A Guide to Fly-By-Wire

Welcome! Today since I can’t think of a plane for PoTP 4 and people don’t seem to understand what FBW is, here’s a guide!

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A simple diagram showing how FBW is able to cut down on the amount of items required, saving weight and maintainace compared to a traditional mechanical set-up.

  1. Origins
    Fly-By-Wire, or FBW was first introduced with a FBW flight control system was the Avro Canada CF-105 Arrow, however it was unfortunately cancelled due to political reasons.

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One of the first commercial aircraft to have FBW was the famous Concorde. In fact, one of the early production aircraft had sidesticks installed! The system could apply auto-stabilisation to the aircraft in flight, smoothing out the ride for the passengers as well as automatically applying opposite yaw if a engine failed. Interestingly, the backup for if the control column somehow got jammed was to have strain gauges fitted to the stick which then sent electrical signals back down the system, ignoring the fact that if the electrical system failed, Concorde would have no way of controlling herself beside using the manual trim wheel.

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Another famous aircraft to use FBW is the F-16, aka the Viper. It was the first production aircraft designed to be inherently aerodynamically unstable; that is when the pilot releases the controls the plane would not naturally return to straight and level flight. Being unstable helps the aircraft be more manoeuvrable as the aircraft would want naturally want to turn instead of “resetting” back to straight and level flight. To counter the tendency to depart from controlled flight and thus avoid the need for constant trim inputs by the pilot, the F-16 has a four-channel FBW system. Just like the well known Airbus style FBW, it manipulates the control surfaces in such a way as to produce the desired result without inducing control loss. The flight control system conducts thousands of measurements per second on the aircraft’s flight attitude to automatically counter deviations from the pilot-set flight path; leading to a common aphorism among pilots: “You don’t fly an F-16; it flies you.” However, this leads to the problem in that if somehow the FBW malfunctions, the pilot is left with no control of the aircraft as there is no mechanical backup whatsoever. This would normally be dangerous but due to the high level of redundancy it is more likely that if the FBW system fails you would already be in a situation which requires you to exit the aircraft immediately. Also like the AB FBW, the flight control computers further incorporate limiters governing movement in the three main axes based on attitude, airspeed and angle of attack (AOA); these prevent control surfaces from inducing instability such as slips or skids, or a high AOA inducing a stall. The limiters also prevent maneuvers that would exert more than a 9 g load on the airframe.

Now to the most well known use of FBW in a commercial jet- the Airbus A320 series of aircraft.

In Airbus’s words: “Incorporating fly-by-wire controls on the A320 allowed Airbus to tailor the aircraft’s computer flight control laws – adapting them to the pilots’ side-stick controllers that replaced previous-generation control yokes – while also introducing flight envelope protection. As a result, flight safety was greatly increased and the crew’s workload was reduced.”

Now this is the controversial part. See http://www.airbusdriver.net/airbus_fltlaws.htm for the full details on how this works, however the gist of the debate is that the FBW system Airbus use is too limiting. A simple example is that just to push the plane past 33 degrees of bank, pressure on the stick is needed. If pressure is removed past 33 degrees, the computer automatically banks the plane back to 33 degrees, no more, no less.

One side argues that the pilot always knows better and that he/she should be in control at ALL times and not be limited by this silly French computer who thinks it knows better. The other side says that the laws such as alpha protection and Vmax protection are necessary to stop people who think they know better to bring the plane out of danger. What do you think?

  1. Pros/Cons

Advantages:

Fly by wire replaces mechanical systems, thus requiring less maintenance and only needing to fix the computers involved.
Weight and space consumed will be comparatively lesser.
Control performance is relatively good considering that all feedback is artificial.
Aircraft’s computer perform functions or corrections without pilot’s input. The stick on Airbii acts more like a directional indicator from what I’ve read, meaning instead of saying “I want to pitch up” you instead tell the computer “I want to climb 20 degrees” and it will hold it for you.

Disadvantages:

It is more costly than a mechinical system
They are more complex as coding is required instead of just a basic knowledge of what goes where.
Loss of electric power due to a double/quadruple engine flameout would mean that all the systems go dead and the aircraft is only controllable with the manual trim wheel.

  1. A vs B
    It would eventually get to this…

In essence, Airbus have hard limits (in Normal Law) that CANNOT be exceeded no matter how hard the control input. The only way to get around this is to put the plane into Direct Law and lose all the protections. I have however heard that the plane would “handle funny” when in this mode as it was designed with the help of the computers to stabilise the aircraft.

Boeing have a different philosophy in that they believe that the pilot(s) should be able to do anything they want, even a barrel roll. Thus, the FBW in the 777/787 have only soft limits that can be easily overridden with more force to the control column.

Both systems have their benefits, however it can be argued that if the Asiana 777 had the Airbus alpha protection, it would not have stalled out and instead survived. Same can also be said against the Air France A332 crash in that the pilots would have tried to recover from the stall by pushing down instead of up. This is however merely speculation and we can only assume that these systems would have helped.

Thanks again for reading, have a great day and constructive comments are always appreciated!

(PoTP 4 is coming tomorrow!!!)

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This is really informative, thanks for sharing!

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Nice guide, really helpful. Thank you for sharing.

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