Why are aircraft hydraulics so important?

Mar 27, 2021

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A few weeks ago, an A320 suffered a hydraulics failure on landing in Burgas, Bulgaria, resulting in the aircraft coming to rest in the grass just off the runway.

In most airliners, the hydraulics system is responsible for powering the heavy-duty systems such as the landing gear, flight controls, brakes, cargo doors and thrust reverses — all essential items when it comes to stopping safely.

So how exactly does the hydraulics system work and how do we deal with a situation if the system fails?

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How hydraulics systems work

In the original days of aviation, the controls in the cockpit were directly linked to the flight controls with cables and wires. The control movements made by the pilot directly controlled the movement of the surface on the wing or tail. Even in some light aircraft today, the same system is employed. It’s simple, it works and it’s safe. So why change it?

Modern airliners are serious pieces of equipment, built to withstand severe forces from the external environment. Manoeuvring hundreds of tons of machinery is beyond human physical capability, so aircraft have been designed to use physics to enable us to fly them literally with a thumb and two fingers.

The basics of hydraulics systems are fairly simple. A small input into the system generates a large output from the system. This principle is used in the brakes of light aircraft, as seen in the video below.

However, when it comes to larger airliners such as the A320 and Boeing 787, the heavy mechanical linkages to the flight controls have been replaced by a number of flight computers and wires — hence the name, “fly-by-wire.”

Any input made in the flight deck by the pilots, for example, the landing gear, is sent via a wire to a computer. These messages are then sent to the computer responsible for controlling the landing gear, which then instructs pumps to pump the hydraulic fluid through the system, generating the action that the pilots needed.

The hydraulics system on the 787

The Boeing 787 has three hydraulics systems: the Centre, Left and Right, each providing 5,000 pounds per square inch (psi) of pressure. They are used to power most of the critical aircraft systems, such as the flight controls, landing gear, nose gear steering, slats and flaps.

As the flight controls are so important, their components are not controlled by a single hydraulics system. Instead, they are distributed across all three so that should one, or even two hydraulics systems fail, there is still sufficient aircraft control available from the remaining system.

Each system has its own reservoir of hydraulic fluid, the level of which we check before each departure. This fluid is then supplied to a pump which pressurises the system.

Related: What’s going on in the flight deck before departure?

The hydraulics system of the 787 on the left and the systems quantities and pressure on the right. (Photo by Charlie Page/The Points Guy)

Centre Hydraulic System

The Centre Hydraulic System is probably the most important of the three, powering the most power-hungry aircraft systems, such as the landing gear, nose gear steering, slats/flaps and also the flight controls.

To control the system, there are two electric pumps, namely C1 and C2. These operate in the same way and alternate in use between flights as the primary pump and the back-up pump. For one flight, the C1 pump will be the primary and then for the next flight, the C2 will be the primary.

Once the engines have been started, the primary pump — whichever one that is — operates continuously powering the system for the entire flight.

The back-up pump then only comes into use at times when a sudden loss of pressure would cause an issue. For example, when the slats/flaps are in motion or during takeoff and landing. It will also activate if the system detects low pressure at any other time.

Left & Right Hydraulic Systems

The Left and Right Hydraulic systems are identical. They only differ in the systems that they operate.

The Left Hydraulics Systems powers the thrust reverser on the left engine and then also some flight controls and wing spoilers. The Right Hydraulics System, unsurprisingly, powers the thrust reverser on the right engine and also some flight controls and wing spoilers.

The hydraulics control panel on the 787 Dreamliner. (Photo by Charlie Page/The Points Guy)

Both systems are primarily powered by their own engine-driven pumps from the engine on the relevant side. Like the Centre System, they both also have an electric back-up pump, which becomes active in certain situations, particularly at times of high demand.

The design of the system also means that should an engine fail, the back-up pump will take over from the primary pump and continue to power that hydraulics system.

So which systems do the hydraulics power?

As mentioned previously, the hydraulics feed the most power-hungry systems on the aircraft. However, the 787 is different to other aircraft types in one way, which we’ll come to in a bit.

Flight controls

Flying an aircraft depends on the pilot’s ability to move the control surfaces on the wings and this is done with the use of the hydraulics. Each time we move the control column in the flight deck, electronic signals are sent to actuators, which move the control surfaces. Even the lightest touch in the flight deck can give full deflection of the flight control surfaces.

As a result, to stop us from inadvertently demanding full deflection of a certain surface, the movement of the control column is “weighted.” This makes it physically more difficult to move the closer we get to the extent of its range of movement.

Slats & flaps

The slats on the front edge of the wing and the flaps on the trailing edge also require a large amount of power so are also controlled by the hydraulics. The operation works in the same way as the flight controls. We select the stage of flap we want in the flight deck and this sends an electronic signal to the slat and flap actuators, which drive the flaps into the desired position.

The flap control lever on the 787 Dreamliner. (Photo by Charlie Page/The Points Guy)

Ram Air Turbine

When you need this, you’re really not having a great day. The Ram Air Turbine (RAT) is a small propeller that automatically drops out of the underside of the aircraft in the event of a double engine failure (or when all three hydraulics system pressures are low). It can also be deployed manually by pressing a switch in the flight deck.

Once deployed into the airflow, the RAT spins up and provides a full 5,000 psi of hydraulic pressure to the flight controls connected to the Centre Hydraulics System. It also provides enough electricity to power basic aircraft systems. This means that even in the extreme case of a double engine failure, there is still enough power to fly the aircraft safely.

Thrust reversers

The massive amount of thrust generated by the engine is also beneficial on landing. After touch down, with the engine power at idle, we pull up a second set of levers to engage the reverse thrust. This causes a vent to open on the side of the engine and barriers to extend in the area where the bypass air flows.

