Alpha Floor Protection and High Angle of Attack Protection

This content has been published after getting repeated queries via site’s contact us page and our A320 technical forum’s topics. To most, in fact all, of the senior Airbus pilots both these terms must be clear but some new ones face challenges in relating both.

Before writing subjective explanation following are main points to understand about differences in alpha floor protection and high angle of attack protection.

Alpha Floor Protection High Angle of Attack Protection
  • It is function of Auto Thrust
  • It is not available in case of Auto Thrust failure or dispatch under A/THR MEL
  • Doesn’t put any restriction on departure of aircraft for any type of operation
  • It is function of NORMAL Flight Control Law
  • It is available in case of Auto Thrust failure or dispatch under A/THR MEL (no relation to auto thrust)
  • This is primary protection and will be available as long as aircraft is in normal law. Aircraft can not depart under any other law than normal law so accordingly this protection puts departure restriction

Alpha Floor and High Angle of Attack Protection

ALPHA FLOOR PROTECTION

ALPHA FLOOR protection commands TOGA thrust regardless of thrust lever position when aircraft speed drops below Vafloor. Alpha floor speed is less than alpha prot speed but more than alpha max speed. ALPHA FLOOR is available from lift-off to 100ft RA on approach.

Indications in alpha floor condition:

  • “A FLOOR” in green, surrounded by flashing amber box on FMA column 1 and amber on engine warning display.
  • When aircraft is no longer in ALPHA FLOOR condition, “TOGA LK” is displayed in green, surrounded by flashing amber box on FMA. TOGA thrust is frozen.
  • Note: To disconnect Alpha Floor flight crew must disconnect autothrust

HIGH ANGLE OF ATTACK PROTECTION

When AOA increases above alpha-PROT, high angle of attack protection activates.

Keypoints:

  • without pilot’s input flight control maintains angle of attack equal to alpha PROT
  • with pilot’s input maximum angle of attack equal to alpha max can be maintained
  • when high angle of attack protection is active normal law demand is modifies to correlate side stick input to angle of attack demand instead of load factor demand (angle of attack can be varied between alpha PROT and alpha Max)
  • at takeoff alpha prot is equal to alpha max for 5 seconds

To deactivate angle of attack protection, pilot must push sidestick:

  • greater than 8 degree forward or
  • greater than 0.5 degree for at least 0.5 second when alpha is less than alpha max

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