Do Airplanes Use Hydraulics?


Today’s commercial airplanes have large control surfaces and when traveling at speed large quantities of force are required to move control surfaces or operate equipment. From flight control surfaces to landing gear and autopilot systems, hydraulic systems provide the power necessary for pilots to control an aircraft. 

Hydraulics are used on larger aircraft to move and operate components that require more force than can be delivered by a human. Electronic control of hydraulic valves and actuators also allow autoflight systems to fly and trim the aircraft. Multiple systems are always installed for redundency.

Without hydraulics, all of todays large aircraft would not be possible. Even small aircraft begin to require some form of power assistance to help reduce the physiclal workload of the pilot. Todays modern airplane have complex hydrualic systems comprising of miles of pipework threading all over the airplane.

Although complex, their design is kept minimal to reduce weight and areas for failure.

To find out in simple explanations how hydraulics are used on an airplane please read on…

Where Do Airplanes Use Hydraulics?

Here are the most common areas where hydraulics are used on an airplane:

Control Surfaces

The flight control systems on an aircraft consist of primary and secondary systems. The three primary flight control surfaces are ailerons, elevators, and rudder.

Hyrdaulically Operated Flight Control Surfaces – Original Image Source: Source: Pedro Ribeiro Simões

The secondary systems consist of flaps, leading-edge slats, spoilers, and trim systems to improve aircraft performance and relieve the pilot of demanding control requirements.

The pilots change the aircraft’s direction and attitude using flight control system.

All of these components face high wind speeds and need considerable force to be moved.

Fly-by-wire Systems

In a fly-by-wire system, an electronic interface completely replaces flight controls that are mechanically connected to the pilots yoke and pedals in the cockpit.

A computer system uses electronic sensors to read the pilot’s control inputs and decides how to much and fast to move the flight control surfaces to produce the desired motion. A fly-by-wire system in an aircraft uses computers to control the hydraulics by opening and closing of the required valves to send fluid to the actuators to move the flight control surfaces.

Auto Pilot/Flight Systems

In addition to the pilots placing inputs into the fly-by-wire systems the aircraft’s autopilot or autoflight systems can place inputs too. When the autoflight systems are activated by the pilots the flight management system will give commands to the hydrualic control system computer to move the flight controls to keep the aircraft in a set parameter.

Each time the aircrafts flight regime needs to be altered the autoflight system gives the command to the hydrualic contol system. The change is then monitored and adjusted until the aircraft is in the desired flight regime.

This happens hundreds of times per second on every flight control whiel the autoflight system is active.

Without this electronic system the aircraft would not be able to be controlled by an autopilot of autoflight system.

Landing Gear

Hydraulics are used in the landing gear as part of the retraction/deployment system and as a damping mechanism. Retractable landing gear is crucial for fast and economical flights since it reduces drag. By leaving the landing gear extended for the entire flight the cruise speed would be reduced and the fuel burn woulf dramatically increase.

The landing gear needs hydraulic systems to retract and deploy because of its extreame weight of each landing gear leg.

Brakes

Friction is used by airplane brakes to slow the aircraft after touchdown, much as it is in a car. Hydraulics press brake pads on to the brake discs to generate massive clamping force.

Airplane Brake Pads and Discs!

Thrust Reversers

When a fast, heavy jet is landing, thrust reversers are essential for bringing it to a halt. Thrust reversers temporarily divert an engine’s thrust in the opposite direction by using hydraulic pressure to open the engine’s reverse thrust deflector doors.

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How Do Aircraft Hydraulic Systems Work?

Hydraulics work based on Pasclas Law – An increase in pressure at any point in a confined fluid will result in an equal increase at any other end in the container. Hydraulic systems utlitize a pump to pressurize fluid and valves to open/close to move actuators. These operate systems with immense force.

It is this immense force that is needed to physically move or adjust parts on an airplane. If you have ever tired to fly a large kite or move a sheet of wood in a windy storm you will know just how much strength or force it takes to control them – If you don’t have enough strength, the wind overpowers you and you lose control.

This is exactly the same on a large airplane, except instead of sheets of plywood the pilot is trying to move and control flight surfaces like rudders, ailerons, elevators and flaps.

When wind is not an issue, the shear weight of components present a problem. Imagine trying to manually wind up the landing gear of an Airbus A380! You have 4 sets of main landing gear containing 20 whels, tires, hubs and brakes plus two at the front – You are not moving that by hand.

This is where hydraulics come in!

Airbus A380 Landing Gear is Immense! – Source: Florian Lindner

In an airplane, hydraulics are primarily used to either control and move large surfaces in contact with high speed airflow or move large and heavy components


For hydraulic systems to work on an airplane they need a few basic components:

Pumps

Engine-driven pumps pressurize the hydraulic fluid and move the fluid throughout the pipework. Electrically-powered pumps provide backup incase of all engines failing.

Typical airplane pumps pressurize the system to between 3000-6000psi.

Hydraulic Fluid

A specially designed fluid that does not compress under load. This tranfers the force generated by the pumps to the movable actuators.

The Airbus A380 has two independant hydraulic systems. One system contains 585 liters while the second system has 545 liters.

Pipes and Fittings

Transports the fluid around the aircraft and connect the pipework to hydrualic components.

Valves

Control where the fluid goes, how much goes and when. They are operated by the electronic hydraulic control unit.

Actuators

Move when hydraulic fluid is pumped into them. They provide the means to mechanically move flight controls and landing gear. These comprise of linear and rotary pistons and rotary gearboxes.

Accumulators

These help keep the system pressurised and prevent any deadspots in the hydraulic system. Think of them like a reservoir providing a hydro dam with a contstant supply of water.

Heat Exchangers

Controls the temperature of the hydraulic fluid in the systems to maintian optimal working performance. Especially important when landing in desert cities like Dubai then climbing to atmospheric temperatures of -50°F.

Electronic Control

Fly-By-Wire and autopilots need to give signals to the hydraulic valves to open and close. This system also monitors temperatures, pressures and flows throughout the hydraulic system.

The electronic control systems take in all the data from the aircraft sensors and issue commands to the various output devices to keep the plane under control.

These systems also monitor the hydraulic systems for temperature, pressure and flow fluctations or failures and control and alert the pilots if manual action is required. Many of the systems have control logic to deal with issues automatically and in an instant.

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Rick James

I am an aviation nut! I'm an ATP-rated helicopter pilot & former flight instructor with over 3500 hours spanning 3 countries and many different flying jobs. I love aviation and everything about it. I use these articles to pass on cool facts and information to you whether you are a pilot or just love aviation too! If you want to know more about me, just click on my picture!

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