The Power of Aircraft Hydraulic System

Aircraft · 6 min read · Oct 05, 2021
Aircraft Hydraulic System

What is a hydraulic system in an aircraft?

In aviation, hydraulic systems are the heart of the airplane systems and are critical for a smooth flight and aircraft functioning. In simple words, a hydraulic system is used to drive machinery or move mechanical components using a fluid under pressure. The aircraft hydraulics are used on aircraft of all sizes to operate most of their equipment, such as landing gears, brakes, flaps, thrust reversers, and flight controls. Thus, the hydraulic system performs the function of moving and actuating the critical and the basic components.

An aircraft jet engine with hydraulic and fuel systems.

Why are hydraulics used in aircraft?

As hydraulic system plays a vital role in the functioning of an aircraft, all aircraft make use of some hydraulically powered components due to the numerous benefits these components can provide. Even in the past, aircraft were still incorporated with hydraulic systems in the form of hydraulic brakes. In small general aviation aircraft, the hydraulic systems are used to provide pressure to activate the wheel brakes and are used to operate the retractable landing gear and a few constant-speed propellers.

With technological advancements, hydraulic systems have also evolved, and their use has become more common and essential, especially in larger aircraft with complex systems to operate all essential and critical flight components. These include flight control surfaces like wings, main landing gear retraction or extension, slats, flaps and doors, and loading ramps on cargo aircraft. Hydraulic systems also provide power to windshield wipers propellers and to move and actuate the brakes.

The basic principles and functionality of the hydraulic systems are the same, regardless of the size and shape of the aircraft. Being very reliable, the hydraulic system provides the perfect amount of pressure without requiring a lot of hydraulic fluid to work, making it convenient and efficient for aircraft of all types. Depending on the aircraft type and size, a single hydraulic system or two or more hydraulic systems can work together for a flight to operate smoothly and safely.

Components and working of the aircraft hydraulic systems

The basic parts of the hydraulic are the same in most aircraft. However, certain redundant systems are embedded into the aircraft that perform very related tasks and are essential to enable safe operation in case of failure of the hydraulic systems. The basic components include :

  • Reservoir – the source of fluid for hydraulic pumps. It contains varying amounts of incompressible fluid or hydraulic oil depending on actuator volume and thermal usage of fluid.
  • Hydraulic pumps –  generate power to pump the fluid from the reservoir to different parts of the system. It may be electric, hand or engine-driven and can include gear pumps, fixed displacement and variable displacement piston pumps.
  • Filters – incorporation of in line filters is essential to keep the hydraulic system clean and the fluid free from contamination.
  • Actuating cylinder – it changes hydraulic power into mechanical power and provides a reversible force in a single direction. It is used to operate the flight control surfaces like flaps and landing gears.
  • Heat Exchanger – keeps the fluid at the proper temperature.
  • Pressure relief valve – used to relieve excess pressure by opening the relief valve and returning the fluid to the reservoir.
  • Control valves – control the hydraulic fluid in response to a manual or automatic system. There are various control valves including flow control valves and direction control valves.
  • Shut off valve – present at the engine firewall, it shuts down the routes of fluid flow in the case of an engine fire.
  • Hydraulic fuses – safety devices designed to automatically seal off a hydraulic line if the operating pressure becomes too low.
A jet engine of an aircraft without its covers, exposing an intricate hydraulic system.
Image source:

How do hydraulics affect aircraft?

The aircraft hydraulic system works on the principle of Pascal’s law and conservation of energy. If you apply pressure to liquid anywhere in a particular system, this action causes there to be equal pressure distributed evenly throughout the entire system. In simple words, this allows force to be transmitted from one point to another in the system and causing displacement of different small systems within an extensive system. Based on this working principle, the hydraulic systems help the pilot function during flight by pressurizing the hydraulic fluids upon every section of vessels and creating pressure to enforce movement in specific aircraft components from one position to another.

The hydraulic fluid is the pressurized liquid used to transmit the power needed to operate an aircraft and is either liquid or oil. The pressure capacity of the hydraulic system can range from a few hundred pounds per square inch to more than 5000 pounds per square inch. This wide range is to accommodate different sizes of aircraft and different loads.

