Hydraulic fluid used in aircraft systems have different requirements.

Hydraulic fluid used in aircraft systems is a topic often overlooked:Virtually all aircraft have hydraulic systems of some description. In small, general aviation aircraft, the hydraulic components may be limited to wheel brakes. In larger aircraft, hydraulic systems can also provide the motive power for many other systems including nose wheel steering, landing gear retraction/extension, flap and slat retraction/extension, flight control actuation, emergency electrical generation and other ancillaries such as airstairs and cargo doors. This means that hydraulic fluid used in aircraft systems is vital to the safe
cost effective operation of these aircraft.

Different types of hydraulic fluid used in aircraft systems.

The following are the types of hydraulic fluid used in aircraft systems today, some of them remain in use in aging aircraft and so may become obsolete as aircraft types are withdraw from service.

Because of the differences in composition, hydraulic fluid used in aircraft systems should not be mixed, even those of the same base type. During the design stage, aircraft manufacturers select a fluid appropriate to the system or aircraft and specify this in maintenance manuals, this is then the only permissible fluid to be used for that aircraft.

Should cross contamination with other fluids occur, such as may be caused by filling with the incorrect fluid type, then the whole system must be drained and flushed out prior to system operation. The manufacturer will usually give advice in the aircraft maintenance manuals regarding what action is required with regard to rubber seals etc. should the system become contaminated in this way.

DTD 900/4081	A natural Castor based oil, golden yellow to brown in colour, it must be used with natural rubber seals.  It is flammable, strips paint and attacks synthetic rubber.  It is toxic in a fine spray mist.
Aeroshell 41/Mil-H-5606/ Def Stan 91-48	A mineral based hydraulic fluid used in aircraft systems, red in colour, must be used with synthetic rubber seals.  It is flammable and attacks natural rubber.  Its density and lubricating properties vary with temperature.
Skydrol 500B/Mil-H-8446	A synthetic, phosphate-ester based oil, purple in colour, its major advantage is that it is fire resistant.  It will strip paint and attacks both natural and synthetic rubber.  Slightly heavier than water, it does however, have a very broad range of operating temperatures.  Seals for use with synthetic oils are made from Butyl, Ethylene, Propylene, and Teflon.  It will absorb moisture from the atmosphere if exposed to it.
Skydrol 500B-4	An improved hydraulic fluid used in aircraft systems, which has the same properties but provides more resistance to wear and erosion of orifices and valve lands.
Skydrol LD-4	A special low-density version, it provides a weight saving of approximately 5% on a volume basis, compared to other Skydrol types.

A hydraulic system uses pressurised hydraulic fluid to drive machinery. Most hydraulic fluids are combustible and a compromised hydraulic system in combination with an ignition source can lead to a fire.

• Hydraulic fluid fires have the potential to lead to the loss of the aircraft.
• In the event of a post crash fire, the hydraulic fluid used in aircraft systems can be an additional fuel source.

• Special hydraulic fluids with fire resistant properties have been developed for aviation use. These fluids are phosphate esters and, unlike mineral oil based hydraulic fluids, are very difficult to ignite at room temperature. However, if the fluid is heated to temperatures in excess of 180 degrees C, it will sustain combustion. The auto-ignition temperature of most aviation hydraulic fluids is in the range of 475 degrees C.
• Fitment of cockpit brake temperature indicators will give the pilots warning of a wheel well fire.