Aircraft Hydraulic Systems: Description, Components and Operation

Aircraft Hydraulic Systems

Aircraft Hydraulic Systems

The operation of aircraft components is made possible by hydraulic systems. Hydraulic power systems primarily operate the landing gear, flaps, flight control surfaces, and brakes. The complexity of hydraulic systems vary from small aircraft, which just need fluid for manual wheel brake function, to huge cargo aircrafts, which have large and complex systems.

The large transport and other business jets have complex control systems, heavy control surfaces that have more dynamic loads, heavy landing gear, brakes, and cargo doors that are almost impossible to operate without hydraulic power. To operate their control surfaces and other heavy loads, all large, high-speed aircraft require hydraulic power.

The system could have a number of subsystems to achieve the required redundancy and reliability. A pump, reservoir, accumulator, heat exchanger, filtering system, etc. are all components of each subsystem. In small aircraft and rotorcraft, the system operating pressure may range from a few hundred pounds per square inch (psi) to 5,000 psi, whereas in big transports it may be higher.

Advantage of Hydraulic Power

As power sources for a variety of aircraft components, hydraulic systems combine the benefits of being lightweight, simple to instal, easy to examine, and requiring little maintenance. Additionally, hydraulic processes are almost entirely efficient, with very little loss from fluid friction.

Depending on the aircraft, any or all of the following parts may be powered by a single hydraulic system, or by two or more hydraulic systems that work together:

  • Wheel Brakes
  • Nose Wheel Steering
  • Landing Gear retraction/extension
  • flaps and Slats operation
  • Thrust reversers
  • Spoilers/Speed Brakes
  • Flight Control Surfaces
  • Cargo doors/loading ramps
  • Windshield Wipers
  • Propeller pitch control

Hydraulic System Components

The hydraulic fluid and three key mechanical parts make up a hydraulic system. These include the "pressure generator," often known as the hydraulic pump, the hydraulically driven "motor," which drives the affected component, and the system "plumbing," which holds the fluid and distributes it as needed throughout the aircraft.

1. Hydraulic Fluid

Based on the aircraft model, their operating environment, and load factors, the following three types of hydraulic fluid are used in the aircraft hydraulic system:

Types of Hydraulic Fluids

The suitable fluid must be utilised to ensure proper system performance and prevent damage to the hydraulic system's nonmetallic components. Use the type of fluid recommended in the aircraft manufacturer's maintenance handbook or on the instruction plate attached to the reservoir or unit that is being serviced when adding fluid to a system.

The following are the most popular three categories of hydraulic fluids:



Phosphate esters

The technician must be sure to utilise the proper type of replacement fluid when servicing a hydraulic system. Fluids used in hydraulic systems may not always mix well. For instance, MIL-H-83282, a fire-resistant fluid, could become non-fire resistant if MIL-H-5606 was included into the mixture.

Mineral-Based Fluids

The oldest, MIL-H-5606, is a mineral oil-based hydraulic fluid. It was developed in the 1940s. It is employed in numerous systems, particularly those where the risk of fire is minimal. Simply said, MIL-H-6083 is MIL-H-5606 that has had its rust prevented. They can be used or interchanged in place of one another. Hydraulic parts are typically shipped with MIL-H-6083 by suppliers. Petrol is converted into mineral-based hydraulic fluid (MIL-H-5606). It is red-colored and has a deep oil-like smell. Depending on the type of synthetic hydraulic fluid, some of them are dyed purple or even green. Petroleum-based fluids are sealed with synthetic rubber.

Polyalphaolefin-Based Fluids

MIL-H-83282, a hydrogenated polyalphaolefin-based fluid with fire resistance, was created in the 1960s to address MIL-flammability H-5606's issues. MIL-H-83282 has a higher flame resistance than MIL-H-5606, however its high viscosity at low temperatures is a drawback. Usually, it is restricted to -40 °F. As with MIL-H-5606, it can be utilised in the same system and with the same hoses, gaskets, and seals. The rust-resistance variant of MIL-H-83282 is MIL-H-46170. Most small aircraft utilise MIL-H-5606, however if they can handle the high viscosity at low temperature, some have upgraded to MIL-H-83282.

Phosphate Ester-Based Fluid

These highly fire-resistant fluids are utilised in the majority of commercial transport aircraft. They are not, however, fireproof, and under certain circumstances, they burn. Additionally, these fluids are highly vulnerable to contamination from atmospheric water. After World War II, the commercial aviation industry became more concerned about the rising number of aircraft hydraulic brake fires, which led to the creation of the first generation of these fluids. The performance requirements of more modern aircraft designs led to the gradual development of these fluids. These new generations of hydraulic fluid, such Skydrol® and Hyjet®, were given performance-based names by the airframe manufacturers.

