Airliners have a lot of moving parts. The control surfaces, brakes, landing gear and many other components need a lot of power to move them. A very simple device is used to do much of the heavy lifting: the hydraulic cylinder (or actuator). It’s a lot of power in a small package…
You’re already familiar with hydraulics. You see and use them every day! The power steering and brakes in your car use hydraulic actuators. The car lifts at tire stores use monster sized hydraulic cylinders. If you have a tractor or large riding mower, there’s a good chance it has a hydraulic system to move certain parts. And, of course, every construction site has backhoes and dozers that use hydraulic cylinders to move their heavy appendages.
Hydraulic power is used to move heavy components on airliners because it’s reliable and packs a lot of power for its weight. The hydraulic system on the Boeing 757 and 767 distributes fluid at 3000 psi to various components. Larger jets, like the Airbus A380, use hydraulic pressure as high as 5000 psi! This super high pressure requires smaller hoses and less fluid which saves a lot of weight on a big airplane.
Take a look at the hydraulic actuator pictured below. This cylinder is the “muscle” that raises and lowers the nose gear. It’s only 18 inches long and has the diameter of a paint can, but with 3000 psi fluid pumping through it, the cylinder has the power to hoist the nose gear into the wheel well while the jet accelerates into the sky. Now that’s a lot of power in a small package!
More hydraulics: This Airbus A-319 uses small hydraulic actuators to raise the spoiler panels on landing. Check out all the plumbing under those panels.
Hydraulics work great, but 3000+ psi is a lot pressure! What if the system springs a leak? No worries. The really important stuff on the jet, like the elevator, ailerons, rudder, and brakes can be controlled by multiple systems. The 767 has 3 separate hydraulic systems and even has backups for the backup systems!
There’s an old saying in aviation that we use to describe dependable systems, and it certainly applies to hydraulic systems: “Works great! Lasts a long time!”
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