Fly-by-wire

[SuperflyTNT]

Hello all,

Can anyone here help explain how the fly-by-wire system works in simple terms?

From what I've understood, it is an Electronic Flight Control System. The pilots inputs on the joystick are fed to the computer, the computer then analyzes these inputs and tells the servo actuator to move the flight control surfaces with the help of a hydraulic system.

The advantages of this system is that it reduces the overall weight of the aircraft and also sees to it that the operating limits of the flight are not exceeding even if the pilot induces erratic changes onto the joystick. Also, the computer makes changes on its own when deemed necessary by the computer without any inputs from the pilot.

Have I missed out on anything?

I am a CPL holder without any jet experience. I am preparing for an interview so would like to know about the fly-by-wire system. Also helps in knowing otherwise as well.

Thank you in advance.

[vilas]

Normal aircrafts can be described as fly by cables and pulleys. Lowest FBW, the inputs on the side sick (Airbus) or yoke( boeing) are converted into elctrical signals which are carried by electric wires to flight control servoes. However once you get into electrics it is possible to involve computers to modify the pilot demand. This are called protections, they can be soft (777) where pilot can override them or hard where he cannot even if he wants to (Airbus). This is basic.

[Wizofoz]

Reduced system weight isn't really the biggest advantage.

The flight Control computers recieve many more inputs than just control column/stick position- they recieve flight data such as IAS, ALT, G, AofA etc, and thus tailor the flight control reponse to the conditions, allowing the designer to program the aircrafts response, making the aircraft easier to fly and with more consistent response across the performance envelope.

This allows for things like Envelope Protection, where the control will either not allow, or give vey clear warning, if certain parameters (approach to stall, overspeed, over bank, "G" exceddence) are exceeded.

Another POTENTIAL advantage of FBW is relaxed stability (with "Active" flight controls), the controls providing input to simulate aerodynamic stability. This can lead to reduced drag.

[FE Hoppy]

Closed loop, open loop lots of variations in protection and input devices. FBW is a huge topic but in simple terms it replaces a mechanical control system with an electronic one.

Weight is a benefit as is servicing time and complexity. Flight control rigging can be a time consuming activity.

As stated, once you have an electronic system you can make the aircraft behave pretty much how you like.

[grounded27]

Can anyone here help explain how the fly-by-wire system works in simple terms?

Most have at least 2 electro/hydraulic systems.

One of manual input, a sensor reads the input, a valve opens, hydraulic power moves the flight control to the desired position, another sensor on the actuator validates the position of the flight control. A FCC (usually 2x redundant), Flight Control Computer monitors and control's this action, it is usually at least 4 times redundant for each axis of control.

The 2nd is similar but the input comes from an autopilot, Otto the Pilot usually resides within the FCC's.

Not all aircraft have flight protection in manual input mode and it can now be turned off on just about all aircraft.

[FlightPathOBN]

Electronic sensor (computer/wire/wireless)to hydraulic actuators, unless with the Firebird, which is electric to electric....

[Uplinker]

Superfly, I think your opening summary sums up FBW very well. I think that FBW is nowadays generally taken to mean control through computers, (others correct me if I'm wrong).

In Airbus FBW, amongst the other non-pilot commanded 'changes' the computers may send to the control surfaces are; Approx half rudder towards a live engine in the event of engine failure; Attitude "stability" involving accelerometers giving feedback to the flight control computers to keep the aircraft in roughly the same attitude the pilot last commanded - (not a perfect system, only the autopilot will keep the aircraft in exactly the right attitude for the particular navigation required); and Turbulence load alleviation/damping, (can't remember the exact name), where spoilers and ailerons are used to reduce wing flex in strong turbulence.

Other 'xxx by wire' systems on Airbus include the nosewheel steering, and of course the engine FADECS.

(The only mechanically operated control surfaces on most Airbus are the rudder and the horizontal stabiliser, but both of these still require hydraulics to work).


Good luck!

[RetiredF4]

@ uplinker
Attitude "stability" involving accelerometers giving feedback to the flight control computers to keep the aircraft in roughly the same attitude the pilot last commanded -

Would you please explain?

.....it maintains 1g flight (flight path stable), which is quite different to maintaining attitude.

[Capot]

Just a word from the cabin....if I may.

Total dependence on computers, 4 parallel systems for safety; hmmmmm....

http://i243.photobucket.com/albums/ff141/picshooter/d86fef5b-ec8e-48d7-9e78-fa65e6c7449c_zps89f158b7.jpg

http://i243.photobucket.com/albums/ff141/picshooter/d86fef5b-ec8e-48d7-9e78-fa65e6c7449c_zps89f158b7.jpg

http://i243.photobucket.com/albums/ff141/picshooter/d86fef5b-ec8e-48d7-9e78-fa65e6c7449c_zps89f158b7.jpg

http://i243.photobucket.com/albums/ff141/picshooter/d86fef5b-ec8e-48d7-9e78-fa65e6c7449c_zps89f158b7.jpg

Just sayin'.........