Fluids of types IV and V are utilised nowadays. Based on their density, type IV fluids can be divided into two groups: class I fluids have a low density, while class II fluids have a standard density. Compared to class II, class I fluids offer advantages in terms of weight savings. Type V fluids are being created in response to industry needs for a more thermally stable fluid at higher working temperatures in addition to the type IV fluids that are already in use. In comparison to type IV fluids, type V fluids will be more resistant to hydrolytic and oxidative breakdown at high temperatures.

Characteristics of Hydraulic Fluid

The medium through which a hydraulic system transfers its energy is fluid, and theoretically, any fluid might be used. However, considering the working pressure (between 3000 and 5000 psi) that the majority of aircraft hydraulic systems produce, as well as the climatic circumstances and stringent safety requirements, the hydraulic fluid used should have the following characteristics.

High Flash Point

Fluid ignition shouldn't happen in the event of a hydraulic leak at the surrounding components' typical operating temperatures. For usage in aircraft, special hydraulic fluids with fire-resistant characteristics have been created. Unlike hydraulic fluids based on mineral oil, these fluids are phosphate esters and are extremely difficult to ignite at room temperature. The fluid will continue combustion, nevertheless, if heated to temperatures higher than 180 degrees C. Most aviation hydraulic fluids have an auto-ignition temperature in the range of 475 degrees C.

Fire Point

The temperature at which a substance releases enough vapour to ignite and burn when exposed to a spark or flame is known as the fire point. Desirable hydraulic liquids must have a high fire point, similar to flash point requirements.

Chemical Stability and Viscosity

Another feature that is crucial in choosing a hydraulic liquid is chemical stability. It is the liquid's propensity to withstand oxidation and degradation over an extended length of time. Under difficult operating conditions, all liquids frequently experience undesirable chemical changes. For instance, when a system runs for a long time at a high temperature, this is the situation. The life of a liquid is greatly impacted by high temperatures. It should be noted that the temperature of the liquid in a hydraulic system's reservoir does not necessarily reflect the actual state of the system's working circumstances. Localized hot spots can appear on bearings, gear teeth, or where a liquid is driven through a small opening under pressure. Even while the liquid in the reservoir may not show signs of an abnormally high temperature, continuous flow of a liquid across these locations may result in local temperatures high enough to carbonise or sludge the liquid.

Hydraulic systems in aircraft must function effectively throughout a wide range of temperatures. The employed fluid must maintain acceptable viscosity at high temperatures while still flowing smoothly at very low temperatures. The freezing and boiling points of the ideal hydraulic fluid will be extremely low and high, respectively.

Lubricant Properties

The system's motors, actuators, and pumps are all lubricated by the hydraulic fluid. The fluid must be both thermally stable and anti-corrosive.

Thermal Capacity/Conductivity

System cooling is accomplished via hydraulic fluid. Heat must be easily absorbed and released by the fluid.

2. Hydraulic Pumps

airbus a320 hydraulic pump
Airbus A320 Hydraulic Pump

Aviation applications use a range of hydraulic pumps that are powered by various energy sources. Pumps consist of:

Gear Pumps

Gear pumps move fluid by utilising meshing gears. Because they move a certain volume of fluid every rotation, gear pumps fall under the category of fixed displacement pumps. Gear pumps are typically not appropriate for high pressure applications, however they may be utilised on low pressure systems (under 1500 psi).

Fixed Displacement Piston Pumps

A piston that moves inside a cylinder is used in piston pumps to pressurise a fluid. Each stroke of a fixed displacement pump moves a predetermined volume of fluid.

Variable Displacement Piston Pumps

On large aircrafts, this kind of pump is the most prevalent. The pump can adjust its fluid flow to match changes in system demand thanks to its variable displacement design. This makes it possible to maintain a nearly constant system pressure.

These pumps' motive power can be produced in a wide range of ways, including:

Manual operation

A manual hydraulic pump is commonly used in light aircraft to supply pressure for the expansion and retraction of flaps or wheel brakes.

Pumps powered by engines are typically positioned on the accessory gearbox for engines.


Hydraulic pumps can be driven by both AC and DC motors, with three phase AC motors being the most popular.


Some aeroplanes use motors that are powered by bleed air to operate hydraulic pumps.


Without any hydraulic fluid being transferred, a Power Transfer Unit (PTU) enables the hydraulic pressure of one hydraulic system to drive a pump to pressurise another hydraulic system. A PTU can be either single- or bi-directional, depending on the implementation.

