What Is a Hydraulic System in Aircraft? – All You’d Want to Know

What is a hydraulic system in aircraft? How does it work?

Have you ever wondered how aircraft have smooth flight and landing? So, well, that’s because of the aircraft hydraulic systems. Hydraulic systems are like the heart of aviation.

The aircraft hydraulic systems are the system that performs a function to move and actuate landing gears, flaps, and brakes. The system acts by pressurizing fluids upon every section of vessels. Thus, creating force and transmitting pressure on every part. However, aircraft hydraulic systems are not the same as pneumatic systems as it is not lightweight.

The aircraft hydraulic system works on a simple principle of Pascal’s law and conservation of energy. This principle allows transmitting force from one point to another in the system. Thus, it causes displacement of different small systems within an extensive system. However, using this principle, aircraft hydraulic systems pressurize the fluid to enforce movement in specific aircraft components from one position to another. Moreover, based on this working principle, the hydraulic system helps the pilot function during flight. 

Furthermore, with that said, the pilot puts the hydraulic system to activation mode by flight control devices. As the hydraulic system gets active, the pump starts operating, causing the actuator to move. Moreover, the actuator moves towards the surface or device that needs to move, creating pressure on it and the surrounding. However, when the systems need to move in the reverse direction, it releases the pressure. Thus, the actuating cylinder changes hydraulic power to mechanical power and completes the purpose of hydraulics.

The purpose of a hydraulic system used in aircraft.

The hydraulic system plays a vital role in the functioning of aircraft. However, the hydraulic system has always been a part of aircraft. Back then, aircraft hydraulic systems were in the form of hydraulic brakes. But, with the introduction of advanced technologies, today, airplanes use a hydraulic system to operate all essential and critical flight components. At present, aircraft uses hydraulic system:

  • For flight controls, thrust reversers, and spoilers in heavy aircraft.
  • To move and actuate landing gears, brakes, and flaps.
  • It delivers constant torque regardless of speed.
  • It provides a smooth and safe ride, especially in critical condition.

Moreover, aircraft hydraulic systems may work together and provide power to:

  • Wheel brakes
  • Wheel steering nose
  • Landing gear (extension or retraction)
  • Slats and flaps
  • Thrust reversers and speed breakers.
  • Flight control surfaces like wings
  • Doors and loading ramps on cargo aircraft.
  • Propellers and windshield wipers

What components do aircraft hydraulic systems include?

The aircraft hydraulic system is complex, depending upon its use. Moreover, it is an open-loop system using a variety of components for smooth and efficient functioning. However, the essential parts are the reservoir, pumps, valves, filters, actuator, etc. So, let’s discuss each component to find how they are crucial for efficient functioning.


The hydraulic oil/ fluid reservoirs are essential as they are the source of fluid for hydraulic pumps. Moreover, the reservoir contains varying amounts of incompressible fluid or hydraulic oil. The volume it contains alters according to actuator volume or thermal usage of fluid. Furthermore, the reservoir protects the liquid from contamination. Thus, it ensures the optimal performance of aircraft hydraulic systems. The makers optimize its size so that the system uses enough oil for proper functioning.

Hydraulic pipes

Hydraulic pipes are the hollow structure that enables the movement of hydraulic fluid to different parts within the system.


The hydraulic pumps are the heart of the hydraulic system. It pumps the fluid/ oil from the reservoir to different parts of the system. Moreover, the aircraft hydraulic system uses different types of hydraulic pumps depending upon their use.

  • Fixed displacement pumps: In this, the flow rate of fluid does not change. Moreover, the same amount of fluid moves at each stroke. Examples are screw pumps or gerotor.
  • Variable displacement pumps: In this, flow rate and pressure change at each stroke. Examples are piston pumps and vane pumps.
  • Manual or hand pumps: In this, the pilot operates it manually.


The aircraft hydraulic systems use valves to control the flow, direction, and pressure of the fluid. Moreover, it is essential to have a controllable amount of fluid in the system. In addition, the valves enable the stoppage of fluid in the circuit in times of hazards. However, there are different types of valves depending upon the need and requirements.

