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I was talking to a pilot, and he said that the alpha floor had been activated, and I did not have time to ask him to explain this. So, can you clarify my doubt, please?

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Alpha floor is a thrust function which provides TOGA thrust regardless of thrust lever position in Airbus. From Airbus Technical Digest FAST 20:

Alpha Floor is a low speed protection (in normal law) which is purely an autothrust mode. When activated, it provides TOGA thrust. As the aircraft decelerates into the alpha protection range, the Alpha Floor is activated, even if the autothrust is disengaged. Activation is roughly proportional to the rate of deceleration.

... at low speeds, if a rapid avoidance manoeuvre is required to avoid terrain, windshear or another aircraft, it is safe to rapidly pull the sidestick fully aft and/or bank and hold it there. The aircraft will pitch up to max Alpha, engage TOGA thrust and climb away.

Note that this protection is only available under normal law and is lost under alternate law.

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  • $\begingroup$ However apparently A310 already had alpha-floor, though it did not yet have fly-by-wire nor normal law. Not sure whether A300 had it too, but it probably did; A310 is not that much different from A300. $\endgroup$ – Jan Hudec Jan 5 '16 at 13:35
  • $\begingroup$ @JanHudec Looks like A300 did have alpha floor. $\endgroup$ – aeroalias Jan 5 '16 at 14:29
  • $\begingroup$ How should the rapid avoidance manoeuvre look in an Aircraft without alpha floor? $\endgroup$ – curious_cat Jan 5 '16 at 14:46
  • $\begingroup$ @curious_cat The pilot will have to add power using the throttle lever, and use the stick to reach the desired attitude without stalling. $\endgroup$ – Dan Hulme Jan 5 '16 at 15:24
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When the angle of attack limit is reached, a value which is dependent on the configuration and longitudinal wind gradient, the FMGC receives the order from the FAC. This order is only taken into account by the FMGC, if:

  • Landing gear shock absorbers are extended at takeoff
  • and up to 100 ft at landing

If these conditions are met, the A/THR is engaged, and the Alpha floor protection is activated. The A/THR is commanded max. consequently.

The only way to cancel alpha floor is by disengaging the A/THR function.

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