Hydraulic systems have for decades been the go-to technology in airplanes, both commercial and military, because of their high power density and proven reliability. Yet, hydraulic systems can be difficult to control, given that sensors are often located far from the brake piston; can require more maintenance, given the propensity for leaks or contamination in the hydraulic lines and the fluid contained therein; and are substantially heavier when compared to modern electric braking systems.
More than 40 years ago, Solutia Inc.’s Skydrol advanced, fire-resistant, aviation hydraulic fluid replaced mineral oil as the fluid of choice for hydraulic systems in aircraft, describes Mike Shaw, electrical engineer at Goodrich Corp. Goodrich is a designer, developer, and manufacturer of various mil/aero avionics, including aircraft wheels and brakes, landing gear, engine control and electrical power systems, sensors and integrated systems, and ISR (intelligence, surveillance, and reconnaissance).
According to the Wikipedia, “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.” That’s fantastic news, for an airliner. For humans, including my fellow geeks, it’s nasty stuff.
“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 studies indicate that Skydrol causes no permanent damage to human tissue. Castor oil can be applied to the affected area to neutralize the burning,” Wikipedia continues.