Originally Posted by
HazelNuts39
IIRC it was not an inconsistency between the three ADR speeds, but a sudden drop in one of them, the "polled" value that the PRIMs were using and considered the most accurate of the three. That anomaly was confirmed at the end of the monitoring period.
Hi Hazelnuts,
Yes, it used polled value, explained by BEA (Interim Report 2) my italics:
Like the FMGECs, the PRIMs consolidate the parameters that they use by means of monitoring mechanisms. Concerning the airspeed, it is the voted value that is used. In normal operation, this is the median value. When one of the three speeds deviates too much from the other two, it is automatically rejected by the PRIMs and the polled value then becomes the average of the two remaining values. But if the difference between these two remaining values becomes too great the PRIMs reject them and the control law switches to alternate 2.
So my reading is:
1) No single value is more imporant than the other in normal operations
2) When one becomes inconsistent with the other two it is rejected
3) When the other two become inconsistent with each other the law goes to ALT 2
It is inconsistent speeds that causes the plane to reject them
Consistent AND valid = Correct
The speeds later became Consistent AND Valid therefore they should have been considered correct. i.e. Normal Law conditions = AoA protections = aircraft breaks the stall.
AlphaZuluRomeo:
Consistent AND valid = Correct
Well, yes it's a good assumption to make that the crew will act correctly once the plane hands over control. Is this what the designers assumed too? Perhaps another flaw in system design?
Normal Law would almost certainly have saved the day. I submit that from the evidence we have, for over two minutes and 30000 ft AFTER the initial problem, that normal law conditions existed. Unfortunately in the interim, ALT 2 had been latched, and the aircraft was unable to break the stall the pilots had caused and failed to recover from.