Hydraulic Fluids

Discuss the ideal properties of hydraulic fluids and explain the characteristics of various type of hydraulic fluids used in aviation.

The efficiency of a hydraulic system is mainly governed by the hydraulic fluid used. Not all fluids have the ideal properties, therefore, the only type of fluid allowed in a specific hydraulic system is that recommended by the manufacturer of the hydraulic components 

Fluids used in an aircraft hydraulics system must have the properties such as incompressible as possible, low viscosity rate, free flowing over a wide operating temperature range, chemically stable, not affect, or be affected by the materials in the system components, must not foam during operation when subject to sudden pressure increases or decreases, good lubrication properties, high flash point, must not deteriorate or form sludge. 

There are three basic types of hydraulic fluids used in aircraft hydraulic systems vegetable base, mineral base, and synthetic base.  Vegetable (Castor oil) base, generally found only in older aircraft types, is Golden yellow (or Blue) in colour, used with natural rubber seals. It is inflammable, strips paint and attacks synthetic rubbers. It is toxic in a fine spray mist. These systems can be flushed with alcohol. 

Mineral base is a kerosene-type petroleum product dyed red and is only compatible with synthetic rubber seals, neoprene seals and hoses may be used with mineral based fluids. While its density and lubricating properties vary with temperature, it does have good lubricating properties, but it is inflammable and attacks natural rubber. Systems and components can be flushed with naphtha, varsol, or Stoddard solvents. 

Synthetic ester base, commonly known by the trade name of SKYDROL, there are currently variants IV and V commercially available. Dyed Purple in colour, this fluid is used with Butyl, Ethylene Propylene, or Teflon seals. It is fire resistant, strips paint but attacks natural and other synthetic rubbers. Skydrol systems can be flushed with trichlorethylene. Components can be cleaned with methyl ethyl ketone (MEK), or isopropyl alcohol. Skydrol will cause irritation of the skin and burning of the eyes, therefore protective equipment and clothing should be worn when handling this fluid.

These fluids are not compatible with each other and must never be mixed, or used to replace each other. If a system has been inadvertently serviced with the wrong fluid, the complete system must be drained and flushed with an approved solvent, and all the seals in the system must be replaced.