Aircraft Hydraulics Explained in 5 Simple Steps

The size of both planes and helicopters have massively increased since the early times of aviation.

Moving a heavy aileron or rotor blade while it’s moving at Mach 0.9 without any help is pretty unrealistic ❌

Aircraft hydraulics fill this very specific need, but there are many other benefits as well!

We’re looking at:

🔸 What are the basic principles of hydraulics?
🔸 How exactly does a hydraulic system work?
🔸 Why use hydraulics rather than alternative methods?

Let’s find out ⤵️

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The Basics of Hydraulics

It all starts with a 17th century Mathematician called Pascal, who probably had way too much time on his hands 😁

He discovered something interesting, which is now called Pascal’s Law (yea, we agree – very original):

“If a force is applied to a liquid in a confined space, then this force will be felt equally in all directions”

To visualise this, have a look at this image, which shows a force pushing a liquid within a container:

The force that we need to move a flight control is caused by this pressure. It’s not created by a hydraulic pump, it’s made when we try to compress a fluid 💢

We need to try to compress fluid in a container where it cannot escape. That’s where pressure will build up! Without ‘restriction’ of fluid, there can be no pressure.

Which means that:

If we apply a force of 1000 Newton to the piston showed above, which has an area of 0.002 m², the result is a pressure of 1000/0.002 = 500 KiloPascal within the fluid.

Let’s see how we can use this ⤵️

How Hydraulics Work in 5 Steps

Right, so how does a hydraulic system actually move an aileron (or anything else)?

Let’s cover the basics first, these are the main components of a hydraulic system:

We have:

🔸 A reservoir with hydraulic fluid, which delivers the fluid to the pump and receives it from the actuators. The fluid itself has a high flash point, lubricates the system, acts as a coolant, and has a viscosity that makes it flow easily across different temperatures 🌡️
🔸 A pump, which can be hand, engine, or electrically driven, to move the fluid around the system
🔸 A selector valve, which enables the pilot to change the flow direction of the hydraulic fluid, and provides a return path from the actuator

🔸 A pressure filter, to keep the fluid clean
🔸 An actuator, to move the actual control surfaces or landing gear
🔸 A relief valve, to make sure excess pressure can be removed

These all work together in 5 basic steps:

Let’s put these 5 steps into the diagram that clarifies how it all works:

1) Pilot Input

It all starts with us moving the controls. This sends a signal to the aircraft’s flight control system.

If it’s a fly-by-wire system, this is done via an electronic signal.

If it’s more old-school, the system is mechanically linked to sensors that pick up the movements from the control system en relay those movements.

2) Selector Valve Operation

The signal from the pilot’s input will arrive at the selector valve. The selector valve will then rotate in the direction that is required to achieve the result the pilot wants ✅

So as an example, if we want to move the actuator down, we need to rotate the selector valve 45 degrees anti-clockwise:

3) Hydraulic Fluid Flow

This movement of the selector valve will allow the fluid to flow from the pump, into the selector valve, which directs it to the top side of the actuator:

4) Actuator Movement

As we’ve discussed before, the added pressure of this fluid within the actuator causes it to move down ⬇️

Meanwhile, the remaining fluid in the bottom compartment will be able to flow back into the reservoir.

5) Control Surface Movement

This actuator is connected to the control surfaces we want to move. So by the actuator moving down, it can be pulling a rod that is connected to an aileron, which makes it pitch up.

4 Reasons Why Aircraft use Hydraulics

We use hydraulics for a few reasons, let’s go over the main ones:

1) High Power Output

Blades and ailerons are not the only components that are getting heavier.

Landing gears, tail rotors, wheel brakes, speed brakes, and many others are often impossible to move on today’s aircraft, unless they’re electrically operated.

Realistically, a human can create a force on a surface of up to a few thousand newtons. To move a B737 aileron at cruise speed, you’d need over 10 thousand Newtons (depending on the airspeed)! Moving it yourself would be impossible ❌

2) Weight Efficiency

Now, there are ways to amplify forces that have nothing to do with hydraulics, like pulleys and other components.

However, hydraulic systems have a high power-to-weight ratio. They’re compact and lightweight compared to other mechanical methods, allowing for higher power outputs without adding unnecessary weight to an aircraft.

A pressurised fluid system doesn’t lose as much energy over long distances compared to mechanical systems. On top of this, it gets rid of a lot of mechanical (and heavy) linkages that would otherwise be required!

3) Precision and Responsiveness

Hydraulics are also pretty good for making sure that the exact amount of force can be applied to the control surfaces.

Because hydraulics use fluid, which is incompressible, the force is transmitted almost instantly to the relevant actuators.

So, when you move a control, it doesn’t take very long to reach the control surfaces at all!

4) Reliability

Not only are hydraulic systems pretty reliable, they also usually have redundant systems in place. If one part fails, another can take over. This is called failsafe:

“Fail-safe generally means a design such that the airplane can survive the failure of an element of a system or, in some instances one or more entire systems, without catastrophic consequences.”

In modern commercial airliners, there are often 2 or 3 hydraulic systems that can continue operations if one fails.

Conclusion

Hydraulics are essential in modern aircraft because they allow us to control heavy components like ailerons, landing gear, and brakes with precision. They rely on Pascal’s Law to use fluid pressure, which makes it possible to move parts that would be impossible for humans to shift on their own, especially at high speeds!

Multiple components like selector valves and actuators all work together to go from the pilot input, all the way to control surfaces movement.

We don’t publish all our Notes from the Cockpit (like this one) publicly, some are shared only by email. Get the next one sent straight to your inbox ⤵️