Aircraft Hydraulic Systems (Authoritative Guide)

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  • Post last modified:February 9, 2021


Aircraft use hydraulic systems as a way of transferring power needed to move essential component such as brakes, landing gear and flight control surfaces.

This power is created by pumps (either electrically driven, or powered by the aircrafts engine) and is then transferred where it is needed through a series of pipes and valves.



Hydraulic use on aircraft

Although you might rightly associate a hydraulic system with large commercial aircraft, even light aircraft use hydraulics – to power brakes for example.


Large complex aircraft have a dedicated hydraulic system (in fact, often more than one) to move flight control surfaces such as flaps and slats, ailerons, rudder, elevator, horizontal stabilizer, spoilers, speed brakes etc.

Landing gear, brakes and cargo doors are also operated by hydraulic power.

Airbus A330 ECAM System Display - Hydraulic System
Airbus A330 ECAM


Aircraft Hydraulic System Components

Basic aircraft hydraulic systems contain 4 main components: the hydraulic reservoir which is a store of fluid, the pump which supplies the pressure, and the selector valve which controls the actuator.

More advanced hydraulic systems, like those on modern commercial jets, also contain accumulators which store energy, the PTU which can transfer that energy from one system to another, and the RAT which is an emergency back-up device.

Let’s take a look at each of those components:


Reservoir

The reservoirs are where the hydraulic fluid is stored. They allow the systems to be replenished and for air to be bled from the system.

Modern commercial aircraft have one reservoir for each system – for example both the Airbus A320 and A330 have three hydraulic reservoirs.


Hydraulic Pump

The Engine Drive Pump (EDP) as the name suggests gets its power from the rotation of the engines. In fact, even in the event of a loss of all engines, the airflow will rotate the engines enough to provide some hydraulic power through the EDPs.

Normally the engine-driven pump has a backup electrical pump which is available in case of failure of the EDP, or engine failure,

Not all hydraulic pumps are powered by the engines or electric motor though – some pumps are powered by airflow or by hand.

For example Airbus A320 and A330 aircraft have a hydraulic hand pump that is accessible to the ground crew and allows them to open or close the cargo doors even when the aircraft’s main hydraulic systems are de-powered.



Accumulator

Aircraft hydraulic accumulators are a vital part of the hydraulic system. These are pressurized containers that store fluid under pressure – i.e. the accumulator a store of energy.

The main functions of hydraulic accumulators are:

  • As a store of energy
  • As a limited store of fluid under pressure available in an emergency – i.e. the Airbus emergency braking memory items calls for use of the parking brake (which is powered by an accumulator)
  • Dampen pressure fluctuations
  • To assist the hydraulic pumps during high load (e.g. landing gear retraction)
  • To account for thermal expansion of the fluid


Actuator

As the fluid pressure is created by the pump and transmitted through the hydraulic system it arrives at the actuator whose job it is to turn the force of the fluid flow into mechanical power.


Ram Air Turbine (RAT)

A hydraulic component that can be powered by airflow is the Ram Air Turbine (RAT). This ingenious device is used in an emergency situation when small propellor is deployed automatically out of the aircraft and into the airflow.

Related: A Pilot’s Guide to The Ram Air Turbine (RAT)

This propellor in turn supplies limited hydraulic pressure to allow the pilots to control the aircraft after the loss of normal hydraulic power. Although this situation is rare it’s happened more than you might think!

Related: What Happens When Aircraft Run Out of Fuel



Power Transfer Unit (PTU)

The Power Transfer Unit (PTU) as the name suggests transfer power from one hydraulic system to another. The PTU is a backup device that allows the transfer of energy from one system to another.

For example if a hydraulic pump was to fail in a particular system, a PTU would allow the hydraulic pressure to be transferred from the other still functioning system.

The Power Transfer Unit (PTU) is a transfer of power only, it does not transfer any fluid between systems.

The PTU can either by selected on manually by the pilots or it can operate automatically if a loss of pressure is detected in one hydraulic system.

For example the PTU on the Airbus A320 operates automatically if it detects a difference in pressure between the yellow and green systems of 500 psi. This arrangement also allows the hydraulics to be powered on the ground with the engines stopped – the yellow system can be powered by an electric pump and with the use of the PTU the pressure is transferred to the green system.

Related Reading: A Guide to the Airbus A320 Hydraulic System



Hydraulic Fluid

Obviously an essential part of the hydraulic system, hydraulic allows the transfer of pressure from the pumps to the actuators.