[Uplinker]

Please note my use of the word "roughly" Not many folk seem to know about this feature because there seems to be a lot of confusion about it whenever I mention it. I am just off to work, but when I get back I will try to reproduce the diagram in the manual which describes what I am talking about.

Capot; An Airbus is an aeroplane flown by 2 PILOTS. There is NOT a "total dependence on computers". There is a lot of automation available, but there are also extensive procedures and back-up systems should anything go wrong. If ALL the computers were to fail, the PILOTS can still fly the aircraft and recover enough systems to make a safe landing. The high level of automation available produces a modern, efficient, safe, and comfortable flight.

[Capot]

At the risk of consigning the thread to a less august forum than Tech Log, can you clarify that?

With any Airbus, my simplistic understanding was always that while in Direct Law control inputs are transmitted unmodified to the control surfaces, and thereby provide a direct relationship between the sidestick and the control surfaces, it still required the computer to receive and transmit those inputs; the main point being that it did not modify them on the way through.

Thus, I have always thought, if all the relevant computers failed (I know, unlikely) the crew would be left with only the elevator trimmer and rudder pedals to control the aircraft, as well as, I suppose, engine thrust. Assuming the hydraulics are OK.

Are you saying that in Direct Law the pilots' inputs go to the control surfaces (or rather the actuators for them, whatever form they have) by a signal that bypasses the computer systems? So the case could never, ever arise where they might have to fall back on the elevator trimmer and so on?

[Uplinker]

No, you are basically right. I'm saying that if all the flight control computers failed, the pilots would still have control of the aircraft through the rudder and horizontal stabiliser. They would be able to keep flying, but they would not be able to land in this state, (or if they did, it would be like Sioux city). They would be able to keep a relatively stable flight path while they reset the flight computers. As long as they got some of the five back, they would be able to land safely.

[Groundloop]

They would be able to keep flying, but they would not be able to land in this state,

I've seen it done in a sim a few times - but would not like to see it done on the real aircraft!

[glum]

Unless you were on board and the other alternative was... :eek:

[Uplinker]

Absolutely !

RetiredF4 ; I have tried to post the Airbus control diagram here for you, as promised, but it has turned out to be too difficult for my limited IT skills, sorry!

Anyway, the point is that there are two separate feedback paths to the flight control computers. One measures the control surface itself - that it has moved where the flight control computer ordered it to go and how far; The second is a "response" feedback which measures how the aircraft moved following the control order, and modifies that order if the aircraft did not respond as required. Both these feedbacks combine to give a sort of attitude stability, meaning that the pilot, when hand flying, does not need to react to every adverse movement the atmosphere causes, (and if s/he does they may find they are 'fighting' the control computers). Better to back off slightly and guide the aircraft rather than actively countering every movement. When hand flying in conditions of heavy turbulence on approach though, the side stick may need to be moved to its' limits to keep to the required flight path.



U

[Groundloop]

Unless you were on board and the other alternative was...

True. But hopefully it would never get to that situation.:ok:

[Microburst2002]

If SuperflyTNT doesn't specify what airplane, the only good answer is, fbw is a flight control system where there is no mechanical linkage between the control wheel or stick or whatever and the surfaces, but electrical signals.

The only fbw systems I know in civil aviation are those of airbus and B777. I don't know what other airliners have fbw

[toffeez]

You overlooked the 787, but that's easily done since it's not doing any aviating right now.

[gums]

Salute!

You have nailed the answer, Micro. 25 words or less.

fbw is a flight control system where there is no mechanical linkage between the control wheel or stick or whatever and the surfaces, but electrical signals.


Confusion exists about FBW systems due to their implementation and not the basic concept described above. A "pure" FBW design would simply use electrical signals to the servos controlling the hydraulics that move the control surfaces. To duplicate the pure mechanical systems that used cables, pushrods, etc., the wheel or stick would move exactly the same amount and have the same forces the pilot feels, but the output would be an electrical signal. This type of system reduces a lotta weight and can also provide redundant paths of the electrons to the control surface actuators.

Confusion exists because along the way, the engineers and operators realized that we could "tailor" the control surface deflections to provide "protections", "dampening" , and even allow aerodynamic "relaxed static stability" planes to be flown by humans. The Airbus "direct law" comes very close to the definition expressed by Micro. But normally, computers "tailor" the signals from the pilot or autopilot to provide safety, performance, etc. So we have gee limits, gee onset rates, angle of attack (AOA) limits, etc.all determined by our particular plane's "control laws" to enhance safety or maximize performance.

See the immense AF447 discussion threads for many posts about "implementation" of FBW.

Hard for me to realize now, that I flew the first "pure" FBW jet 34 years ago, and the jet had been flying using a pure FBW system for 5 - 6 years before that. Only other system like it was the shuttle orbiter.

Viper pilot '79 sends....

[toffeez]

gums, do you mean Concorde?

