydrol PE-5
Eastman describes it as the fluid with the longest fluid life of any commercially available fluid, with low density and low viscosity at low temperature. It works well in high-moisture conditions and requires less frequent fluid replacement, reducing disposal volume and decreasing your environmental footprint.
Skydrol 5
It was introduced in 1996. Eastman describes it as a lighter-weight hydraulic fluid that allows users to save fuel. It’s the first aviation hydraulic fluid to show high-temperature erosion resistance, decreasing downtime. Its high-temperature erosion resistance reduces wear on hydraulic components, extends maintenance intervals, and minimizes aircraft downtime – making it a popular choice for operators looking to balance performance with cost efficiency.
Skydrol LD-4
Eastman describes it as the world’s best-selling Type IV aviation hydraulic fluid. It is a low-density product that prevents valve erosion and controls deposits. It features an anti-erosion additive and acid scavenger to maximize its protective properties. Type IV is an older standard hydraulic fluid, as compared to the more modern Type V used today, but it does not compromise on qualifications. In fact in some cases, the two variants can be intermixed within the same system.
Skydrol 500B-4
This variant has been in service for longer than all the others, but still carries a proven track record in high-performance hydraulic systems that has kept it a staple of aviation fluid programs for decades. According to Eastman Company, it is the only commercially available, standard-density aviation hydraulic fluid.
Skydrol today faces competition from Mobil HyjetTM V, which is also a Type V fire-resistant phosphate ester hydraulic fluid produced by ExxonMobil.
Besides phosphate ester-based hydraulic fluids (used in commercial airliners), other types of hydraulic fluids exist, primarily for use in general aviation aircraft – mineral oil-based hydraulic fluid, such as MIL-H-5606, and polyalphaolefin-based
fluid, such as MIL-H-83282 and its rust-inhibited version MIL-H-46170.
An aircraft maintenance technician must take care never to contaminate a hydraulic system approved for one type of hydraulic fluid with another, such as using MIL-H-46170 in place of Skydrol 500B-4. Appropriate seals, gaskets, and hoses are specifically designated for the type of fluid in use. Phosphate ester-based type V fluid is compatible with natural fibers and with a number of synthetics, including nylon and polyester, which are used extensively in most aircraft. Petroleum oil hydraulic system seals of neoprene or Buna-N are not compatible with phosphate ester-based fluids and must be replaced with seals of butyl rubber or ethylene-propylene elastomers. Contamination, both particulate and chemical, is detrimental to the performance and life of components in the aircraft hydraulic system, therefore must be avoided at all costs.
