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IANA Pilot. I do make aerodynamically correct fixed-wing and lifting-body type paper airplanes, however, and I have had a LOT of fun playing with aerodynamics

IANA Pilot. I do make aerodynamically correct fixed-wing and lifting-body type paper airplanes, however, and I have had a LOT of fun playing with aerodynamics in this hobby. One thing to keep in mind regaring stall is that it's 100% dependent on angle of attack, not speed (something Popular Mechanics gets entirely wrong). What happens basically is that at a high angle of attack the air layer doesn't track the wing surface properly and so you are deprived of standard lift. With certain aircraft designs (SR-71 for example) this is very hard to make happen (but the SR71 can stall it's engines before the wings stall due to AoA). If you are faced with a stall, I would expect the first thing to do is to pitch down to reduce angle of attack then accellerate and pitch up to get out of it. T-tail designs are generally disfavored because the elevators can get blanked by the wings in a deep stall, but with the A330 this isn't an issue as it doesn't have this tail design. I find it puzzling that a professional pilot would pitch up in response to a stall warning. Popular Mechanics is right to flag that is as difficult to understand.

StavrosK 15y ago
Is it possible that the warning speaker was broken? It's odd that none of the pilots would even acknowledge it. Or does the recorder record the stall warning noise too?
- gnaffle 15y ago
  From what I read from the preliminary report, it's established that the stall warning sounded. If it had not been audible on the CVR, they would probably have noted it. One problem was that the stall warning stopped due to high angle of attack even though the plane was stalled, and it started again when the nose was lowered and the AOA was in the "valid" region again. This might have confused the pilots in a situation when they already had inconsistent airspeeds etc. to deal with.
  - StavrosK 15y ago
    Ah, yes, I read that too. It was so frustrating that the pilot kept pulling up all the time, it makes no sense. You'd think that at some point he'd stop and reassess, but I guess he was too confused and shocked to do it...
    - gnaffle 15y ago
      Yes, maybe a combination of shock, confusion or distrust in the instruments was a factor. It will be interesting to see what the human factors group of the investigation comes up with. I'm sure that the flight training will also be investigated, there has been concerns that improper stall recovery technique is being taught by some instructors: http://www.caa.co.uk/docs/33/012010.pdf http://www.caa.co.uk/docs/33/012010.pdf (And of course, in planes that are not certified for stalls, you can't really practice full stalls and have to train on approach to stalls instead.)
      - jrockway 15y ago
        (And of course, in planes that are not certified for stalls, you can't really practice full stalls and have to train on approach to stalls instead.) Of course, very little airliner training happens outside a simulator, which can you can stall without damaging anything.
        lutorm 15y ago
        On the other hand, it's not obvious to me that the simulator would behave correctly in this regime either. Having the simulator work correctly in a deep stall is probably not high on their list of priorities. It's possible that this was even the first time ever that an Airbus had been stalled like that, so maybe no one actually knew how it would behave.
        jrockway 15y ago
        Yes, that's possible. I remember reading an article by the author of X-Plane about how stalls are mostly simulated by trying it in the real aircraft and programming something similar, because the airflow dynamics are too difficult to model in real time. (Incidentally, the Cessna 172 is nearly impossible to stall in X-Plane, but somewhat easier to stall in FlightGear. I wonder who's closer :)
bambax 15y ago
A commenter below (cellularmitosis) makes a very interesting point: Bonin may not have realized he was in "alternate law" (let's call it "mode" instead of "law"). In "normal mode" the computer will not let pilots stall the plane, whatever they do; it will accept the commands up to what it considers dangerous. There's an "envelope" of acceptable plane movements; pilots can move inside this envelope but not outside of it. In "alternate mode", the envelope is much wider and you can actually stall the plane. If you're in normal mode, it makes sense to pull the stick all the way so that you're at the edge of the envelope: you climb as fast as you possibly can (as fast as the computer will let you). And you can probably fool yourself when the stall alarm rings: the computer is telling me I'm near stalling -- I'm at the edge of the envelope, THIS IS WHAT I WANT!! In fact you're not in normal mode anymore, and the computer is telling you that you're way past the envelope. But you can't register that, because for you that is simply impossible. If that's what happened, the cause of the crash is insufficient training in alternate mode.
- einhverfr 15y ago
  yeah, I agree with that. Also there's another factor in that stall warnings may have been seen as an ADIRU malfunction rather than an actual stall. In those cases, I suppose pulling back might make some sense if you think you have bad ADIRU input and the plane is in normal law.
  - tankenmate 15y ago
    I think the whole point is that in almost all circumstances an ADIRU failure will lead directly or indirectly to alternate law. Hence the correct procedure should be to assume alternate law and cross check. Once your instruments start failing left, right, and centre you should go into what I call "advanced free fall" mode, check horizon (true, false or otherwise), check altitude, check parachute, repeat... If you hit gimbal lock (or similar INS failure) in the dark, well just bend over and kiss it goodbye.
    - einhverfr 15y ago
      I was thinking of the Qantas and Malaysian Airlines mishaps where faulty ADIRU's lead to sudden uncommanded changes in altitude and eventual stall warnings.
Florin_Andrei 15y ago
> One thing to keep in mind regaring stall is that it's 100% dependent on angle of attack, not speed (something Popular Mechanics gets entirely wrong). What happens basically is that at a high angle of attack the air layer doesn't track the wing surface properly and so you are deprived of standard lift. So the air layer on the back "peels off", and you have turbulent flow on the back, instead of laminar, is that right? So the negative pressure is greatly diminished.
- mike_ivanov 15y ago
  Exactly. http://en.wikipedia.org/wiki/Stall_(flight) http://en.wikipedia.org/wiki/Stall_(flight)
vannevar 15y ago
One thing to keep in mind regaring stall is that it's 100% dependent on angle of attack, not speed... While the degree of stall with respect to the wing is independent of airspeed, the effects of stalling on the aircraft are very much dependent on speed. Even with the wing in the process of stalling, it can still generate enough lift to keep flying if you're going fast enough.

