Airbus Alpha protect details

[Machinbird]

A phrase in the interim ATSB report on the QF72 accident indicates a deeper level of complexity to Alpha Protect and flight control than is revealed in FCOM.

Flight envelope mechanisms
• First upset was close to worst possible
scenario:
– 4 degrees alpha prot, 6 degrees anti pitch-up
• AOA processing algorithm using just two
sensors only on A330 and A340
– different algorithms used on other Airbus
aircraft

-4 degrees alpha prot? Is this an authority limit? If so, how is this calculated. Is it constant?
Anti pitch-up? How does this tie in? What does it do exactly, and when?
FCOM Flight controls hasn't been particularly enlightening. I hope someone with access to the maintenance manuals might be able to enlighten. How do these algorithms work? What other parameters are at work?

[PBL]

Machinbird,

I can't answer all your questions, but I can answer some.

-4 degrees alpha prot? Is this an authority limit? If so, how is this calculated. Is it constant?


QF 72 was in cruise, so between 0.78M (design manoeuvring) and 0.86M (M_mo). If you read the BEA AF 447 Interims, you will see that stall warning is triggered at 4.2° AoA at Mach 0.8. So it seems at cruise airspeeds, the protections need to start kicking in somewhere around 4°. Such a figure for alpha prot at cruise airspeeds seems consistent with that.

No, alpha prot is not an authority limit. Alpha max is an authority limit, and AFAIK alpha prot is less than alpha max in any flight regime. No, it is not constant. Stall AoA varies with Mach number at higher Mach numbers, and presumably alpha max and thereby alpha prot vary concomitantly.

I hope someone with access to the maintenance manuals might be able to enlighten.

This has to do with aerodynamics and flight control. I don't see a reason why any of it should be addressed in the maintenance manual.

How do these algorithms work? What other parameters are at work?

In my experience the precise details of the aerodynamics and flight control of high-performance commercial aircraft is mostly proprietary information. That considered to be appropriate info for flight crew is in the FCOM. Whatever is not there is thus likely to be proprietary.

PBL