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On Air France 447 the crew had experienced complete failure of the pitot static system, which meant they lost their readings on their airspeed indicators, but according to the Mayday / Air Crash Investigations episode the aircraft had detected that the plane was about to enter an aerodynamic stall and the stall warning systems were activated just before the crash.

How did the A330 in question detect that the plane was on the verge of stalling without the airspeed indicators working?

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There are no stick shakers on A330. In normal law it will automatically work the elevators to avoid stall, but in alternate law a stall warning is all they have. – Jan Hudec Mar 7 '14 at 7:55
Oops, you're right. Made the edit :d – shortstheory Mar 7 '14 at 12:17
This is sad irony, we have a potential repeat of AF 447 occurring a day after I asked this question :( – shortstheory Mar 8 '14 at 8:32
up vote 25 down vote accepted

Stalls occur based on a wing's angle of attack rather than the aircraft's airspeed. (In fact, one of the basic facts that all pilots learn in their initial training is that an airplane can stall at any airspeed). The A330 measures angle of attack using vanes mounted on the fuselage:

enter image description here

However, below 60 knots, these vanes become ineffective. During the AF447 accident, the stall warning only engaged intermittently because the computers detected that the airspeed readings from the blocked pitot tubes were invalid, and therefore silenced the stall warning system.

Furthermore, because of the lack of valid airspeed data, AF447 was no longer in normal law (and therefore had no automated angle-of-attack protection), a fact which the pilots may not have been aware of. By the time the deicing equipment had cleared the pitot tubes, the aircraft was at such an extreme pitch that airspeed indications were still considered invalid, and the stall warning system did not activate. The computers only activated the stall warning system when the crew pitched down, which brought valid airspeed data back into the cockpit.

An Airbus press release suggests that this may have contributed to the accident:

We also note that the aircraft, or more specifically the design of the stalling warning system, misled the pilots: each time they reacted appropriately, the alarm triggered inside the cockpit, as though they were reacting wrongly. Conversely, each time the pilots pitched up the plane, the alarm shut off, preventing a proper diagnosis of the situation.

(Emphasis mine)

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Which does not explain why the alpha protection can't use the angle-of-attack vane. It uses airspeed, weight and wing loading (from accelerometer) instead. – Jan Hudec Mar 7 '14 at 7:58

Aircraft can stall at any airspeed, the only real indicator for a stall is an angle of attack indicator. This is because weight, icing, flaps, G-forces being pulled, and speed all effect the aircraft's stall speed.

Most large aircraft, including the A330, have angle of attack indicators that detect the high angle of attack which precedes a stall. Although the other instruments had failed, the angle of attack meter was still functioning, though not very well.

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To backup flyingfisch, the angle of attack (AoA) sensor is the only sensor used to determine aural stall warning. The AoA sensors were operating correctly in the case of AF447. The failure mode of sensors due to ice crystal icing is typically limited to those probes which collect by way of geometry such as pitot and TAT probes. Static ports and alpha probes seem unaffected. The aural Stall warning system in Alternate law is purely a limit AoA, and that limit is primarily determined by Mach; it reduces significantly at high mach due to compressibility.

For AF447, the stall warnings stopped when the limit of the AoA probes were exceeded. This was presumably due to a validity checking algorithm of the system to discount extreme AoA beyond which an airliner would never conceivably achieve in its lifetime whilst airborne. Although in their case, that AoA was actually correct and the aural stall warning should never have ceased. As AoA probes become increasingly inaccurate beyond certain angles, some manufacturer s choose to temporarily derive AoA from other sources such as inertials.

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I don't think an AoA probe would be inaccurate unless it's passing it's max deflection. If there are angles that become increasingly less accurate that would seem to be a serious design flaw. – Rhino Driver Apr 7 '15 at 14:26
They become inaccurate due to aerodynamic influences created by the fuselage, often in the presence of even small amounts of sideslip. Flight testing is conducted with probes mounted well forward into the airflow where the influences of the airframe are negligible. The results of such testing will map the effects of aforementioned influences, however they would certainly not have mapped them on an airliner at extreme AoAs evident on Af447. You would expect a fighter jet to be pretty accurate over the range of possible AoA values due to their innate requirements. – busdrivingtupperware Apr 8 '15 at 11:52

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