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.

Aircraft Hydraulic Systems

Most aircraft utilize some form of hydraulic system, which may range from simple hydraulic wheel brakes, to very large and complex systems operating a broad range of services.

Hydraulic actuation offers many advantages over conventional mechanical and electrical systems, the major advantages being:

•   Provision of smooth and steady movement
•   Hydraulic power is confined to pipelines and components, and does not require widespread significant structural strengthening.
• The installation of hydraulic systems and components is simpler than mechanical power transmission systems.
• Variations in speed and power output can be made without the need for complex and heavy gearboxes etc
• Power for hydraulic systems can be provided from many separate sources, for both normal and emergency operation.

 

There are a number of fluids that have been used in aircraft systems over the years, they are in the form of an oil, which usually have a natural, mineral or synthetic base.  There are advantages and disadvantages for each of these fluid types, and selection of the fluid used in a particular aircraft or system will depend upon which properties are required.  Listed below are the properties required of an ideal hydraulic fluid, but it must be remembered that no one fluid provide all of them.

• It should be incompressible.
• It should have a reasonable density with little or no variation due to temperature changes.
• Its viscosity should have a low rate of change with changes in temperature.
• It should have a large working temperature range.
• It must provide good lubrication properties.
• It should not present a significant health hazard to operators.
• The flash point should be above 100°C, but should preferably be non‑flammable.
• It should not foam.
• It should be chemically stable under all operating conditions.
• It should neither harm, nor be harmed by materials used in the system pipelines and components.
• It should have a good storage life.