Hydraulic Systems


A hydraulic system uses a fluid under pressure to drive machinery or move mechanical components.


Virtually all aircraft make use of some hydraulically powered components. In light, general aviation aircraft, this use might be limited to providing pressure to activate the wheel brakes. In larger and more complex aeroplanes, the use of hydraulically powered components is much more common. Depending upon the aircraft concerned, a single hydraulic system, or two or more hydraulic systems working together, might be used to power any or all of the following components:

A hydraulic system consists of the hydraulic fluid plus three major mechanical components. Those components are the “pressure generator” or hydraulic pump, the hydraulically powered “motor” which powers the component concerned and the system “plumbing” which contains and channels the fluid throughout the aircraft as required.

Hydraulic Fluid

Fluid is the medium via which a hydraulic system transmits its energy and, theoretically, practically any fluid could be utilized. However, given the operating pressure (3000 to 5000 psi) that most aircraft hydraulic systems generate in combination with the environmental conditions and strict safety criteria under which the system must operate, the hydraulic fluid that is used should have the following properties:

  • High Flash Point. In the event of a hydraulic leak, fluid ignition should not occur at the normal operating temperatures of the surrounding components. 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, they 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.
  • Adequate Viscosity. Aircraft hydraulic systems must work efficiently over a broad temperature spectrum. The fluid used must flow easily at very low temperatures but must also maintain adequate viscosity at high temperatures. The ideal hydraulic fluid will have a very low freezing point and a very high boiling point.
  • Lubricant Properties. The hydraulic fluid acts as a lubricant for the pumps, actuators and motors within the system. The fluid should have anti-corrosion properties and be thermally stable.
  • Thermal Capacity/Conductivity. Hydraulic fluid acts as a system coolant. The fluid must be able to readily absorb and release heat.

Hydraulic Pumps

Several types of hydraulic pumps driven by a variety of power sources can be found in aviation applications. Pumps include:

  • Gear Pumps. Gear pumps use meshing gears to pump fluid. Gear pumps are fixed displacement type pumps as they move a specific amount of fluid per rotation. Gear pumps may be used on low pressure systems (under 1500 psi) but are generally not suitable for high pressure applications
  • Fixed Displacement Piston Pumps. Piston pumps utilize a piston moving in a cylinder to pressurize a fluid. A fixed displacement pump moves a specific amount of fluid with each stroke.
  • Variable Displacement Piston Pumps. This is the most common type of pump on large aircraft. The variable displacement design allows the pump to compensate for changes in the system demand by increasing or decreasing the fluid output. This allows near constant system pressure to be maintained.

The motive power for these pumps can be generated by a wide variety of options inclusive of:

  • Manual. In many light aircraft, a manual hydraulic pump provides pressure for wheel brakes or flap extension and retraction.
  • Engine Driven. Pumps are frequently mounted on the engine accessory gear box.
  • Electric. Both AC and DC motors are utilized to power hydraulic pumps with three phase AC motors being most common.
  • Pneumatic. Bleed air powered motors are utilized on some aircraft to drive hydraulic pumps.
  • Hydraulic. A Power Transfer Unit (PTU) allows the hydraulic pressure of one hydraulic system to drive a pump to pressurize a second hydraulic system without any transfer of hydraulic fluid. Depending upon the installation, a PTU can be single or bi-directional.
  • Ram Air Turbine. In the event of an emergency, some aircraft have a Ram Air Turbine (RAT) that can be extended into airstream to generate hydraulic pressure.

Hydraulic Motors and Cylinders

Hydraulic motors and cylinders utilize pressurized fluid to do mechanical work.

  • Hydraulic Motors. A hydraulic motor is a mechanical device that converts hydraulic pressure and flow into torque and angular displacement or rotation. Various types of hydraulic motors such as gear, vane and radial piston motors are available. On aircraft, hydraulic motors are most often used to drive jackscrews which can in turn be utilized to power flaps, stabilizer trim and some vertically extending landing gear applications such as found on the LOCKHEED C-130 Hercules aircraft.
  • Hydraulic Cylinders. A hydraulic cylinder, sometimes referred to as a linear hydraulic motor or a hydraulic actuator, is a mechanical actuator that is used to provide a reversible force in a single direction. The hydraulic cylinder consists of a cylinder barrel within which a piston connected to a piston rod ues hydraulic pressure to move back and forth. Aircraft applications include landing gear extension and retraction, cargo door operation and movement of flight control surfaces.

