[einhverfr]

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.

[bambax]

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.

[brazzy]

Bonin would have had to realize that the plane is in alternate mode in the first place in order to react correctly. It probably never crossed his mind since he never trained in it, and conversely if he had trained in it, he may have been more likely to at least check.

But to me the bigger problem seems to be that such an important change in the plane's behaviour could happen without anyone noticing. I'd consider the mode to be something the pilot must be made aware of, not something he has to deduce from the fact that the airspeed isn't available.

Perhaps the mode is shown prominently and the pilots just didn't notice it in their state of panic. Making it more prominent probably leads right into an insane arms race - the stall warning was as prominent as anything can be and still got ignored.

I don't envy the person who has to design a airliner cockpit's user interface and decide which of a hundred potentially vital pieces of information should be displayed how.

[presto8]

Bonin had to have known that the plane was in alternate law.

In the flight recorder log, at 2h10m05s, there was an audible "cavalry charge" alarm that indicated to everybody in the cockpit that the autopilot was disconnecting (plus message at the same time on the ECAM).

Then, on the ECAM message console 1 second later, the message "F/CTL ALTN LAW (PROT LOST)" was displayed: alternate law, protection lost. At the same time, Bonin said "I have the controls", which to me indicates that he knew that the autopilot was off and that alternate law was engaged.

References: [1] Page 45, http://www.bea.aero/docspa/2009/f-cp090601e3.en/pdf/f-cp0906... [2] Page 88, Ibid.

[waqf]

Of course Bonin knew that the autopilot was off. I don't think that's the same as being in alternate law.

[gldalmaso]

What boggles my mind is that pilots are being trained in anything less than every mode an aircraft operates on.

[kristaps]

A mode switch as important as this should be very visible. An idea: put a two-color led under every indicator light and switch to the other color when the plane falls into alternate law. This should make it sufficiently clear that something very notable has just happened.

[Maakuth]

Something like that was my initial thought. However, you have to remember that there is massive amount of very notable things going on and data shown. As brazzy pointed out, the stall warning was as prominent as possible and still was pretty much completely ignored. I don't think they would've noticed any amount of leds in that point.

[earnubs]

There's a school of thought that says that user interfaces should be modeless, I wonder if the concept of a mode here is a fault in the design of the HCI as well as insufficient training?

[mtts]

Oh no ... we are NOT turning this into another Vi vs Emacs flamefest

;-)

[einhverfr]

Actually the normal vs alternate law thing might be a mode to build into the displays. I am usually a modeless fan and a VIM user, but this is an exception ;-)

[kelnos]

When in normal law, if you pull the nose up all the way, and are at the edge of the envelope, will the computer even give you a stall warning? I would guess (hope!) not, since the plane is never actually in danger of stalling. Assuming that's the case, a stall warning from the computer should always be heeded.

If that's not the case, and the stall warning sounds even when there's no real danger of stalling (because the controls are operating in normal law), I feel like that's a terrible user interface.

[ansible]

I really don't like the idea in general that 'normal law' mode lets the pilots yank the controls any which way and the computer (supposedly) prevents the plane from leaving the safe flight envelope. I believe that sets up the wrong attitude in the pilot's mind about having to carefully and thoughtfully control the aircraft.

If the flight computer is having to intervene and change the flight controls, then at the very least there should be a force-feedback mechanism in the stick which tells the pilot he's doing something wrong, and that he really shouldn't be yanking back the stick that hard.

The other bad part of the user interface is that the two sets of flight controls are not linked, like they were in the old days. With side sticks, it is not easy to see what the other pilot is doing. And averaging the control inputs of the two pilots is INSANE, in my opinion. Only one pilot should be flying the plane, and it needs to be quite obvious who that is at all times.

The CRM mechanism to take over flight controls should not be saying the words "I have control", it should be flipping a big switch on the center console that visibly indicates who has control.

[acqq]

I like the idea of linked control, like on the small planes. That's "the principle of least surprise" and also gives the copilot the right information of what another guy is doing -- that was obviously missing here!

However if you add force feedback of the plane computer "correcting you" you'd never know if it's plane or an another guy. Therefore, force feedback from the computer doesn't sound to me as a good idea. Some kind of feedback would be a good thing, but in panic, it wouldn't be noticed. I guess I'd put something like something "protruding up" on the stick when in another mode -- you'd feel and see it.

Finally, switch flipping is unnecessary if you have a force feedback. It think that's really the major feature missing!

[einhverfr]

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]

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]

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.

[StavrosK]

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]

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]

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]

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

(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]

(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]

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.

[Florin_Andrei]

> Of course, very little airliner training happens outside a simulator, which can you can stall without damaging anything.

Maybe they should apply mild taser shocks to pilots stalling in the simulator. I'm not being snarky. There should be some kind of physical consequence of making bad mistakes, otherwise it's too disconnected from reality.

[onli]

It's especially frustrating for me because i remember all the times i crashed in a simulator due to stall until i understood what was happening. I don't even remember the name of the simulator, i'm not even an amateur-pilot - but that makes that report almost unbelievable, that something like this happened in a real flight to a real pilot.

[lutorm]

unbelievable, that something like this happened in a real flight to a real pilot

Actually, most fatal crashes are "stall-spin accidents", where pilots stall the airplane near the ground without sufficient altitude to recover. But those are not cases where you keep the airplane in a deep stall for 90s. When you stall an airplane in VFR, it's obvious what happens.

No one would persist in keeping the airplane at 20 degrees positive pitch while descending at thousands of feet per minute without realizing the airplane is stalled. But here, without outside references and with obvious confusion about the state of the airplane, it was apparently beyond these guys. (Except the captain, whose comment about "no, don't climb" seems to indicate he was on the right track, but by then it was too late.)

[chc]

Actually, only the less experienced copilot, Bonin, appeared to be in favor of climbing. The other repeatedly told him to level out or dive, and apparently thought Bonin had listened (which would explain why he was so baffled). As the OP points out, neither that copilot nor the captain seemed to realize that Bonin had been futilely trying to climb the whole time until that moment at the end when the captain ordered him to stop.

[Florin_Andrei]

> 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]

Exactly.

http://en.wikipedia.org/wiki/Stall_(flight)

[vannevar]

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.