AF 447 Thread No. 11

[Chris Scott]

Hello vilas,

Thanks for listing many of the areas we have been discussing in relation to the A340 AIRPROX and (since yesterday) the AF A340 Incident, and summarising your understanding of them.

I particularly welcome your explanation of the Alpha Lock and Alpha Floor functions in relation to the captain's statement to the UK AAIB after the A340 AIRPROX incident. Some of us are still trying to confirm that Alpha Floor could be engaged at a speed greater than M0.53. Do you have any information?

Some of your points, however, contain significant differences from my understanding. I shall now quote them in the order of your post, and respond.

"In normal law from Valpha prot to alpha Max side stick commands alpha directly..."
I think you are referring to AoA (Protection) Law, not Normal Law? ***

"Once in alpha prot with stick neutral AOA does not increase but is decreased to Valpha prot."
I think that would be true if AoA (Protection) Law was triggered as the AoA was increasing through Alpha-Prot, but not necessarily true if the trigger criterion was phase-advanced Alpha-Prot, i.e., below Alpha-Prot.

"...phase advanced means it [is] triggered before 5 degrees may be 2or 3 degrees. So FBW will try to maintain that and not increase as you suggest."
How could it do that? I think you may be misunderstanding the principle of phase-advanced Alpha-Prot. Even if you do understand it, I think it may be helpful at this point to share my own (tentative) understanding in case it helps some of our readers. No doubt better brains than mine will correct me if necessary.
"Phase-advanced" stall-protection existed, to my knowledge, nearly 50 years ago on the VC10 and BAC 1-11. On Airbus FBW, when the AoA is increasing rapidly in Normal Law, the EFCS/FBW needs to anticipate the exceedance of Alpha-Prot and trigger/engage AoA (Protection) Law in "advance". So it is constantly extrapolating what the AoA may be a short time after the present. We do not know the precise figure of that time-period, but it is probably between 1 and 2 seconds. That means that the precise AoA at the moment of engagement is a function of the rate-of-change of AoA (i.e., delta-alpha), as well as the current value of Alpha-Prot. Delta-alpha obviously depends on a number of variables, including recent/current pilot inputs and gusts. Therefore, for the EFCS to use the engagement AoA as the target for the duration of the period of AoA Law would seem unlikely and inappropriate.
I stand to be corrected, but my money is on the EFCS targeting current Alpha-Prot - which is variable for the reasons you have implied.

"When the speed is below V alpha prot you should know that side stick is commanding alpha there is no need for any further display."
True but - sitting comfortably here on terra-firma - I would simply remind you that the speed indications on the PFD, excellent as they are, may be quite difficult to interpret when you and they are jumping up and down in severe turbulence?


*** (added by edit)
Apologies, vilas! I now see from DonH's post (below) that you are right. Although the AAIB Bulletin into the A340 AIRPROX incident of 2001 refers to it as "AoA (Protection) Law", the FCOM extracts from both 2001 and 2010 that DonH displays clearly refer to it as the "High Angle of Attack Protection" mode of Normal Law.