Control valves

These valves are essential to control the hydraulic fluid in response to a manual or automatic system. Moreover, there are various control valves:

  • Flow control valves: It controls the rate of fluid flow. Moreover, it works by altering the diameter of the hydraulic fluid opening.
  • Direction control valves: It is the check valve. Moreover, it works by allowing hydraulic fluid to flow in one direction. Thus, it blocks the other direction of fluids.

Pressure relief valve

The pressure valve relieves the system pressure if beyond the optimal limit. It works by opening the valves, reducing the stress, and thus making the fluid return to the tank.

Shut-off valve.

These valves shut down the routes of fluid flow in the case of an engine fire. Therefore, it is present at the engine firewall. In addition, it prevents damage from occurring due to ignition in fluids.


Hydraulic fuses are the safety device in aircraft hydraulic systems. Moreover, it automatically seals the hydraulic line once the pressure becomes too low.


Actuator changes hydraulic power into mechanical power in the aircraft hydraulic systems. Moreover, the actuator may be the cylinder or motor, depending upon the movement.


Maintaining the hydraulic system clean and free from contamination is essential. Therefore, the aircraft hydraulic systems have in-line filters. It removes and filters out contaminants from the hydraulic fluid. It has two types of filters:

  • Suction line filter- It stops contaminants from getting into the different parts.
  • Return line filter- It stops contaminants from getting into the return line.

Pros and cons of hydraulic systems in aircraft?

The aircraft hydraulic systems are no doubt essential for a smooth flight. But, with lots of advantages, there are few cons related to the hydraulic system.

Pros of the aircraft hydraulic systems

There are many advantages of using hydraulic systems in aircraft.

  • They are easy to control and give accurate results.
  • It is easy to check if there is any leakage in the system.
  • It has automatic lubrication systems and protects from corrosion.
  • The hydraulic system does not cause sparks and is safe in flights.
  • It is easy to repair and maintain.

Cons of the aircraft hydraulic systems

There may be few disadvantages of the hydraulic system, which is essential to know.

  • Using inappropriate fluids may damage the components of the hydraulic system.
  • Leakage of fluids may cause ignition of fire hazards.
  • Contamination in the system or fluids may affect the performance and even damage the system.

How many hydraulic systems does the aircraft have?

The aircraft hydraulic systems are of two types:

  • Basic hydraulic system
  • Power hydraulic system

Basic hydraulic system

The basic hydraulic system includes basic components like a reservoir, pumps, valves, and many more. Moreover, it has two approaches:

Open coil hydraulic system

This system consists of fluid flow but no pressure in the system. Thus, the actuator in the system remains idle. Moreover, the pump forces the fluid in the circuit. Therefore, the fluid flows from the reservoir through selector valves and returns to the reservoir. In this system, the valves are in series with each other.

Closed coil hydraulic system.

In this system, there is pressure on the fluids. Thus, there are three actuators in the action and parallel to each other. In this system, the valves are parallel, thus causing pressure to vary.

Power hydraulic system

The hydraulic power system is a small-unit system of the pump, filters, reservoir, valves, and relief valve. Moreover, it offers power supply flight control popular in modern aircraft. In addition, this system eliminates the use of heavy components and hydraulic fluids. Thus, they are the latest modifications of hydraulic systems.


The aircraft hydraulic systems are complex yet very crucial for the functioning of airplanes. However, talking about the hydraulic system in aircraft, they are the heart of planes that pump hydraulic fluid for smooth flight and landing. Moreover, they work on a principle where they apply pressure on the fluid and generate power. And, then this hydraulic power turns into mechanical energy by the actuator present in the system. In this article, you get that aircraft hydraulic systems are crucial for flight controls, brakes, flaps. In addition, it helps to move and actuate landing gears, thrust spoilers, and many more. Thus, an aircraft hydraulic system has many advantages that overcome disadvantages as they provide an easy and safe ride.

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