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AP4ATCO - Hydraulic Systems

From SKYbrary Wiki




To gain an understanding of:

  • How an aircraft hydraulic system works
  • What aircraft services are powered by hydraulics
  • The impact of loss of hydraulics on the performance and handling of an aircraft.


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 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.

Potential Hydraulic System Problems

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.

Anticipated Impact on Crew

The crew that experiences problems with hydraulics might face the following:

  • Downgraded flight characteristics - the crew could have difficulties controlling the aircraft that might vary from light to severe if all hydraulic systems are not present;
  • Increased stress in the cockpit - the crew would need to perform some preliminary checks to estimate the extent of reduction of the normal characteristics of the aircraft;
  • Increased time allowance - additional time might be needed in order to check the status of the problem and to initiate the required mitigations. It should be noted that some of the situations are present in SOPs or other documents with proper check-lists, but others may necessitate an ad hoc solution;
  • Manual flying - major damage of hydraulics would usually affect the auto-pilot functions;
  • Immediate landing - after assessing the situation, the crew could decide to carry out an immediate landing at the nearest (suitable) airport.

Quiz Questions:

Q1: A Hydraulic system consists of 3 components

  1. Battery, Motor, Cables
  2. Reservoir, Pump, Undercarriage
  3. Pump, Motor, Plumbing
  4. Oil, Gauge, circuit breaker


Q2: A crew experiencing a hydraulic failure may notify a controller...

  1. of a need for an immediate landing
  2. of a need to enter the hold while they manage the situation
  3. of a need to land on a specific runway



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