Expert: Capt Paul - 1/18/2007

Question
Hi Capt. Paul.
Just wondering how you can get an Airbus into Alpha Floor?
Thanks
Ricky

Answer
Hello Ricky,

To understand “Alpha Floor" we have to take a short look at some of the Alpha protections that the Airbus offers,  “Alpha Prot" vs. "Alpha Floor" and “Alpha Max”.


“Alpha Prot".  This is a condition that the aircraft has reached, and as a protective measure, the following limits are consequently activated - 45 bank limit, speed brakes retract, side stick commands AOA, pitch up trim inhibited – all of which will cancel  when the aircraft HAS safely left the condition that triggered the “A-Prot” protections.  

“Alpha floor" (which triggers TOGA A/THR, speed brakes retract, etc.) occurs at a "pre-determined" AOA (Angle of Attach) between "Alpha Prot" and "Alpha max.” , but this condition must be manually cancelled by the crew by manipulation of the thrust levers  

According to the FCOM, AOA triggers ALPHA FLOOR at 9.5 degrees in CONFIG 0; 15 degrees in CONFIG 1 & 2; 14 degrees in CONFIG 3; and 13 degrees in CONFIG FULL.  

Alpha floor (and therefore automatic TOGA thrust and speed brake retraction) is NOT directly triggered by airspeed bleed.  Alpha floor is strictly a function of angle of attack, and you can put the simulator into alpha floor at speeds in excess of 250 knots.  If you command full back on the side stick, you will quickly command an angle of attack close to the maximum value.  This triggers alpha floor, TOGA thrust, and speed brake retraction within a second or two.  Bleeding off the airspeed is another way to increase angle of attack and trigger alpha floor.


You can consider Alpha Floor as a mode of the Auto thrust system, and Alpha Prot a mode of the flight control system.  

When the aircraft’s angle-of-attack goes beyond the ALPHA FLOOR threshold,this means that the aircraft has decelerated significantly (below ALPHA PROT
Speed): A/THR activates automatically and orders TOGA thrust, regardless of the thrust lever position.

When the aircraft accelerates again, the angle-of-attack drops below the ALPHA FLOOR threshold.  TOGA thrust is maintained or locked. This enables the flightcrew to reduce thrust, as necessary.  TOGA LK appears on the FMA to indicate that TOGA thrust is locked.  The desired thrust can only be recovered by setting A/THR to off, with the instinctive disconnect pushbutton.

ALPHA floor is available, when the flight controls are in NORMAL LAW, from liftoff to 100 ft R/A at landing. It is inhibited in some cases of engine failure.


Incidentally, Alpha protection can also be entered at high speed when it gives protection at the buffet boundary.  Here, the aim is to allow the pilot to use all the available manoeuvring capability while staying outside the deterrent buffet and countering any tendencies to pitch up.  The high-speed stall occurs at much lower Alpha than the Alpha-max adopted as the low-speed limit, and Alpha-Prot and Alpha-max are shifted down as a function of Mach when in the clean configuration.  Thus, full back stick will result in the aircraft stabilizing at the buffet boundary with a small margin to the deterrent buffet.  Because the margin is tiny, a small pitch-down pre-command is introduced progressively at a given Alpha between Alpha-Prot and Alpha-max (function of Mach) to prevent dynamic overshoots into the deterrent buffet with rapid entries into Alpha protection at high Mach.


I hope that this answers your question.

Cheers