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Airbus Fly By Wire System and Backups

  1. Jun 4, 2009 #1
    Hi, I am currently studying aerospace engineering and recently have been doing some research on the airbus fly by wire system, (this was before the Air France crash although it has spurred me on a bit). I am posting this here as I am not certain whether it is truly an aerospace issue.

    Basically, I have a few questions on the system that I cannot find the answer to anywhere despite hours of searching:

    On the A320, there was a mechanical backup system, but on the A330-40 aircraft I have been unable to find any information regarding what happens in various power failure situations (however unlikely any may be, they are all still possible).

    I understand the backup computer/electrical/hydraulic systems, how many there are and how they work with each other. My question comes from a number of different scenarios which may arise.

    Firstly, what happens if all flight computer systems fail? Can the pilots still provide input to the hydraulic systems? Are the electrical signals still transmitted simply without the computer monitoring?

    Secondly, if there is complete electrical failure (including all backups), however the engines are still running. Can they still be controlled? If so, I understand this would allow the pilot basic pitch control, but what would (s)he do about yaw/banking? Do the hydraulics work off of the engines or do they require electrics?

    Thirdly, as above except with the engines offline. How would pitch/yaw/banking be controlled? (I understand that an answer for yaw/banking may be dependant on the answer to number two).

    The questions above stem from my main issue with the fly by wire system, which I never see someone answer directly. I see everywhere that the hydraulics can work even during electrical failure. But is this with/without backup power (Batteries and RAT)? If there is a complete loss of electrics (again all backups lost), even if the hydraulics work, how can an electrical signal be sent from the flight controls in the cockpit to the hydraulic systems?

    And finally, if there is a complete failure, how would the pilot fly the aircraft without electrics and hydraulics? Does a 767, which is not fly by wire require hydraulics? Or can it be flown without them (anyone who has seen Freefall - Flight 174, a true story, will know what I am aiming for here). The pilot managed to glide the aircraft with, as far as I am aware, no hydraulics or power what so ever (until the RAT kicked in after a certain amount of time although he did manage it without anything for quite a while). Is this possible in an Airbus A330-40 without the above mentioned systems or dead stick I should say? Even without the RAT the 767 was gliding quite well, would an Airbus?

    I do apologise for the long windedness of the questions but I have spent hours researching it and cannot find anything that even comes close to answering my questions.

    Any help very much appreciated, thanks in advance.
    Jared James
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  3. Jun 4, 2009 #2


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    Wether fly by wire or not you need power to operate the control surfaces. In a 'conventional' aeroplane you push the stick and it pushes a hydraulic cylinder which moves the surface in a fly by wire you move a small computer control that tells the computer to move the hydraulics. In either case without power to the hydraulics the surface isn't going to move. To move the surfaces on a 747 with old fashioned wires and the strength of the pilot, as in a Cesna, would require a tug-of-war team on the flight deck.

    In a fly-by-wire you're SOL, the only input to the system is a little computer joystick.

    Depends if you including the engine controls in the 'everything failed' they are also controlled by computer, there isn't a little throttle wire to the carburettor!

    Both, the hydraulic pumps are driven by the engines (or the RAT in an emergency) but they also require electronics to control them on a fly by wire.

    In a traditional aircraft if you have no electronics but the hydraulics still work you have some control of the surfaces but not the engines and no instruments. In a fly by wire you don't have control of the hydraulics.
    However failure of the hydraulics is MUCH more common than failure of the electronics. It's very easy to duplicate electronic systems, it's much harder to duplicate hydraulics and even when you do you still end up with single point failures at the pumps and actuators.
    The boeing 7E7 is doing away with hydraulics completely - it will have electric motors riving the control surfaces, commanded by a computer. This is mostly to save weight but should be more reliable.

    Without power you don't have hydraulics, no power - no pump - no pressure. You might have a little remaining capability in the reservoir but again you are SOL.

    All large commercial aircraft glide well, if you have no hydraulic or electrical power then there is no pilot messing with the control surfaces to spoil this. It doesn't matter if it is fly-by-wire or not.

    Similairly all require power to run the hydraulics and require hydraulics to move the surfaces. Fly by wire doesn't fundamentally change this. In theory it adds an extra layer of failure, but this failure is so unlikely that it is made up for by the increased safety of having the computer check everything else.

    Some military FBW aircraft are deliberately unstable for extra maneuverability, in this case they will generally stop flying in seconds if the control systems fail.
  4. Jun 4, 2009 #3
    Hi, thank you for such a detailed response. It has certainly answered a lot of questions I had.

    Based on your responses I'm just wondering, is fly by wire safer than having the traditional mechanical link?

