What is “Fly-by-wire” technology in aviation? What benefits does it have and why is it used in modern airplanes today?
In: Engineering
In classical (pre fly by wire) planes the various parts of the plane used to control the flight, are connected directly to the control elements in the cockpit with a long rod or steel rope. So if the pilot pushes the control stick, it pushes the rod, which is connected to the rudder of the plane. So the movement of the joystick controls directly the rudders. For this you need pretty complex mechanics and it’s not so easy to make put this through large airplanes.
In the fly by wire technology the control stick in the cockpit is basically just an electronic sensor, which converts the position of the stick to an electronic value. This value is transfered by an electric wire the rudder, where a motor moves the rudder of the plane to the position set by the electronic value.
This way you just have to put electric wires through the plane and not complex mechanics, which is easier and requires less maintenance. And when you have electronic values it’s much easier to integrate everything with computers, which helps the pilot to fly the plane.
The downside is that fly by wire technology requires electricity. Therefore these airplanes have an emergency battery or a small wind turbine, which can provide emergency power. That way the pilots can still control the airplane, even if the main power from the engines is not available (e.g. all engines broke)
Aircraft used to have the control surfaces directly, physically connected to the control inputs in the cockpit via cables. Any movement the pilot made in the controls was transferred directly to the control surfaces to make the plane move.
Modern aircraft have performance capabilities that far exceed what the human body can tolerate, so computer intervention is required. In fly-by-wire systems, the control inputs are captured by the flight control computer, audited, and altered as needed to maintain stable flight, optimize the movement, and protect the pilot. The the tailored signals are digitally transmitted to the motors that control the aircraft’s moving parts.
Fly by wire or drive by wire simply means a mechanical component of your car or plane is controlled by an electronic device rather than mechanically.
For example, throttles in cars used to have a physical cable that ran from the pedal to the throttle body, when you pushed the pedal down the cable tightened up and opened the throttle body to let more air and fuel in (speed the engine up).
Drive by wire on the other hand means the press the gas pedal down, an electronic signal is sent to the throttle body, and a servo motor in the throttle body actuates to open it.
As others have explained – it means the pilot’s controls send a signal to a motor that moves the control surface. As opposed to an old direct mechanical connection from the controls to the control surfaces or a hydraulic connection.
Benefits can include:
– less effort to move the controls (since the resistance on the wheel/stick/pedals is just ‘fake’ feedback and not the actual force to move the control surfaces against the wind)
– not prone to wear/stress/leaks like physical or hydraulic connections.
– can be overridden. The onboard computer can override inputs that would endanger the aircraft.
– can allow for unstable designs.
The last one is interesting. Historically, aircraft would be built to be stable (i.e. they’d naturally return to stable horizontal flight unless the pilot is acting on the controls). If you grab a kite by its nose and pull it through the air, that’s a fairly stable scenario. But this kind of design kinda means if you’re trying to turn, you’re having to fight against the aircraft that wants to go back to level, straight flight.
Modern fighter aircraft are sometimes designed with “relaxed stability”. This is a bit like grabbing a kite by the rear and trying to push it through the air – it’ll just buck all over the place trying to get away from stable level flight. Such a fighter would be near impossible to fly with physical controls – instead fly-by-wire systems are used which can compensate for this and keep the aircraft stable, while also allowing for greater manoeuvrability when the pilot wants it.
The major reason that is not addressed in most of the other posts is that it is a major weight savings to have wires connected to the control surface motors instead of hydraulics that go all the way to the cockpit.
This also allows more room for cargo space and fuel, since the hydraulics are no longer there.
The fly-by-wire system also allows the flight control computer to override errant pilot inputs, but this comes with the downside of allowing the flight control computer to override *valid* pilot inputs when the flight control computer receives faulty data from sensors.
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.
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.
Fly by wire aircraft still use hydraulic actuators for flight controls, there is just no control cables going from a yoke/stick to the power control units, just wires.
Non fly by wire aircraft with hydraulic flight controls also do not require any appreciable amount of effort to move the controls during normal operation.
A bicycle is not fly-by-wire. You turn the handle and the wheel turns.
Riding a bike in a video game is fly-by-wire. You move the joystick, the game translates that electrical signal to turn the bike.
Benefits of fly-by-wire is you don’t have to be physically strong to control a giant airplane, machine does it for you based on the electrical signal you send it with your controls.
An aircraft is aerodynamically stable along an axis of rotation if disturbances will tend to attenuate over time and converge on a steady position on that axis.
An aircraft is aerodynamically unstable along an axis of rotation if disturbances will tend to amplify over time, preventing the aircraft from maintaining a steady position on that axis.
An aircraft is statically unstable along an axis of rotation if steady flight tends to become unstable as a matter of course regardless of control input or disturbances. An aircraft can be statically stable and dynamically stable, statically stable and dynamically unstable, or both statically unstable and dynamically unstable; an aircraft cannot be statically unstable but dynamically unstable, that is impossible.
If the aircraft is dynamically stable, the pilot can relax the control surfaces and maintain steady flight. The impacts of turbulence and other disturbances will require only error correction
If the aircraft is unstable, the pilot has to constantly try and prevent the aircraft from wanting to go all over the place.
Intuition tells us that instability along one or more axis of rotation would be a bad thing, right?
In most cases yes, but instability allows for much better maneuverability because the airframe’s design isn’t trying to return to steady flight while the pilot is deliberately trying to depart from it. This instability is so severe that human operators cannot be expected to overcome it while still maintaining any degree of mission effectiveness. Thus, a flight computer sits between the pilot and the control surfaces. The flight computer is constantly parsing flight instrument data, pilot control data, and control surface positional data to adjust the control surfaces as needed.
The F-14 Tomcat was mechanically controlled and aerodynamically stable.
The F-15 A/B/C/D are similarly mechanically controlled and aerodynamically stable but the nearly identical F-15E is fly-by-wire.
The F-117 Nighthawk, despite not being a fighter in any sense, was aerodynamically unstable in all 3 axis and a right bastard to fly even with a digital flight system.
The F-16 is the first purpose-built fly-by-wire only combat aircraft. It was designed to be unstable from the beginning and that contributed immensely to its famed agility. The F-16 is both statically unstable and dynamically unstable; static instability means that it will tend to become turbulent even from steady and undisturbed flight, there’s no controlling it without a flight computer.
Some fighters, such as the F-22, F-35, and F-18 are unstable in some axis (mainly pitch) but stable or neutral in other axis.
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