Reading about Air France Flight 447, to quote Wikipedia (emphasis added):

... A second consequence of the reconfiguration into alternate law was that stall protection no longer operated. Whereas in normal law, the aircraft's flight management computers would have acted to prevent such a high angle of attack, in alternate law this did not happen. (Indeed, the switch into alternate law occurred precisely because the computers, denied reliable speed data, were no longer able to provide such protection – nor many of the other functions expected of normal law).[55] The wings lost lift and the aircraft stalled.

But whether an aircraft stalls or not has nothing to do with its airspeed - the only things that matter are the wing's angle of attack and configuration (e.g., are the slats extended? is the leading edge damaged? have we got ice on the wings?). The aircraft's configuration was completely normal, and the AoA vanes were unaffected by the blocked pitot tubes (as evidenced by the fact that the aircraft's stall warning correctly sounded due to its excessive AoA during the period of time that the pitot tubes were blocked and valid airspeed data was unavailable). Why does the A330/A340 disable high-AoA protection in alternate law if airspeed data is lost - even if AoA data remains perfectly valid?

  • $\begingroup$ I've read a lot about the airbus stall protection and I am still not sure if it really controls angle of attack. I think it is more of a attitude and speed stabilization, which would be lost when pitot tubes fail. See this video: youtube.com/watch?v=9fqy8uPzW90 at 10:20 he says: aoa may vary even with full aft stick but its the pitch and speed that they want to be stable $\endgroup$ – Jan Jan 27 at 7:49

1. AoA vanes are speed sensitive

An AoA vane is in essence a weather vane, but due to various aerodynamic interactions it's not perfect. From flight testing they would have worked out how speed affects the position, and based on that the sensed position is corrected by speed input.

From the A330/A340 FCTM (Flight Crew Training Manual) section 8.110.4:

The ADRs provide a number of outputs to many systems and a blockage of the pitot and/or static systems may also lead to the following:

  • (...)
  • Alpha floor activation (because AOA outputs from the sensors are corrected by speed inputs)
  • (...)

2. Alpha floor inhibition

The above suggests alpha floor could have been activated due to the blockage, but unlike the A320, alpha floor is inhibited on the A340 above Mach 0.53 (no reason is given). (Source: A340 FCOM 1.22.30)

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3. Crickets + stall

That stall warning in alternate law is based on a computed VSW (V Stall Wanring), and not the AoA. (A340 FCOM 1.27.30)

enter image description here

The A340 FCOM and FCTM are obscure as to what goes into the VSW calculation, but one of the inputs for that speed for the A320 is the calculated gross weight.

  • $\begingroup$ Still, if the aircraft is stalling, that indicates that the angle of attack is too high; so why not force a pitch-down anyway when something sets off the stall warning? $\endgroup$ – Sean Jan 28 at 3:48
  • 2
    $\begingroup$ @Sean - The computer can't know for sure it's stalling, all it knows is that it is being fed contradictory data, which renders the data invalid, and that's why when ADIRUs detect a fault, the control is given to the pilots. As to what the pilots were supposed to do, that's another question, which IIRC was answered before. Basically, with proper training there are memory items to carry out for such ADIRU faults that would ensure the plane is not stalled by the pilots. $\endgroup$ – ymb1 Jan 28 at 3:59
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    $\begingroup$ @Sean - Here's a question about a crash where 2 of the 3 AoA vanes froze: aviation.stackexchange.com/q/36205/14897 -- when the computer decides the data is faulty, it can't know what is good, marginally acceptable, or bad. $\endgroup$ – ymb1 Jan 28 at 4:06

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