System "Plumbing" Components

Aviation hydraulic systems, in general, are of the "open loop" variety drawing fluid from a reservoir, pressurizing it and making it available to the various user components before returning the fluid to the reservoir. The primary components of the "plumbing" portion of the hydraulic system include the following:

  • Reservoir. Hydraulic fluid reservoirs are required by most aircraft systems to provide a ready source of fluid for the hydraulic pump(s) and to contain a varying volume of fluid. This variance results from differential actuator volume (dependent upon whether the actuator is extended or retracted) and for fluid thermal contraction or expansion. The reservoir size is optimized so that only the amount of fluid needed for proper function is carried. In many installations, bleed air is used to pressurize or "bootstrap" the reservoir to help prevent hydraulic pump cavitation.
  • Filters. Hydraulic fluid cleanliness is essential to proper system function. In-line filters are incorporated into the hydraulic system to remove any contaminants from the fluid.
  • Shut Off Valves. Hydraulic shut off valves are usually installed at the engine firewall. In the event of an engine fire, the shutoff valve is closed to prevent possible ignition of the hydraulic fluid.
  • Control Valves. Hydraulic motors and actuators have an associated control valve which is positioned in response to a manual or automated system selection such as moving the flap lever. The control valve responds to that selection by positioning to allow pressurised hydraulic fluid to flow into the motor or actuator in the appropriate direction.
  • Pressure Relief Valve. In some systems, especially those utilising a fixed displacement pump, pressure relief valves are incorporated to ensure that nominal system pressure is not exceeded. If system pressure becomes too high, the relief valve opens and fluid is returned to the reservoir.
  • Hydraulic Fuses. Hydraulic fuses are in-line safety devices designed to automatically seal off a hydraulic line if pressure becomes too low.
  • Accumulators. A hydraulic accumulator is a pressure storage reservoir in which hydraulic fluid is held under pressure by an external source of energy. The external source can be a spring or a compressed gas. An accumulator enables a hydraulic system to cope with extremes of demand using a less powerful pump and to respond more quickly to a temporary demand. It also acts as a system shock absorber by smoothing out pulsations. In the event of a hydraulic pump failure, the energy stored in an accumulator can provide a limited number of brake applications after landing.

Hydraulic System Redundancy

Hydraulic system redundancy is achieved by two primary means - multiple systems and multiple pressure sources within the same system.

  • Multiple Pressure Sources. Hydraulic systems often have more than one pump available to pressurise the system. It is quite common for a system to have one or more engine driven pumps plus one or more electric pumps. In some cases, a manual pump is also incorporated. Some systems only use the electric or manual pumps while on the ground when the engines are not operating. Others use the electric pump(s) to provide an additional pressure source during high demand situations such as gear retraction or as the primary pressure source in the event of the loss of the engine driven pump(s). When an electric pump is used as the primary pressure source, a second electric pump or a Ram Air Turbine might be incorporated into the system as a backup source of hydraulic pressure. Provision of multiple pressure sources helps to ensure that the entire hydraulic system is not lost in the event of a single component failure.
  • Multiple Hydraulic Systems. In many aircraft, flight control surfaces are hydraulically actuated. In these cases, multiple actuators on each surface, powered from multiple hydraulic systems, are essential to ensure that the failure a hydraulic system will not result in loss of control. In modern commercial aircraft, it is common to power the flight control surfaces from three independant hydraulic systems. The control surface architecture allows for failure of two of those systems without compromising control.


Hydraulic systems are subject to several significant threats. These include:

  • System Overheat. The system exceeds its maximum allowable operating temperature and must be de-energized.
  • Loss of System Pressure. Loss of system pressure can occur in two different ways; loss of fluid or failure of a hydraulic pump.
  • Hydraulic Fluid Contamination. Contamination can be chemical or particulate in nature and can be caused during fluid production, by improper servicing of the hydraulic system or by a component failure.


Hydraulic system overheat, loss of pressure or fluid contamination can all result in the loss of the hydraulic system and the loss of function of those components that it powers. Fluid contamination can also result in loss of hydraulic system efficiency, fluid leaks, excessive component wear and premature component failure.


The primary defense against hydraulic fluid contamination lies in robust maintenance practices. Any fluids used to service the system must be as specified in the AOM and fluid types should not be mixed. Care should be taken to ensure that the fluid is not contaminated prior to use and that no contaminants are introduced to the system while topping up the fluid. System filters should be cleaned or replaced as per manufacturer's guidelines.

In the event of a System Overheat or Loss of Pressure, following the Quick Reference Handbook (QRH) or ECAM checklists may result in recovery of the system. If the loss of pressure was as a result a total of loss of hydraulic fluid, the system is not recoverable.

Typical Scenarios

  • A transatlantic flight gradually loses hydraulic fluid from one of the hydraulic systems. When the Electronic Centralized Aircraft Monitor (ECAM) annunciates the system Loss of Pressure, appropriate actions are completed to secure the system.The Captain contacts Company maintenance to discuss options. Due to the fact that there is no significant loss of aircraft capability due to system redundancy, that a few hours of flight time to destination remain and that there were ample diversion airfields enroute, it is decided to continue to the planned destination. The flight lands without further incident.
  • Just after top of climb, a turboprop aircraft suffers a complete loss of the Utility hydraulic system due to a rudder actuator failure. Controlability of the aircraft is not an issue as the Booster hydraulic system also provides pressure to the flight control surfaces. The Captain electes to make a turnback. An alternate (gravity) gear extension is carried out and a flapless approach and landing is made. The Auxillary hydraulic system is used to provide pressure for the brakes and full reverse propeller pitch is used to help bring the aircraft to a stop. The aircraft is shut down on the runway and then towed to the ramp.

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