There are a number of highly-desirable properties for aircraft hydraulic fluid:

  • Incompressibility
  • Non flamable
  • Resistance to corrosion
  • Resitance to heating/freezing
  • Good lubrication
  • Low viscosity

Cars and light aircraft generally use a mineral-based fluid. However the flammability of these fluids make them unstuitable for large commercial aircraft.

Modern airliners use a synthetic fluid which is more resistant the high pressures and temperatures on these aircraft that can cause pump cavitation. A common brand of hydraulic fluid used is Skydrol which is light purple in color.


Hydraulic Filters

Hydraulic filters ensure the fluid is cleaned as it passes around the system.

Hydraulic systems tend to have high pressure filters installed on each system.

Additionally, there are normally filters installed on the filling system of each reservoir, in the braking system of the landing gear and “case drain filters” on engine-driven and electric pumps.

The case drain filters allow the fluid to be checked for the presence of metal particles which would indicate pump wear.


Hydraulic Valves

There are numerous types of valves used in aircraft hydraulic systems, generally falling into the categories of Flow Control Valves or Pressure Control Valves

Flow Control Valves allow the pressure to be directed to where it is needed and come in the form of selector, sequence, shuttle and restrictor valves.

Selector valves allow you to select the direction of flow of the fluid whereas sequence valves are used to create a sequence of events – for example during landing gear retraction.

Shuttle valves can be used in emergency situations to isolate the main supply and instead route hydraulic pressure from an emergency supply.

This can serve to supply hydraulic pressure to essential services even in the event of a leak in the main system.

Restrictor valves control the speed of operation – for example they can be used to regulate the speed of flap deployment.


Priority valves

These valves are used to protect vital aircraft systems in the event of a serious hydraulic failure.

During normal operation the pressure entering the priority valve is sufficient to allow the fluid to flow out to both primary and secondary services.

In the case of a reduction in pressure the valve partially closes and cuts off supply to the secondary (less important services) thereby protecting critical controls.

For example, the Airbus A330 hydraulic system has a a priority valve system that, in the event of low hydraulic pressure, will cut off “heavy load users” such as the landing gear, nosewheel steering and emergency generator to protect the pressure supply for the flight controls and normal braking.

A similar system (known as a “pressure-off brake system”) installed on the THS (Trimable Horizontal Stabilizer), slats and flaps ensures the same result.


Hydraulic Fluid Cooling

Due to the immense pressures exerted on hydraulic fluids cooling them is critical.

If the fluid becomes too hot it may cause the fluid to expand or break down causing damage to the entire system. This temperature is usually measured at the pumps and at the reservoir.

A “heat exchanger” is used to dissipate heat and cool the fluid.

On light aircraft this cooling is often achieved with airflow. On larger aircraft the heat exchanger operates to cool the fluid using cold aircraft fuel.

Often during cruise the temperature of the fuel can become very cold. For example, the Airbus A320 FUEL LO TEMP ECAM triggers at about -45ºF/-43ºC.

Related: A Guide to ECAM/EICAS Monitoring Systems on Aircraft



Real World Examples

Now that we have a good understanding of the various parts of an aircraft’s hydraulic system, lets take a look at a real-world example:



Boeing 737-800 Hydraulic System

The Boeing 737-800 has three hydraulic systems known as A, B and Standby.

Either the A or B system can power all the required flight controls without a decrease in aircraft controllability.

If hydraulic pressure is lost in either the A or B system the Standby system is used.

The components powered from System A & B include: the rudder, yaw damper, elevator, ailerons, slats and flaps, spoilers, autopilot A & B, landing gear, brakes, nose wheel steering and PTU.

Both main systems can be supplied with pressure from an engine-driven pump or an AC electric pump.

Note: the engine-driven pumps (EDPs) supply approximately 4 times the amount of fluid volume of the electric pumps.

The Standby Hydraulic System on the Boeing 737-800 is used as a backup to the A & B main systems. It can be activated automatically or manually by the pilots, and is powered by an electric motor-driven pump.

The Standby System powers: leading edge flaps and slats (extension only), thrust reversers the standby yaw damper and the rudder.


Related Reading: Airbus – Dual Hydraulic Failure on the A320


Airbus A330 Hydraulic System

The Airbus A330 hydraulic system is made up of three individual systems referred to as Green, Blue and Yellow.

Each system is supplied with power from an engine-driven pump (EDP) and has a backup electrical pump.

The A330 green hydraulic system is also further backed up by a Ram Air Turbine (RAT).

Normal system pressure is 3000 psi, reducing to 2500 psi when the RAT is supply the power.


For more on A330 Hydraulics take a look here: The A330 Hydraulic System – A Pilot’s Guide


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