/Secondary Systems
   /Hydraulics & Pneumatics

Aircraft Hydraulic Circuit

Hydraulic Components

The complexity of any hydraulic system used on aircraft depends largely on the functions it needs to perform. If designed for wheel brakes only or to extend/retract the landing gear, this last application will result in a more or less complex system.

The principles are based on Pascal's Law: It says that pressure exerted anywhere in a confined incompressible fluid is transmitted equally in all directions throughout the fluid such that the pressure ratio remains the same.

Larger aircraft will use more complex hydraulic systems, homebuilt aircraft will normally use hydraulics in their foot operated brake system or use them for operating the landing gear in some type of aircraft. But they are usually not found in homebuilt aircraft due to the complexity of the system.

Below we will describe the main components you will find in these systems and the items to be checked by the pilot in command during pre-flight.

System Components

As with all systems working with fluids they, at least, will have a reservoir, pump and filter (to keep the fluid clean), some valves / actuators and a pressure gauge to monitor the working pressure. We will describe these parts and their functions below.


This must contain enough fluid so that all actuators can operate at the same time. There must be some amount of reserve in case of a leak, so that the system can operate for a period of time and the aircraft is able to land.

The reservoir functions as an expansion chamber (when the fluids heats up) and traps air bubbles should they enter the system somewhere. It can be pressurized for aircraft flying at high altitudes. Returning fluid must enter the reservoir without causing foaming and bubbles.


Aircraft Hydraulic Gear Pump

The main pump is driven by the engine or by an electric motor. Pressure is held in an accumulator. With hydraulic landing gear aircraft you will find a hand operated backup pump in case the gear fails to extend by the main pump. This will require a large number of manual pump action from the pilot at a time where stress is higher than normal.

These pumps are available in different types depending on the volume and pressure requirements: vane, spur gear and the fixed angle piston type.

Vane & spur gear

This is a constant displacement low pressure/high volume pump (vane) or medium volume/pressure (gear) pumps. Both require a pressure relief to prevent damage to the system due to increased RPM of the pump which would increase pressure.

Fixed angle

Some are constant displacement types but others are a variable displacement/constant pressure pumps and the latter obviously will not need a pressure regulator. They are capable of very high pressures up to 3000 - 3500 psi (207 - 241 bar/hPa) but with low volumes of fluid transferred.

Pressure regulator

To prevent damage we need to keep the pressure within the design limits of the system. Normally if a pump moves fluid and there is no restriction, there will be no pressure. The fluid just moves around. But when there is a restriction (such as in a closed circuit) the pressure will build up until the regulator kicks in.


Aircraft Hydraulic Accumulator

A two part pressure vessel in which the sections are divided by a bladder. One parts contains a gas (air or nitrogen) and the other half contains the working fluid. The gas is pressurized to half the working pressure of the system.

Constructed this way the gas will act as a damping device and levels out pressure fluctuations and it also serves as backup pressure should the pump fail. An electrical comparison is a power supply combined with a battery for backup.


There are three types of valves used: check, pressure relief and selector valve. The check valve is a non-return type, basically a hydraulic form of the electronic diode. The pressure relief valve limits the amount of pressure if it exceeds a preset level. And the selector valve is operated by the pilot to initiate the movement of an actuator.

Actuators & filters

Actuators are the main moving parts, they convert pressure into a mechanical movement to do useful work. They come in different sizes and shapes, this depends, of course, largely on the size and weight object it needs to move.

Filters keep the operating fluid clean from foreign contamination as microscopic particles can ruin valves, pumps resulting in a leak and possibly worse. Some filters have a bypass should the working material become clogged, much like the bypass in the Rotax oil filter.

Written by EAI.

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