Basic Hydraulic System

In the basic hydraulic system, as shown below the basic components that all hydraulic systems may consist of, 

Actuator

Sometimes referred to as a hydraulic jack, this provides the output force required to operate various aircraft services.  Consists of a piston,on which the hydraulic pressure acts, which is secured to a rod, also known as a ram, which imparts the force developed by the piston.

Selector 

This component provides control of the system, and may be manually or electrically operated from the cockpit.  The terms ‘pressure’ and ‘return’ may only be applied to pipelines up to this component, the pipelines from the selector to the actuator may be referred to by their purpose ex: UP/DOWN,OPEN/CLOSE,RETRACT/EXTEND

Pump

The pump provides a flow of fluid to the system.  its should be mentioned pump provides flow, not pressure, as it is the resistance to that flow that causes a pressure rise,without making a resistance to the flow the pressure cannot be created.  In the illustration a simple hand pump is shown, many aircraft systems use engine or electrical motor driven pumps to provide greater flow rates and so speed up system operation.

Reservoir

The reservoir provides a means of holding surplus fluid, its purpose is to receive, store and supply fluid to and from the system.  It maintains a surplus, so that in the event of a leak the system will still have sufficient fluid to operate correctly.

It stores fluid that is temporarily not required by the system, as can be seen the actuator does not have equal volumes on each side of the piston.  When retracted (piston rod fully into the body) the actuator will hold less fluid than when fully extended (piston rod fully out of the body) and so this fluid must be stored when not in use.

The reservoir will also supply fluid to the pump to maintain a flow within the system and receives return fluid from the system.

Pipelines

The system pipelines are used to direct fluid to the appropriate components.  They may be made from various materials depending upon the pressure at that point in the system and the operating environment.  Within the system they are normally referred to by the nature of their oil flow ie. RETURN LINE or PRESSURE LINE.

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.