Hydraulics in the plane

Fluid flying: the hydraulics

Tim Takeoff
1 picture
5 minutes

As most people are aware, aircraft fly on aviation fuel. But there is another fluid that is essential for the operation of modern commercial aircraft: hydraulic oil.

In today’s light aircraft and during the infancy of commercial aviation, all of the on-board controls were operated using cables or sliding rods. This had many advantages, as it was possible to transfer forces without delay or loss. Since the aircraft were relatively small, the pilots were able to operate everything with simple muscle power. While flying a modern wide-body aircraft on a long-haul flight with the strength of one’s own “guns” would certainly look amusing, it is practically impossible. It would be like driving a modern SUV without power steering. Only that the SUV weighs more than 300 tonnes instead of three and flies through the air. The forces of the rudder alone, or the raising or lowering of the under-carriage and landing flaps requires considerable pulling and retaining forces.

The power of fluids

As early as 1795, Englishman Joseph Bramah had devised a hydromechanical machine powered by pressurised water. This allowed him to use the power of the water to increase the force applied 2000 fold. In other words, it had the same effect as the power steering in our cars. A fluid flows through narrow pipes that are under constant pressure. This fluid does not have to be water; in theory, it could function with any fluid. However, thanks to their chemical properties, modern hydraulic oils have prevailed, even in aircraft manufacturing.

Hydraulics as a transmission system

Hydraulics make it possible to transfer forces over variable distances and even “around corners”. A simple, mechanical transmission system always requires a shaft or sliding rods and cables to guarantee drive. In an aircraft, space is obviously at a premium. This is why hydraulics quickly became very useful. It is possible to maintain control by a wide variety of means. Since the entire system has to remain pressurised, pumps are required on board that guarantee permanent or function-related pressure. In modern aircraft, sometimes certain hydraulic pumps are only used for short periods, e.g. to raise or lower the landing gear. For the rest of the flight, they are simply locked away in the body and thus “depressurised” in order to prevent them from being subjected to a permanent load.

The basic principle of the hydraulic system

An individual hydraulic system essentially comprises a reservoir that contains the hydraulic oil and can be pressurised. Pipes lead out of this to the motors to be controlled, which are also known as actuators. This is where valves are located, which feed pressure to or release pressure from the controls requiring operation. Now there are essentially two different principles by which the control commands of the pilot (or autopilot) can reach the actuators. These not only control the rudders on the wings, but also the under-carriage, thrust reverser, landing flaps, brake flaps or the yaw rudder.

1. Conventional flight control

In a conventional control system, such as that of a Boeing 737, the control pressures are transmitted purely mechanically via cables to the actuators or directly to the rudder. Of course, simple circuits are also already being used here to lower or turn off certain rudders at specific speeds. However, this is a very rudimental and heavy setup, as the entire mechanical system also has to have a certain amount of redundancy. This results in a lot of heavy metal in the aircraft, which it would be best not to have.

2. Fly-by-wire

In order to save weight, engineers came up with what is known as the “fly-by-wire” system. This turns the input of the pilot into “smart” input, which is only sent direct to the rudder in an emergency. During normal operation, all of the commands are sent to a sort of primary flight computer. This checks the input in fractions of a second. If the input complies with the logic, the computer sends the commands via a cable to the respective hydraulic actuator. The hydraulic system adjusts and the rudder moves. Other sensors check the executed movement and send the signal back to the primary flight computer. If the values of the command and the sensor correspond, the computer is happy. However, if something doesn’t add up, it’s possible to readjust at once, switch off or immediately inform the pilot. Little is seen of all these processes in the cockpit, because the control system is as fast as lightning. It feels just like flying a conventional aircraft. Only one that is considerably more “clever”.

The advantages

It is already clear to see that flying by hydraulics offers many advantages due to the flight control alone. Countless safety features can be installed using software. Hydraulics also offer additional mechanical advantages in addition to simply being weight-saving. The system has extremely high positioning accuracy and is capable of transferring huge forces in a small space. It is infinitely variable and has a long service life, as the oil used is self-lubricating and cooling. By measuring the pressure, it is also easy to monitor and to react immediately in the event of faults.


As no system is error-proof, an aircraft must have as much in-built redundancy as possible. A hydraulic system can lose some or all of its pressure as the result of a leak, which leads to a loss of the connected control system. As one can well imagine, one system is not enough. This is why in modern aircraft, there are two to three systems in operation, and sometimes even multiple closed-loop circuits, in order to guarantee reliable operation. For example, there is a left-hand, a right-hand and a central system. However, this does not mean that the entire left-hand or right-hand side is operated via one system. That would of course be counterproductive in the event of a fault. There is therefore a mix within the systems and the control surfaces, so that in an emergency, only parts of the control system fail and the “job” can be taken up by other surfaces. For example, each side of the pitch elevator is connected to a different system, so that if the worst comes to the worst, one side always remains operable.

Cables and canines in the cargo hold

And if it really does come to the crunch, some aircraft often still have a residual pair of cables, just like in the good old days, to use as a last resort. But until such a scenario occurs, a well-trained crew will have certainly already managed to land the aircraft. On your next flight in an Airbus A320, pay attention to the typical “barking” when the engines start up. It sounds as though a pair of dogs have been stowed away in the cargo hold. The noise is particularly loud when sat in the central rows of seats. This “barking” is simply the hydraulic systems being brought to life by the pumps in the engines. Don’t panic – now you know what this is!

by Tim Takeoff

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