StavrosK 15y ago

Is it possible that the warning speaker was broken? It's odd that none of the pilots would even acknowledge it. Or does the recorder record the stall warning noise too?

gnaffle 15y ago
From what I read from the preliminary report, it's established that the stall warning sounded. If it had not been audible on the CVR, they would probably have noted it. One problem was that the stall warning stopped due to high angle of attack even though the plane was stalled, and it started again when the nose was lowered and the AOA was in the "valid" region again. This might have confused the pilots in a situation when they already had inconsistent airspeeds etc. to deal with.
- StavrosK 15y ago
  Ah, yes, I read that too. It was so frustrating that the pilot kept pulling up all the time, it makes no sense. You'd think that at some point he'd stop and reassess, but I guess he was too confused and shocked to do it...
  - gnaffle 15y ago
    Yes, maybe a combination of shock, confusion or distrust in the instruments was a factor. It will be interesting to see what the human factors group of the investigation comes up with. I'm sure that the flight training will also be investigated, there has been concerns that improper stall recovery technique is being taught by some instructors: http://www.caa.co.uk/docs/33/012010.pdf http://www.caa.co.uk/docs/33/012010.pdf (And of course, in planes that are not certified for stalls, you can't really practice full stalls and have to train on approach to stalls instead.)
    - jrockway 15y ago
      (And of course, in planes that are not certified for stalls, you can't really practice full stalls and have to train on approach to stalls instead.) Of course, very little airliner training happens outside a simulator, which can you can stall without damaging anything.
      - lutorm 15y ago
        On the other hand, it's not obvious to me that the simulator would behave correctly in this regime either. Having the simulator work correctly in a deep stall is probably not high on their list of priorities. It's possible that this was even the first time ever that an Airbus had been stalled like that, so maybe no one actually knew how it would behave.
        jrockway 15y ago
        Yes, that's possible. I remember reading an article by the author of X-Plane about how stalls are mostly simulated by trying it in the real aircraft and programming something similar, because the airflow dynamics are too difficult to model in real time. (Incidentally, the Cessna 172 is nearly impossible to stall in X-Plane, but somewhat easier to stall in FlightGear. I wonder who's closer :)

bambax 15y ago

A commenter below (cellularmitosis) makes a very interesting point: Bonin may not have realized he was in "alternate law" (let's call it "mode" instead of "law"). In "normal mode" the computer will not let pilots stall the plane, whatever they do; it will accept the commands up to what it considers dangerous. There's an "envelope" of acceptable plane movements; pilots can move inside this envelope but not outside of it. In "alternate mode", the envelope is much wider and you can actually stall the plane. If you're in normal mode, it makes sense to pull the stick all the way so that you're at the edge of the envelope: you climb as fast as you possibly can (as fast as the computer will let you). And you can probably fool yourself when the stall alarm rings: the computer is telling me I'm near stalling -- I'm at the edge of the envelope, THIS IS WHAT I WANT!! In fact you're not in normal mode anymore, and the computer is telling you that you're way past the envelope. But you can't register that, because for you that is simply impossible. If that's what happened, the cause of the crash is insufficient training in alternate mode.

einhverfr 15y ago
yeah, I agree with that. Also there's another factor in that stall warnings may have been seen as an ADIRU malfunction rather than an actual stall. In those cases, I suppose pulling back might make some sense if you think you have bad ADIRU input and the plane is in normal law.
- tankenmate 15y ago
  I think the whole point is that in almost all circumstances an ADIRU failure will lead directly or indirectly to alternate law. Hence the correct procedure should be to assume alternate law and cross check. Once your instruments start failing left, right, and centre you should go into what I call "advanced free fall" mode, check horizon (true, false or otherwise), check altitude, check parachute, repeat... If you hit gimbal lock (or similar INS failure) in the dark, well just bend over and kiss it goodbye.
  - einhverfr 15y ago
    I was thinking of the Qantas and Malaysian Airlines mishaps where faulty ADIRU's lead to sudden uncommanded changes in altitude and eventual stall warnings.

Florin_Andrei 15y ago

> One thing to keep in mind regaring stall is that it's 100% dependent on angle of attack, not speed (something Popular Mechanics gets entirely wrong). What happens basically is that at a high angle of attack the air layer doesn't track the wing surface properly and so you are deprived of standard lift. So the air layer on the back "peels off", and you have turbulent flow on the back, instead of laminar, is that right? So the negative pressure is greatly diminished.

mike_ivanov 15y ago
Exactly. http://en.wikipedia.org/wiki/Stall_(flight) http://en.wikipedia.org/wiki/Stall_(flight)

vannevar 15y ago

One thing to keep in mind regaring stall is that it's 100% dependent on angle of attack, not speed... While the degree of stall with respect to the wing is independent of airspeed, the effects of stalling on the aircraft are very much dependent on speed. Even with the wing in the process of stalling, it can still generate enough lift to keep flying if you're going fast enough.