    Ignoring Power By Wire for now, shall we say we have an A330 and a 767 both flying at cruise altitude. Both run out of fuel, what would be the difference in the two aircraft regarding control.
    The best way I can put this question is to say:
    In a mechanical system, you have a direct link to the hydraulics so you have some control regardless of whether or not the flight computers are working or there's power to the cockpit.
    Whereas with a fly by wire system if you lose power to the flight computers or the cockpit, how do you still retain control? As in, what would send the electrical signal to the hydraulic system?

    I'm just taking the flight 147 example for the above question, as the pilot managed to maintain stable flight in a 767 without any form of electrical power, he had no 'help' with the hydraulics (he commented in a documentary I saw on it how difficult it was to maintain a stable glide path) until the RAT kicked in and provided some power. It just seems to me that if you ended up in a similar situation in an A330-40 then there would be no chance of a stable glide as you would not be able to operate the hydraulics without electric to the computer systems.

    Is it even possible to fly those aircraft without any power? I say fly I suppose it's more of a glide, but if you see what I mean. It just seems a step backwards to me.

    Also, as a side note, could someone clear up what the hydraulic or power 'packs' are on the A380 and VC10? Not sure if thats what they are called but I heard they both have them.
    Last edited: Jun 4, 2009
  5. Jun 4, 2009 #4


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    A traditional mechanical link is a wire from the stick to the control surface, this is how your parking brake works and isn't possible in anything larger than a Cesna.

    Very little, both would glide until the RAT kicked in and powered the hydraulics. During that time the backup battery would power the computers and instruments on both.

    If you have no flight control systems you cannot control the plane, the point is that the failure of half a dozen redundant computers is less likely than the failure of the hydraulics.

    You need power to drive the hydraulic pumps, in the short time until the RAT starts you have reserve hydraulic pressure in the system, you also have electrical power from batteries. A stable glide doesn't need any control because the best thing to do in a glide is nothing.

    It's not possible to fly (as in manouver) an aircraft without hydraulics, except for a limited amount of yaw by using engine thrust. In a fly by wire you also need some computer system to control the hydraulics.
    It's a backward step in the same way as the anti-lock brakes in your car, they require a functioning hydraulic system and a computer. With the original brakes on a cart you just pushed a lever and held a wooden block against the wheel. Simpler but not quite as effective.

    Not sure what a hydraulic power pack is.
    The ram air turbine (RAT) can either drive an electrical generator (ELRAT) which among other things drives a backup hydraulic pump or a completely hydraulic system (HYRAT) where the turbine drives a hydraulic pump directly.
    Originally RATs were only fitted to military aircraft, you are more likely to lose an engine on a military aircraft because the engines are much more stressed and people shoot at them - early military aircraft had hydraulic RATs for simplicity, weight and space.
    The VC10 (according to some searches) was going to have a HYRAT but all were fittted with modern ELRATs, as far as I know the A380 has a conventional (although huge) ELRAT.

    It's like claiming 4 engined aircraft are safer than 2 engine because if 2 engines fail they can still fly. but the chances of two engines failing for separate reasons is very very low. All the cases where two engines on a twin have failed were due to circumstances like running out of fuel which would also have stopped 4 engines, or 6 or 8 !
    Last edited: Jun 4, 2009
  6. Jun 4, 2009 #5
    Thank You, you have cleared up a lot of things I was curious about. I personnally am for Fly By Wire/Power By Wire, but after doing research for my course I have been having a debate with my grandfather (aircraft engineer through 50's - 2001) as to which is better. He is old school as doesnt like the sound of no direct link.

    Once again, many thanks. I will try to dig up the link to the regarding the 'packs'.
  7. Jun 4, 2009 #6
    Last edited by a moderator: Apr 24, 2017
  8. Jun 4, 2009 #7


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    It sounds like the VC10 main flight controls had electric hydraulic pumps mounted near to them to drive the actuators with power to the pumps supplied by the aircraft electrical system. This is very like the all electrical system for the 7E7 and saves the weight and complexity of running hydraulic lines through the wings.

  9. Jun 4, 2009 #8
    Ah, I see. So does the A380 have these?

    You have been most helpful.

    Jared James
  10. Jun 4, 2009 #9


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    It looks like the A380 has electrically powered hydraulic pumps.

    Most aircraft have the pumps powered mechanically from the engines, this is in theory simpler but means a lot of plumbing of hydraulic lines into the wings and engines. It also reduces redundancy since many lines have to pass through the same point.
    Planes must also have an electrical backup hydraulic pump in case the engines fail - in the case of the A380 it looks like they have simplified it by having only electrical pumps.

