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For example, something similar to Qantas Flight 72.

Assuming the Angle of Attack sensors are faulty and give bogus data - for example, that the Angle of Attack is 20 degrees, or the maximum Angle of Attack when the plane is flying normally and in no danger of stalling.

The flight protection systems are going to kick in and try and pitch the plane down because the computers will think the plane is stalling or about to stall because of the (bogus) Angle of Attack readings.

Would the computer still be restricted in how far it can pitch the nose down? For example would it suddenly angle down but stop at 10 degrees nose down, or would it go into a ballistic, unrestricted and dangerously steep dive?

I read a pilot in an Airbus cannot make the plane pitch down more than 10 or so degrees in ''Normal Law'', would this apply if the plane's computers are trying to pitch the plane down due to faulty AOA sensor data?

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  • $\begingroup$ Maybe I'm misunderstanding the question, but: If the angle of attack data fed into the flight protection systems is wrong -- unknown to the protection system, obviously --, how could it know when the limit is reached? Wouldn't it always think the airplane is pitched at whatever the faulty data indicate and desperately try to "correct" that? $\endgroup$
    – Peter - Reinstate Monica
    Nov 10 '18 at 2:23
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    $\begingroup$ No, Angle of Attack is separate from the Attitude of the plane. Angle of Attack is measuring airflow angle over the wing, wheras Pitch Attitude is the angle of the plane itself relative to the horizon. The Angle of Attack sensor could be saying 20+ degrees but the pilots would still see the Artificial Horizon saying the plane was in level flight as the Artificial Horizon would be unaffected. $\endgroup$
    – Willy A
    Nov 10 '18 at 2:24
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    $\begingroup$ Thanks for the clarification. I let the comment stand for others wondering the same though. $\endgroup$
    – Peter - Reinstate Monica
    Nov 10 '18 at 2:28
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The answer below is for the A320, as this is the only Airbus manual I have access to.

would it go into a ballistic, unrestricted and dangerously steep dive

It'd likely stop somewhere before that. "Abnormal attitude law" engages when:

  • Bank angle above 125°
  • Pitch above 50° nose up or below 30° nose down
  • KIAS below 60 to 90, or above 440
  • Mach above 0.91
  • AoA above 30° to 40°, or below -10°

I believe the FAC will also give up at some point as the computed KIAS is so far away from what is measured. If it is a double ADR failure or double FAC failure it will go down to Alternate Law. If it is a triple failure it would go to Direct Law, as was the case of GXL888T.

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  • $\begingroup$ Could you clarify if abnormal attitude law engages when all of those conditions are met, or any of them? $\endgroup$
    – 0xdd
    Nov 12 '18 at 15:20
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    $\begingroup$ @Jules, any of them (it is the only interpretation that makes any sense). $\endgroup$
    – Jan Hudec
    Nov 12 '18 at 17:33
  • $\begingroup$ 125° bank angle limit is insanely high. Is there ever a reason to keep running in normal or alternate law with e.g. 90° bank angle? $\endgroup$
    – Mikko Rantalainen
    Feb 8 '21 at 10:34

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