JD-EE
A bit of an update on the concept of a bogus Vmo/Mmo pitchup causing a stall.
1.A bit of study on the Vmo/Mmo protection mode indicates that activation of this protection will disengage autopilot as it pitches the aircraft up. This means, that the soonest such an event could have occurred would be at the very beginning of the ACARS sequence, thus perhaps explaining this message at the very beginning:
2:10:10WRN/WN0906010210 221002006AUTO FLT AP OFF
Very shortly after this message we have:
2:10:23WRN/WN0906010210 279100506F/CTL ALTN LAW
So if AF447 encountered such a pitchup, it would not have been very long in duration before the aircraft was no longer in normal law so the duration of a common mode bogus overspeed indication would not have had to stay in sync for very long either before dropping out of sync and causing ALT2 law. Perhaps 10 seconds max.
2. The 2nd BEA report, page 46 describes the airspeed elaboration function used by the PRIMS as follows:
"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. Furthermore, another monitoring procedure is applied to the
value of the voted airspeed and triggers switching to alternate 2 law when it
falls by more than 30 kt in one second."
Note also from the second BEA report regarding other aircraft incidents:
In seven cases, the autopilot was reconnected during the event. In two of
them, the re-connection occurred when the two speeds were consistent
with each other but were erroneous; My personal conclusion: Bogus airspeeds can be encountered for significant periods (minutes) on an Airbus.
The question now is, how long would it take for such a protection caused pullup to put AF447 on a trajectory for a certain stall.? How long does it take for an Airbus to switch flight control laws?