    Another interesting feature is that to reduce the size and weight of the lines they have gone to 5000psi, instead of the normal 3000psi.
    Even 3000psi - the normal max pressure of scuba tank is a lot of pressure, 5000psi if something sprung a leak would slice through metal like a cutting torch.
  11. Jun 4, 2009 #10


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    One clarification on "fly by wire". It typically isn't just used to refer to non-mechanical control actuation, but electronic/hydraulic with computerized control. Ie, there were hydraulic control systems back in the 50s and 60s that wouldn't generally be considered "fly by wire". The difference? Fly by wire doesn't necessarily send to the control surfaces exactly the signals the pilot tells it to.
  12. Jun 4, 2009 #11
    Hi Russ, funny you should mention that, I was just looking at concorde and its so called fly by wire. It seems to me that it depends on what you consider fly by wire to be.

    To me it's when there is no direct mechanical link between control surface and flight yoke. So for that purpose, concorde is fly by wire. However I feel I am wrong in that respect based on your above post, as it had no computer control as far as I am aware.

    Any clarification on this would be appreciated, I'm not entirely sure how concorde worked but according to my books, (yes a 19 year old that still prefers books over the internet), it was servo controlled or something along those lines?

    Also, can anyone provide me with a few other aircraft that are fly by wire aside from Airbus, I have been looking at a fair few so far, but am interested in older aircraft that were considered fly by wire, or at least a similar concept.

    Jared James
  13. Jun 4, 2009 #12


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    Have you read the wiki on it? It has a lot of good info on the subject:

    Either way, computer control is a significant and separate innovation that should be discussed on its own.
  14. Jun 5, 2009 #13


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    While googling about the Air France crash, I found an interesting article debating the merits of allowing the pilots to override the flight control system and execute direct commands on the aircraft controls: http://www.aviationtoday.com/av/categories/atc/13015.html

    Basically, Airbus argues that it does more harm than good to allow a pilot to exceed the design flight envelope (ie, +2.5g), whereas Boeing (and others) believes in an emergency, it should be allowed.
  15. Jun 5, 2009 #14
    330 still does have mechanical backup.

    Depending on the model, and date of manufacture, pilots have mechanical control over either Horz Stab, or Horz Stab + Vert Stab.

    Many airbus models have crashed due to the system taking over, however, the system can be over-ruled by pilots if need.

    There is no audiable reference the plane is taking over in most cases, and the adi does not clearly state this either.

    Think a good, well documented event was the crash of a russian aeroflott airbus 300/310 where the computer took over and cause the aircraft to go out of limits.

    An airbus demonstrator crashed due to this too, and since they airbus have tried to make it more clear that there are ways to fly out of the envolope, as demonstrated by airbus several times.

    Boeings have came down due to flying out of the envolope, airbus' have came down due to system input. So no real right or wrong in the manufactures views.

    Dont quote, but sure the B777 has FBW aspects, as does many, many millitary planes.
  16. Jun 5, 2009 #15
    The 777 is 50/50 as far as I am aware. I didn't know 330 has mechanical backup (or at least some).
    I'll be honest with you Russ, after the past two years in uni being told "Never, ever use wikipedia as a reference source!!!" I'm not a fan of using it. I know a lot of stuff on there is good, but it's the fact anyone can edit the articles that just means there's an air of uncertainty about it. So, probably have come accross the article at some point but may not have taken much notice of it. I do appreciate the help though, that second article was very interesting. Going to do a bit of looking in that area now I think.

  17. Jun 13, 2009 #16
    As an airline mechanic i need to point out a few misconceptions you have.
    first the 7e7 witch is now the 787 will have hydraulic systems three of them in fact . it will have a very limited pneumatic system witch one is pressurizing the hydraulic reservoir. the other engine cowl ice protection.

    you talk about old wire on the 747 flight control. it is not wire it is cable

    in fbw airplanes the rat will drop to give you electric power . are hydraulic pressure. if the engines are running you have a engine driven hyd pump. and on the b777 you will have a back up generator that is for the essential fly by wire buss. and most new airplanes have airborne running apu witch will give you electric power.

    and gliding requires pilot input there controlling the plane using the rat are other backup system. a good example of this is in 2001 was Air Transit Flight 236 witch ran out of fuel. due to a broken fuel line the rat deployed so the were able to control the aircraft to land.
    this happened august 24th 2001. with the use of the rat the crew landed the airplane controlling the essential flight controls
  18. Jun 13, 2009 #17
    The Boeing 777 is a full flyby wire airplane. but is very different from the 330 system.
    the 777 in normal fbw will have all the control laws . if problems happen the computers will go into other laws. but the 777 you can disconnect the primary flight control computers and goto direct law witch will get you home. the 330 has no disconnect . also on the 777 there is a back up cable system with will give you pitch and roll.
  19. Jun 13, 2009 #18
    I'm sorry, but I can't for the life of me understand this. It makes no sense. I DID NOT talke about the 747, I have already mentioned the RAT, please clarify.
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