These barriers block the bypass air from passing out the back of the engine and direct it forwards through the vents. Whilst the reverse system doesn’t directly increase the braking effectiveness, it does help the braking system by reducing the energy that the brakes have to endure.

Landing gear

The landing gear system on the 787 Dreamliner consists of two main landing gear assemblies and one nose gear assembly. Each main gear setup has four wheels, each of which has an electronic brake. The nose gear has two wheels, neither of which have a brake.

When airborne, to reduce drag, the gear is retracted and folded away into the belly of the aircraft, waiting to be used for landing. However, leaving the retracted gear exposed to the elements would still create a large amount of drag, drastically increasing both fuel usage and noise. To stop this from happening, the gear bays have doors.

Related: Brace for impact! How the landing gear on the 787 Dreamliner works

With the aircraft on the ground, or in fact in the air, it’s difficult to notice the gear doors. It’s only when the pilots move the gear lever in the flight deck that it’s possible to see them in action.

The landing gear lever in the flight deck is situated on the centre panel within easy reach of both pilots.

Close up view of engine and landing gear on Boeing’s 787 Dreamliner aircraft. (Photo by RyanFletcher/Getty Images)

During World War II, hundreds of accidents were attributed to pilots inexplicably raising the landing gear just before landing. On closer study, it was found that instead of lowering the flaps for landing, pilots were instead raising the landing gear. Why?

It was found that, combined with severe fatigue, the identically shaped levers for the landing gear and flaps were being confused and the wrong ones were being used.

Aircraft designers decided to change the shape of the levers so the landing gear lever felt like a wheel and the flap lever felt like a flap. As soon as these changes were made, these kinds of accidents stopped virtually overnight.

To raise the gear, we simply move the gear lever to the up position and this starts the gear retraction sequence. Firstly, the Centre Hydraulics Systems opens the gear bay doors. Next, hydraulic actuators drive the gear up into their stowage positions in the belly and nose of the aircraft. With the gear locked in position, the hydraulics system then closes the gear bay doors.

To lower the gear, we simply do the opposite. When the gear lever is moved to the down position, the gear bay doors open, and the wheels free-fall out of their stowage without the use of the hydraulics system. When in the down position, they lock into position to stop them from folding on touchdown.

What if the wheels don’t come down?

One of the worst fears some passengers have is the landing gear not coming down for landing. Whilst problematic, like all systems onboard the aircraft, there is a back up to the main system should it not work properly.

As the gear uses gravity to deploy, even in normal operation, the only thing hindering them is the gear doors, which are powered hydraulically. Should there be a problem with the normal system, we can use the alternate extension system.

This consists of a dedicated electric pump and fluid from the centre hydraulic system to extend the gear. Selecting the alternate gear switch to the down position releases the gear doors and locks which lock the gear in the up position.

This enables the gear to fall out of the gear bay naturally and lock into the down position.

Nose-wheel steering

Once moving, pilots need to be able to steer the aircraft around corners and this is done through two different sets of controls. Limited steering is possible using the rudder pedals, which sit underneath the pilot’s feet. Unlike in your car, the pedals under our feet have very little to do with acceleration and a lot to do with braking and steering.

A tiller sits outboard of each pilot and allows them to steer the nose-wheel up to 70 degrees. (Photo by Charlie Page/The Points Guy)

These primarily control the rudder on the tail (mainly used in crosswind landings) and also control the brakes. However, the main source of steering comes from the nosewheel steering tiller, which can turn the nose wheel up to 70 degrees.

Using power from the Centre Hydraulics System, we can use the tiller to turn the nose wheel up to 70 degrees in either direction. Should there be a failure in the Centre System before landing, it may leave us unable to taxi off the runway. In this situation, we would have to ask ATC to arrange for a tug to attach to the aircraft and tow us to the parking stand.

A word on the brakes

Stopping a 200-tonne aircraft landing at 160 mph requires a lot of braking force and to do this, the 787 has eight wheels on the main gear assembly — each of which has a brake unit. On other aircraft types, the brake units are powered by the hydraulics system. An electrical signal is sent from the flight deck to hydraulic actuators near the main landing gear. Here, hydraulic fluid at 3,000 psi is used to force the brake unit against the wheel, thus slowing it down.

This system works fine, but the pipes and actuators that form this part of the hydraulic system come at a considerable weight cost. Extra weight means more fuel burn, which in turn increases costs and carbon emissions. What if the brakes could be powered a different way?

The brakes on the 787 are electrically powered. (Photo by Charlie Page/The Points Guy)

That’s the case on the Dreamliner. Designers did away with the use of the hydraulic system and all its associated architecture and instead used electricity to power the brakes. When the pilots press on the brake pedals, an electrical signal is sent directly to the brake unit on the wheel. Here, electrically powered actuators are used to press the carbon brake disc against the wheel, slowing it down.

By changing to electric brakes, a 787-8 saves 64 kilogrammes (141 pounds) per aircraft and a 787-9 saves 111 kilogrammes (244 pounds). The brakes are also known as “Plug and Play” because electrical wiring replaces the traditional hydraulics and it’s much easier and quicker to change the brake units when needed. Smart features also allow engineers to monitor the brake performance more closely, giving a real-time measurement of wear on the carbon disks.

Bottom line

Hydraulics systems are a great way to exert more pressure on areas some distance away, using a smaller input pressure. The reality of modern airliners means that the forces which the pilots can generate by hand are not enough to move the aircraft systems. As a result, hydraulics systems are employed to generate extra power.

They are so important that not only are there backups in case of a failure, there are backups for the backup. Even if one, two or all three systems were to fail, the architecture is designed in a way that the aircraft can still fly safely with whatever systems remain.

Featured photo by Charlie Page/The Points Guy.

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