Ram Air Turbine

Some aeroplanes include a Ram Air Turbine (RAT) that can be extended into the airstream to produce hydraulic pressure in an emergency.

3. Hydraulic Motors and Cylinders

Pressurized fluid is used by hydraulic motors and cylinders to do mechanical work.

Hydraulic Motors

A hydraulic motor is a mechanical device that transforms hydraulic flow and pressure into rotation and torque. There are several types of hydraulic motors, including gear, vane, and radial piston motors. Hydraulic motors are frequently used on aircraft to power jackscrews, which in turn can power flaps, stabiliser trim, and some applications for vertically extending landing gear, such as those seen on the LOCKHEED C-130 Hercules aircraft.

Hydraulic Cylinders

A hydraulic cylinder, also known as a linear hydraulic motor or a hydraulic actuator, is a type of mechanical actuator that delivers a single-direction, reversible force. The hydraulic cylinder consists of a barrel-shaped cylinder inside of which a piston attached to a piston rod moves back and forth using hydraulic pressure. Applications for aircraft include moving the flight control surfaces and extending and retracting the landing gear.

4. System "Plumbing" Components

In general, "open loop" aviation hydraulic systems pull fluid from a reservoir, pressurise it, and then make it available to the various user components before returning the fluid to the reservoir. The following are the main parts of the hydraulic system's "plumbing" segment:


Most aircraft systems need for hydraulic fluid reservoirs, which must hold a range of fluid volumes and serve as a ready source of fluid for the hydraulic pump(s). This variation is due to the thermal expansion or contraction of the fluid as well as the differential actuator volume (which depends on whether the actuator is extended or retracted). Only the amount of fluid required for proper operation is carried since the reservoir size is optimised. To help minimise hydraulic pump cavitation, bleed air is frequently utilised to pressurise or "bootstrap" the reservoir.


Clean hydraulic fluids are necessary for a system to operate properly. The hydraulic system includes in-line filters to purge any impurities from the fluid.

Shut Off Valves

In most cases, hydraulic shut-off valves are mounted at the engine firewall. The shutdown valve is closed in the case of an engine fire to avoid the hydraulic fluid from possibly igniting.

Control Valves

An connected control valve for hydraulic motors and actuators is positioned in response to a manual or automatic system choice, such as raising the flap lever. In response to that choice, the control valve adjusts its position to let pressurised hydraulic fluid enter the motor or actuator in the desired direction.

Pressure Relief Valve

Pressure relief valves are sometimes included in systems to make sure that the nominal system pressure is not exceeded, especially in systems that use fixed displacement pumps. Fluid is restored to the reservoir when the relief valve opens due to excessive system pressure.

Hydraulic Fuses

By automatically cutting off a hydraulic line if pressure drops too low, hydraulic fuses are in-line safety devices.


An external source of energy maintains the pressure of hydraulic fluid in a hydraulic accumulator, which is a pressure storage reservoir. A compressed gas or a spring may serve as the external source. A hydraulic system can respond more quickly to a brief demand and handle extremes of demand with the help of an accumulator. It also dampens pulsations, acting as a system shock absorber. The energy in an accumulator can only deliver a certain amount of brake applications after landing in the event of a hydraulic pump failure.

Hydraulic System Redundancy

There are two main ways to achieve hydraulic system redundancy: by using several systems and multiple pressure sources inside a single system.

Multiple Pressure Sources

There are frequently multiple pumps available to pressurise hydraulic systems. One or more engine-driven pumps and one or more electric pumps are extremely prevalent in systems. A manual pump is occasionally also included. Some systems only employ manual or electric pumps while the engines are not running when they are on the ground. Others employ the electric pump(s) as the main pressure source in the event that the engine-driven pump fails or as an extra pressure source during high demand circumstances like gear retraction (s). When an electric pump is utilised as the main source of hydraulic pressure, the system may also include a second electric pump or a Ram Air Turbine as a backup. In order to prevent the failure of the complete hydraulic system due to a single component, numerous pressure sources are provided.

Multiple Hydraulic Systems

Flight control surfaces are frequently hydraulically activated in aircraft. In these situations, it is crucial to have numerous actuators on each surface, each powered by a different hydraulic system, to prevent loss of control in the event of a hydraulic system failure. Three independent hydraulic systems are frequently used to power the flight control surfaces in contemporary commercial aircraft. Two of the systems could fail according to the control surface architecture without compromising control.


Particularly, aviation hydraulic systems are employed to assist in managing and controlling apparatus like brakes, flaps, thrust reversers, flying controls, and of course, landing gear. Because they provide the ideal amount of pressure to operate these devices, hydraulic systems are the recommended system.

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