[FE Hoppy]

Embraer Ejets fbw has a direct mode which does nothing other than replace the cables with electrical signals with no software interference. I think it's similar on the 777.

In normal mode this direct signal is mixed with software generated signals to dampen, limit, shape the control output.

[flapsupdown]

That's correct - the 777 "normal mode" takes pilot input and mixes it with inputs from the Primary Flight Computer. In the "direct mode" the flight computer is skipped altogether and pilot inputs are sent directly to the flight controls. However, even in direct mode it is still considered FBW.

But even still it is not a "pure" FBW system as the controls are mechanically linked to one spoiler on each wing for redundancy

[gums]

Salute!

Not sure how much "backup" the Concorde had, but it sure as hell had a lotta FBW concepts implemented.

Our Viper had ZERO mechancial connections of any kind to the flight control surfaces.

Pprune has a great thread about the Concorde, including engineers and at least two pilots contributing.

Thanks for calling me out, Tofeez.

old Viper pilot sends....

[Uplinker]

RetiredF4, thanks for the PM. Maybe I am confusing people by talking about attitude - I am referring to pitch and bank. Nothing to do with engine thrust.



U

[sierra_sotiropoulos]

In total, an A320 has 7 flight control computers that process pilot inputs according to 4 different modes of operation, called LAWS:
1) Normal Law
2) Alternate Law
3) Direct Law
4) Mechanical Back Up

In Normal and Alternate Law, pilot commands are processed from the computers and then converted to flight surface movement. In reality an Airbus pilot when moving the sidestick asks for “G’ load” in Pitch and “roll rate” in Roll. The computers will calculate the appropriate control surface deflection and actuate them. Furthermore, the aircraft doesn’t need the pilot to trim it to maintain a given attitude. The FBW will auto-trim the control surfaces to maintain both pitch and bank. (It is strange though that the B787 has trim controls on the control wheel…)

In Direct Law, the control surfaces movement is directly proportional to the sidestick movement.

All control surfaces are electrically controlled and hydraulically activated.
All primary control surfaces have at least two hydraulic actuators (for redundancy) which receive electrical signals (orders) from the flight control computers.

Only the Rudder and the Trimable Horizontal Stabilizer have a direct mechanical link to be used as a last resort, provided hydraulic power is available.

[Control Eng]

Sorry gums, but I have to trump both of those!

Our Viper had ZERO mechancial connections of any kind to the flight control surfaces.

You are about two decades late!

An aircraft with 'ZERO mechancial connections of any kind to the flight control surfaces' first flew on 30th. August 1952. It was the VX770 prototype for what became the Avro Vulcan.

Avro_Vulcan (http://en.wikipedia.org/wiki/Avro_Vulcan)

The term used for electrically controlled hydraulics with feedback was 'Powered Flying Controls'. The 'Controls' even had 'autostabilisation in the form of pitch and yaw dampers' and later an 'auto mach trimmer'.

As with FBW, loosing electrical power was critical - 'Because there was no manual reversion, a total electrical failure would result in a loss of control.'

Interestingly the Vulcan was powered by Olympus engines, which later were to power Concorde.

A Vulcan airframe was used as a testbed for the Concorde engines.

Re-reading the Wiki highlights that many safety features that are currently used were developed for the B2 variant of the Vulcan (RAT, AAPP->APU).

The Vulcan is of the same vintage as the De Havilland Comet - the first commercial jetliner, yet some of it's engineering developments took many decades to appear in commercial jetliners!

[DozyWannabe]

Total dependence on computers, 4 parallel systems for safety; hmmmmm....

http://i243.photobucket.com/albums/ff141/picshooter/d86fef5b-ec8e-48d7-9e78-fa65e6c7449c_zps89f158b7.jpgx4

...

From a Software Engineer's perspective, your worry is pretty far from the reality. The relationship between the computer systems in aircraft flight control compared to the computers we interact with in a home or business sense is somewhat akin to the relationship between an old pick-up truck or marine diesel engine versus the engines used in Formula 1.

By which I mean that the former are designed to plod along doing their thing until doomsday without anything going wrong, whereas the latter are tuned for performance with the expectation that they will crash occasionally. There is no complex operating system involved in flight control logic, and the software has been exhaustively tested - it won't necessarily be perfect, but it won't do anything in and of itself to put the aircraft in danger.

Most of the previous posts are correct - digital FBW is implemented in the Airbus models of the A320, A330/A340 and A380 series, as well as the Boeing B777 and B787. Analogue FBW was implemented in Concorde.

Fundamentally, FBW means that the connection between the flight controls and the flight surfaces is electronic rather than electro-mechanical - that's about it.

[Capn Bloggs]

But even still it is not a "pure" FBW system as the controls are mechanically linked to one spoiler on each wing for redundancy
So then the A320 is not "pure" FBW because it too has mechanical connections to the rudder and THS "for redundancy"... Methinks you need to redefine "pure".