Originally Posted by Chris Scott
Can Alpha-protection be activated while the AP is engaged?
Re the NAT-TRK A340, think you and NOD are right to consider the possibility that the initial pitch-up was caused by overspeed protection, which also disconnected the AP. With speed back in limits, Normal Law would be restored. As NOD implies, absence of sidestick movement by the PF would allow a 1G (approx) trajectory, with speed decaying (and AoA rising) until Alpha-Prot was activated.
Chris; I must apologize for airing a couple of thoughts which I quickly withdrew after re-reading the AAIB Bulletin, the Fcom and the AF447 ACARS messages.
The NAT incident shows that Alpha-prot can be activated with AP engaged, but causes AP disconnect. Therefore this scenario doesn't fit the AF447 ACARS messages, which indicate that AP disconnect was immediately followed by reversion to ALT law.
In the NAT incident M=0.882 was reached at 14:21:43, but the pitch up began at 14:21:50 when Alpha-prot law was engaged. Once Alpha-prot is engaged, it remains active until a nose-down command is given with the sidestick (see the FCOM and the AAIB Bulletin). Until that occurs, "the AoA protection law seeks to hold the angle of attack constant at alpha prot".