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Comment by dmm | Hacker News Reader

> As if the buffet weren’t enough of an indication, the stall warning erupted again, alternating between STALL STALL STALL and a chirping sound.

Off topic but how does the airplane know it's stalling if the airspeed indicators are not working?

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Airspeed is actually a secondary and unreliable indicator of a stall. It's commonly said that "you can stall an airplane at any airspeed and at any attitude". This is not strictly true, but it gets the point across that there's a wide range of situations in which you can stall. You can be going near zero speed and not be in a stall if you're pulling near zero gees (e.g. going over the top of a parabolic arc), and you can be going well above the normal stall speed and still stall if you're pulling a lot of gees (e.g. making a steep turn, this is called an accelerated stall).

As sibling replies mention, angle of attack is the key indicator. There's a critical angle where you're stalled if you go beyond it, and not stalled below it, regardless of airspeed.

>how does the airplane know it's stalling if the airspeed indicators are not working?

The A330 has vanes which measure the airflow direction

http://aviation.stackexchange.com/questions/2094/how-does-an...

Light aircraft usually just have a metal flap on the leading wing edge which blows upward if the plane is angled up too much.

Edit: Having read the stackexchange explanation I think there is a big part of the reason for the crash right there. The warning system was operated by the computer taking data from the vanes and air speed indicators which gave weird results which surely confused the crew. Had the plane had a basic metal flap with switch and buzzer the crash may not have happened.

I can't find a source for this right now, but I believe that the pitot tubes were only iced over briefly, and that normal airspeed readings did resume long before the end, but it seems the crew never reached any state of confidence about whether (or which of) their instruments were correct.

It raises an interesting point about instrumentation - it's easy to make a display that looks like a gauge, a needle, or whatever. But if the system behind the display can have some knowledge of whether it's getting good data or not, then really there is (at least) another dimension to the information. How do we represent that so that operators can make sound judgements in unfortunate circumstances?

Article mentions that the aircraft also provides GPS-based speed, so maybe that's how? That's just an educated guess, I have no real idea.

Edit: Reading through the official investigation document:

"The angle of attack is the parameter that allows the stall warning to be triggered; if the angle of attack values become invalid, the warning stops. "

So like said in another comment - even with no valid speed values, the angle of attack can trigger a stall warning.

Yes, and the angle of attack sensor also stops the stall warning if it reads a value above a certain threshold, discarding the value as invalid.

But apparently the airplane can still fly for a short time even at this crazily large angle. Therefore when the pilot did the right thing to reduce the angle, he triggered the stall warning again and completely got confused. I think this is a major design flaw in Airbus's system.

Yeah, even though it's suicidal, the angle of attack can physically be larger than 40 degrees, so I am not sure why would it be discarded as invalid. Most likely Airbus engineers decided that since no one would try to ascend at such ridiculous angle(especially since the autopilot will correct if the angle of attack is larger than 15 degrees), then such high values must indicate a broken sensor. A decision which contributed to the catastrophe, unfortunately.

Ground speed, which is what a GPS would indicate, doesn't have very much correlation with the aerodynamics of a plane. The true factor is airspeed which includes the winds around the plane. Taken to an extreme you could have a ground speed of zero and be flying perfectly normal if there's a head wind that cancels your forward velocity. (Won't happen with a commercial airliner but it illustrates the disconnect between the two factors)

A stall warning occurs when the angle of attack (angle between the wing and the airflow) is too high to maintain lift. There is a separate instrument for detecting this (which can also be disabled by ice build up, but appeared to be working here). Low airspeed is not the only condition for a stall - it can happen even at high speed!

Angle Of Attack sensors would be my thought.

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