Types of Hydraulic Fluid

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

Due to the differences in composition, hydraulic fluids may not be mixed, even those of the same base type.  During the design stage, aircraft manufacturers will 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 oil, 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 type, 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.