What is “Fly-by-wire” technology in aviation? What benefits does it have and why is it used in modern airplanes today?
In: Engineering
So fly-by-wire (FBW) means a computer sends an electronic signal to the things that move the control surfaces. (An ELI5 of conventional primary flight controls is a whole other thing…)
Previously, steel cables were pulled one way or the other when pilots moved controls in the cockpit. On the other ends of those cables in recent planes, there would be hydraulic cylinders that would move the control surfaces according to how the cables pulled on them.
FBW takes the cables out, along with a bunch of pulleys and bellcranks and mount brackets, and replaces them with just a few electrical wires. Sensors on the cockpit controls, tell a computer what the pilots are trying to do. The computer sends signals to electronic servovalves (sorry, not very ELI5 term there; will try to ELI5 it below) on the hydraulic cylinders at the control surfaces, which then make the cylinders move the same way the steel cables would have.
Why bother then? Well for one thing, we save some weight and space by removing those cables and the stuff that mounts them. Even with the extra sensors and computers, the system weighs less and takes up A LOT less space. And you can put the computers anywhere you like on the plane, so you have a little flexibility with balance and fit. Also, those old steel cables would stretch over time or even break (I’ve seen an autopilot aileron cable on a CRJ700 holding on by a no-shit single strand at C check), but there’s nothing pulling on the wires to damage them in normal use. And a FBW system can more easily include autopilot, so you can get rid of the additional servos and stuff from that system. It’s a big positive.
Even Apache helicopters had a FBW system as a backup to their mechanical-linkages flight controls since the late 1980s. The idea was that a computer watched the controls and cylinders, and if a pilot was trying to move controls but the linkages didn’t move, then the computer would take over and tell the cylinders what to do. Boeing improved this system a lot in the AH-64D.
So, a servovalve…. In our situation, it takes and electronic signal as a command and uses electricity to move a valve. It’s kinda like how your cruise control moves the throttle in your car’s engine. But here, the valve is letting hydraulic fluid pressure go one way or the other to move a cylinder, or stopping it so the cylinder doesn’t move.
Any why do we need the hydraulics? Well, on small planes we don’t. But then they don’t have FBW either, and just one of the computers can cost more than your Grandpa’s Cessna 150. On a plane weighing a couple hundred thousand pounds moving at 500 mph, moving an aileron or elevator or rudder enough to actually affect the airplane some time today, takes some torque. What’s more, if we just geared an old cable system to gear down pilot input and make it torquier (is that a word?), then they’d have to move the controls really really far to do anything. If we don’t gear them down then pilots would have a hard time moving the controls once they’re moving fast enough to take off. And no one wants a slow airplane. So we use the small miracle of hydraulics to push hard enough to move those control surfaces without pilots having to be super strong.
Long, I know. Sorry. Hope it helps.
Source: Currently a commercial aircraft maintenance tech, and been working in aviation a long time.
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