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In the last couple chapters we looked at the how the flight controls on an airliner work. But makes the ailerons, flaps, spoilers and other controls actually move? In most airliners these are powered by hydraulics.

Most major systems on an airliner work off the hydraulic systems. The primary flight controls as well as the flaps, slats, spoilers, stabilizer trim, landing gear, nose wheel steering, and even the brakes are usually hydraulically powered.

It's pretty important stuff. If you lost all the hydraulics on a modern airliner you'd be in a world of hurt.

So let's take a look at this very important system.

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At one time, all flight controls were powered by "armstrong". The pilot's yoke was connected by rods or cables to the ailerons, elevators and rudder. The main reason airliners have a yoke instead of joystick (like on a fighter plane) is the extra leverage was needed just to move the larger control surfaces.

As planes got bigger and faster the control surfaces had to be larger and, due to the higher speeds, the air loads on those control surfaces became greater. Engineers were able to use some fancy aerodynamic tricks like control tabs to compensate.

On the KC-135, when I moved the ailerons, there was a miniature flight control on the back of the aileron called a "control tab". It would move in the opposite direction and help to "fly" the aileron in the direction I wanted it to go.

Control Tab (sometimes called a servo tab)
This worked, to a degree. The KC-135 flew like a truck. Checking the flight controls on the ground was like working out at the gym. We did have hydraulic assist on the rudder. It failed on me once, and landing the jet was like working out on the stair-master. Try moving a garage door with your legs sometime.

After the 707/DC-8 era, the engineers decided that they'd pushed manual flight controls about as far as the could and they needed something better. Everything from the 727 onwards has had hydraulically powered flight controls. The 727 still had manual backup, but even that has been dispensed with on newer jets.

So how do hydraulics work? At its most basic it's a way of transferring force. Liquids cannot be compressed. If you have a pipe or a hose full of liquid and you push the liquid from one end - it's going to push at the other end. Simple enough.

On an aircraft, we generate force using hydraulic pumps. These pumps can be driven by the plane's engines or electrically from the plane's electric system or both. We're talking about a lot of force. More than 3000 pounds per square inch. Don't ever get near a control surface or gear door when a plane's hydraulics are powered - it moves with enough force to cut you in half.

When I move the yoke or rudder pedals a series of valves meter some of that force (fluid under pressure) through lines out to the appropriate flight controls. The fluid goes to a device called an "acuator", which is essentially a piston. That piston can make the control surface move in either direction.

Hydraulic system operation (simplified)
So instead of my puny muscles, I've got 3000 pounds of hydraulic force courtesy of the plane's engines moving the flight controls.

So far so good, but I don't want to bet my life on a hydraulic actuator. That's why we've got 3 of them on each flight control, each powered by a separate hydraulic system. Mister Boeing (and Monsieur Airbus) don't like a single point of failure. If it's important we've got at least two and probably three of 'em.

Let's take a look at the hydraulics on a 757. This diagram has been greatly simplified so that pilots can understand it.

Boeing 757 Hydraulic Systems
We have three systems left, center and right. The left system is powered by a pump driven by the left engine and the right system is powered by the right engine. The center system is powered by two electric pumps. The left and right system also each have a backup electric pump.

Most of the important stuff like the landing gear and flaps are on the left system. Everything else is on the right system. All three systems power the flight controls independently. That's what gives me a warm fuzzy. Each flight control has 3 actuators each powered by a different hydraulic system.

Airbus uses a very similar setup except they use Blue, Green and Yellow. Doesn't really matter. They could have called them Moe, Larry and Curly. Actually I kind of wish they had.

Here's what the 757 hydraulic control panel looks like. Just some switches to turn the pumps on and some warning lights to tell us if something is wrong. Mister Boeing likes a dark cockpit. If nothing is lit up then everything is working the way it's supposed to.

757 Hydraulic Panel
The only other indications we have is a digital readout of system pressure and how much fluid is in each hydraulic reservoir.
Hydraulic System Status
The RF means the left system fluid is low and needs a Refill
Normally you turn the hydraulics on and they work. No further action required. There are some things that can go wrong. The most likely culprits are either a bad pump or a leak somewhere.

A pump can either overheat or fail entirely. If it overheats we're going to turn it off so the end result is the same. No big deal. We've got an electric backup pump so that system is still going to work.

Since the left system on the 757 has all the really important stuff we have yet another means of powering it. We can power it from the right hydraulic system.

Now, we never ever want to connect one hydraulic system directly to another because a leak in one would empty them both. Instead we have a clever device called a PTU (Power Transfer Unit). The right system drives a hydraulic motor, which is mechanically connected to a hydraulic pump which can power the left system.

A leak in a system is a bigger problem. At 3000 psi a leak will probably empty the system of fluid in a very short time, possibly just a few seconds. That hydraulic system is probably gone and we're not getting it back. Still not a crisis. We have two other hydraulic systems. Even if it's the left system we have alternate means of lowering the landing gear and flaps.

So what if we lose both engines? Well then we've got big problems, like finding a place to land. We can still power the hydraulics, however. The Auxiliary Power Unit (APU) has a generator which can power our electric backup pumps.

OK smart guy, what if the APU doesn't work? Now what?

We're not completely screwed just yet. We've got one last trick up our sleeve. The RAT!

Not THAT kind of rat.
RAT stands for Ram Air Turbine. It's a little propeller with a hydraulic pump attached to it. If we lose both engines this little guy is spring loaded to pop out and power the center hydraulic system. This lets us steer (important) while we hopefully get an engine restarted or find a place to land.

Air Canada 143 (Gimli Glider)

This kind of RAT
Could we ever lose all three hydraulic systems? It's exceedingly rare but it has happened. Most famously United Flight 232, a DC-10, lost all hydraulics after its center engine failed catastrophically and took out hydraulic lines for all three systems. The crew was able to steer the aircraft to a crash landing by using the engines and more than half the people on board survived. Tragic, but it could have been worse.

United 232 Sioux City Iowa

On a happier note, in 2003 a DHL Airbus 300 was hit by a missile after taking off from Baghdad. The missile took out all three hydraulic systems. Once again the crew was able to use engine thrust to steer the plane, this time to a successful landing.

DHL A300 Baghdad

So what's next? Newer aircraft like the Boeing 787 and Airbus A380 are starting to move away from hydraulics and use electrics to power more systems. The manufacturers claim savings in weight and greater efficiency over hydraulically powered systems. So next time around we'll take a look at the electric systems.

Extended (Optional)

Originally posted to Major Kong on Wed May 01, 2013 at 05:20 AM PDT.

Also republished by Aviation & Pilots, Kossack Air Force, Central Ohio Kossacks, and Community Spotlight.

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