mm43;
Thank you for providing the document and thank you to Andy Tracy for providing his kind permission. I think there is great value in posting such a comprehensive, well-written paper on important aspects and understandings of the A330/A340 flight control system. The target audience for the document is clearly A330/A340 pilots but a bit of reading, thought and patient enquiry will assist understanding of the abbreviations, relationships and reasons why, etc.
machinbird;
If you had actual angle of attack sensor derived protection in Alternate Law (1) or (2)-Why would you lose it with loss of the VS1g computation?
I believe the usefulness of Andy Tracy's paper may already be apparent. From the paper:
Pitch air mode is a load factor demand law with auto-trim and full flight envelope protection.
Not sure I can help but I'll try.
First, in the same way that the sidestick does not control the ailerons and instead requests roll-rate as previously described, the sidestick does not directly move the elevators in Normal Law. The pitch request of the sidestick is a 'g' request moderated through the PRIMs/SECs as available in Normal Law and normal law alpha protections.
As per a previous post on VLS, the references (essentially) are to VS1g in Normal Law.* Since pitch is referenced to 'g' and since the FMGECs (
Flight Management, Guidance and Envelope Computers) require both 'g' and aircraft weight/CG inputs for VS1g computation (as well as Slat/Flap position as VLS computation changes with such extensions), when such data is lost, reversion is to speed reference.
Obviously with loss of inputs the FMGECs must have "some place to go". The default reference is airspeed even though AoA itself may be available; In direct answer to your question
machinbird, I am unsure how, if at all, any use or blend of AoA from one or all of the three sensors, is done - it may be, I don't know.
One outcome of loss of alpha protections in any reversion to Alternate 1/2 Laws (as described in the Tracy paper) which is immediately visible to the pilots is the change in the speed scale, (schematic posted in the earlier thread on AF447) which no longer displays the alpha speeds but reverts to displaying a red-and-black strip the top of which is VSW**.
*My thanks to HazelNuts39 for an extensive and informative discussion which helped me greatly in understanding this certification issue. Perhaps he may be coaxed into posting a suitable, brief discussion on VS1g and VLS certification (which is slightly different than VS certification)? I think it is germane.
From an A330 FCOM:
**Vα MAX : Maximum angle of attack speed.
Corresponds to the maximum angle of attack that may be reached in pitch normal law.
Represented by the top of a red strip along the PFD speed scale, in normal law.
VSW : Stall warning speed.
Represented by a red and black strip along the speed scale when the flight control normal law is inoperative.