What is “Fly-by-wire” technology in aviation? What benefits does it have and why is it used in modern airplanes today?
In: Engineering
avionics technician here, a bunch of right/partially right answers in this thread but I’ll expand where I can.
Most modern jets (anything built in the past 40ish years) even if they aren’t fly-by-wire, they’re partially fly-by-wire. There is a transducer on the rudder pedals as well as the control stick/yoke that measures pilot input and either adds to takes away authority depending on conditions.
Fly-by-wire aircraft allow you to have fewer moving parts, there, it makes the aircraft easier to maintain. Control surfaces can be combined so you can have a larger control surface. For example, the entire trailing edge of the F-16’s wing is a flaperon. It combines the function of a flap and aileron. You only use the flaps for take-off and landing and you’re only going to be making minor corrections with using the ailerons during those stages of flight, but, if you need maximum maneuverability during a dog fight, your roll rate is limited by the size of your aileron (technically, in every modern fighter, the horizontal stabilators will move asymmetrically). Without fly-by-wire, the trailing edge of the wing would be split between a flap and an aileron, each half the size giving the pilot less control surface. Aircraft like the B-2 would not be possible (or mindbogglingly complicated) since it doesn’t have a rudder, its directional controls work by opening the speedbrake on one side which also acts like an aileron.
The other benefit would be, each aircraft will fly exactly like the next one. In fly-by-wire aircraft, the cables must be checked after a certain number of flight hours for tension because, just like the brake cables in your bike or the throttle cable on your car, it stretches over time. When it stretches, it essentially takes away authority from the pilot denying him/her full deflection of the control surface. Each cable has to be checked for tension and if you can’t get it tight enough, it needs to be changed. After the cables are tightened, you need to make sure pulling back on the stick deflects the horizontal stabilators x inches and kicking the rudder pedals full right or left deflects them by a certain amount but the manual gives you some leeway with how tight the cables need to be so each aircraft is slightly different than the next. In a fly-by-wire aircraft, each aircraft is identical.
Thirdly, you can adjust gains more easily. In a Cessna 172 (or any general aviation aircraft that’s not fly-by-wire) if I pull back on the yoke all the way on the ground at take-off, the elevators will give you full deflection. If you pulled back on the yoke at 15,000 feet or whatever the ceiling of a Cessna 172 is going it’s top speed, the elevators will move the same amount and you might be in for a bad time. In an F-18, if you were taking off, pulling back on the stick while on your takeoff roll, a computer will calculate how fast you’re going, the density of the air, and your angle of attack and decide how much the stabilators will deflect. Since you’re going relatively slow it will probably give you the full or close to the full amount. If you’re at 15,000 feet going 800 knots, pulling all the way back on the stick and the control surfaces reacted the same way, you’d instantly over-g the aircraft and cause significant structural damage, the flight control computer will not allow you to do that. If you’re about to crash into the side of a mountain, there is an override switch. The fight control computer will do something called gains scheduling which will give or take away inputs depending on what phase of flight you’re in. If you’ve got the gear down and coming in for a landing, it will give you all the authority you want. If you’re flying straight and level at cruising altitude/speed, it will take some authority away to be less twitchy.
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