The pilot puts the particular hydraulic system to activation mode by switching on the input or flight control devices. The flight control device can include a rudder, ailerons, or elevators. As the pilot activates the hydraulic system, the pump starts operating, pressuring the system and setting the actuator in motion.

The actuator moves towards the control surface or device that needs to move in the desired motion, for example, the brakes or landing gear, creating pressure on it. This moves the desired surface in the correct position. When the systems need to move in the reverse direction, it releases the pressure and the direction changes. Therefore, the actuating cylinder changes hydraulic power to mechanical power and completes the purpose of hydraulics.

What are the basic types of hydraulic system in aviation?

The aircraft hydraulic system has two types, namely the Basic hydraulic system and the Power hydraulic system.

The Basic hydraulic system consists of the basic components: the reservoir, pumps, valves, and many more. This hydraulic system has two further working approaches; the open coil hydraulic system and the closed coil hydraulic system. In the open coil system, there is no pressure within the system, but only the fluid flows, due to which the actuator in the system remains idle. The pump forces the fluid in the circuit, and it flows from the reservoir through the selector valves, which are in series with each other and then return to the reservoir.

In the closed coil system, the fluid is under pressure, and three actuators work parallel to each other, along with the valves that are also in parallel. This causes the pressure to vary.

The other type of hydraulic system is the Power hydraulic system, which is the latest modification of hydraulic systems. This hydraulic power system is a small-unit system of the pump, filters, reservoir, valves, and relief valve. While it eliminates the use of heavy components and hydraulic fluids, it also provides a power supply to flight control popular in modern aircraft.

Hydraulic fluids

The hydraulic system transmits its energy via the fluid, which conveys power between components. The fluid also acts as a coolant, lubricates the system components, and has to accurately transmits the pressure. It must have a high flash point with adequate viscosity to fill aluminum pipes and optimum thermal capacity, along with thermal stability and anti-corrosion properties. The three main types of hydraulic oil include vegetable oil, made out of alcohol and castor oil and typically found mostly in older aircraft, synthetic oil, and mineral oil. Skydrol is the most advanced and widely used hydraulic fluid in aircraft.

Airman Logan Wilson, of Plant City, Florida, works on a utility hydraulics system reservoir on a C-130 trainer
Image source:

Advantages of hydraulic system

Aircraft hydraulic systems are essential for a smooth flight and safe operations. Although hydraulic systems have a few threats, many advantages outweigh them, making the systems reliable. For aircraft to operate, they can utilize either hydraulic or pneumatic systems, but due to the many advantages they can provide, hydraulic systems prevail over pneumatic systems.

While pneumatic systems use compressed gas or air to transmit power, aircraft hydraulic systems work using a liquid or hydraulic oil as the hydraulic fluid, making them a better choice. This is because the hydraulic fluid is not susceptible to compression, and it does not change pressure with the aircraft taking off or landing, which means there is no delay in the movement. This is very important in critical flight situations where certain actions have to be executed without being concerned about how long those actions might take. Pneumatic system is less efficient due to delays in the system, high noise levels, and their sensitivity to vibrations.

Aircraft hydraulic systems also offer more force and are easy to control, and give accurate results. With easy repair and maintenance, they are safer and more economical to utilize. They are very cost-effective to install and can operate at maximum efficiency even at the toughest flight conditions. Leakage in the system can be easily checked and detected, and its automatic lubrication systems protect from corrosion. The hydraulics have maximum efficiency with negligible frictional losses due to fluid dynamics. The components within the system are typically lightweight and simple to install with a small number of moving parts. The fact that hydraulic systems respond very quickly to control inputs enhances the efficiency and safety of the flight.


Contamination in the system or hydraulic fluid can be caused due to several reasons. It can lead to the loss of hydraulic system efficiency, excessive component wear, and damage to the overall system. While inappropriate fluids damage the system components, fluid leakage may cause ignition of fire hazards. Overheating of the system can occur, which causes loss of the hydraulic system and the loss of function of those components that it powers. All these factors call for adequate and routine preventative maintenance.

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Jet pilot @NASA

1 comment

  1. Asghar says:

    If aircraft hydraulic sytem operating at 3000 psi and During landing gear extension or retraction how much hydraulic pressure will be droped

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