From the Airbus FAST Magazine #20 (refer to flight manuals for latest info).
“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.
Alpha Floor is inhibited:
- below 100 feet radio Altitude,
- if autothrust unserviceable,
- following double engine failure on an A340 (or one engine out on the twins),
- following certain system/auto flight failures,
- above Mach 0.53.
Subject to the above, 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. Such precise manoeuvring around the low speed edge of the flight envelope is virtually not possible in any conventional aircraft.”