View Full Version : A320 pitch normal law-AOA protection

28th Apr 2010, 12:19
Huy guys,

hope someone can clear this up for me:

I understand that when speed goes below AlphaProt:
a) Stick-elevator becomes direct relationship
b) AP disconnects
c) you can pull the sidestick all the way back to maintain AlphaMax, but when released, it returns to AlphaProt.

my confusion is, if you can do (c), then when does the ALPHA FLOOR kick in?

if i pull the sidestick all the way back to AlphaMax, how do i know if the alpha floor is going to engage or not?

why in some cases you can pull all the way back to alphaMAx, then release and it returns to alphaProt (no alphafloor activation)? and in other cases, if you pull back and your speed drops below alphaProt (but speed is still above AlphaMAx) then alpha floor activates?

Appreciate any help on this!

Thanks guys!

28th Apr 2010, 13:18
I think its a rad alt issue.Too long ago since I was current but there is a rad alt threshhold where the aircraft either will or will not give you alpha floor.

28th Apr 2010, 13:24
I believe this is to do with low energy activation.
ie pull it slowly all the way back it will maintain alpha max. Yank it back and it senses a high rate of change and alpha floor is triggered once it gets to alpha prot.

I'm sure there is a more technical explanation but pretty sure this is the gist.
Alha floor is available above and a below radalt so don't think thats linked.

28th Apr 2010, 13:28
Hi C4,

My only experience is watching it in the simulator (honestly), but it seems to depend on the speed trend vector. If the system thinks you will go below Alpha max in "X" seconds, then Alpha Floor with TOGA LOCK is activated.

So gentle flying will let you get close to Alpha Max, but a sharpish pull with consequential delta "g" (which is not felt in the sim) and increase in Alpha max, can trigger Alpha Floor unexpectedly.

CONF iture
28th Apr 2010, 15:59

The way things are in your point C, ALPHA FLOOR will trigger.

A possibility to fly at ALPHA MAX without ALPHA FLOOR activation would be to have earlier disconnected the A/THR system, and this is how Airbus pilots proceed for the high AoA presentation during airshow.


28th Apr 2010, 16:15
Hello guys,


Alpha floor will trigger (no faults) somewhere between alpha prot and alpha max. In case (c) you have now TOGA (alpha floor) and with stick fully back you will climb at alpha max. If you release stick, you will climb with TOGA (alpha floor) and alpha prot. If you push the stick, you will accelerate and exit alpha protection (exit also alpha floor conditions), ALPHA FLOOR reverts to TOGA LOCK, you can now exit Alpha floor by disconnecting autothrust.
By the way case (a) is stick-AOA direct relationship at α>αprot.
It would be more clear if you consider alpha protection and alpha floor as independent protections. In an autothrust failure alpha floor would not engage but alpha protections still apply. Thrust setting would be your job then.


29th Apr 2010, 17:04
cheers guys all your input has made the following a lot clearer to me.

thnx for taking the time, much appreciated :ok:

2nd Jul 2010, 13:27
Just to clarify, if Autothrust is not engaged (at idle for eg) and the sidestick is pulled back all the way to Alpha Max, the flight control computers would still command thrust to maintain Alpha Max? The only difference being Alpha Floor protection not kicking in due to Autothrust not being engaged?

2nd Jul 2010, 14:23
the flight control computers would still command thrust to maintain Alpha Max?

aristoclis is correct. The elevators will move to maintain Alpha Max.
Alpha Floor is a thrust mode.

2nd Jul 2010, 15:23
Thanks for pointing that out cheers!

3rd Jul 2010, 13:17
ch4 wrote I understand that when speed goes below AlphaProt:
a) Stick-elevator becomes direct relationshipWrong. Sidestick now commands Alpha instead of g, so stick neutral gives AlphaProt, stick fully back gives AlphaMax, with progressive Alpha in between.

Alpha Floor is an A/THR function, not FBW. In a controlled deceleration with the sidestick moving progressively back it will engage somewhere between AlphaProt and Max (exact point not quoted in FCOM because it varies with pitch rate). It will engage at normal IAS if you pull agressively.
Hope that helps to make it clearer.

3rd Jul 2010, 13:32
There is a lot of misunderstanding here so far. I'll try to clear it up for you.

1. A-floor is an ATHR function, not F/CTL, as has been correctly stated.

2. It will activate whether or not the ATHR is active.

3. It activates at the A-floor threshold which is dependant on the aircraft configuration (Flap 0, 1, 2, 3 or FULL).

4. The A-floor threshold is an angle of attack. Without looking it up I think it is 9.5 deg AoA in flap 0 and 13-15 deg AoA in other configs. The precise threshold depends on the 320 variant.

5. A-floor reverts to TOGA LK when the aircraft AoA returns to a value below the A floor threshold. This is at no precise point on the speed scale which can be identified by the pilot.

6. Any sharp pitch up input can provoke A-floor at speeds well above VLS/A Prot because there will have been an instantaneous increase of the AoA above the A-floor threshold.