Hello,
HN39;
My tentative explanation of these events is that the airplane was traversing the fringe of an area with significant updraft velocity (1000 fpm?). The AP pitched the nose down to maintain altitude, and the A/THR reduced engine thrust to maintain airspeed. After disconnect of AP and A/THR, the FCS increased pitch to regain 1 g normal acceleration, and thrust was locked at significantly less than required to maintain airspeed in level flight in still air.
More analysis to be done by BEA (and perhaps me). Why did AP and A/THR disconnect 2 seconds before the speeds dropped?
Yes, I agree with your assessments concerning conditions just prior to AP-A/THR disengagement.
On your question regarding disengagement of the AFS, I suspect that the AP - A/THR disengagement occurred two seconds before the loss of airspeed because the Mach parameter dropped to M0.30 for one sample, two seconds prior to the CAS dropping and the engagement conditions for the AP-A/THR-FD systems were temporarily not met.
On the flight conditions, aircraft response etc, in my view the variations in roll, pitch and engine thrust would be considered minor and common in light to more than light turbulence. Even in moderate turbulence at altitude, manual flight in Normal Law is not a problem. I haven't done it in anger in Alternate Law but in the simulator it is a non-issue but as I've mentioned, one has to be gentle. I think the control inputs by the PF were exceedingly brisk and resulted in the 1.5g initial vertical accelerations. But eight-degrees of roll in two seconds, (as noted in the BEA Report's graphs near the beginning of the Report), is not in itself a "control" issue, nor is the slight pitch down then recovery by the autoflight system just prior to disconnection.
What I wish to convey here is, with a loss of auto-flight capability, one simply flies the airplane, maintaining stable flight while sorting out why the system disconnected, then reconnecting when re-engagement conditions are met.
On why the PF's seatbelt may not have been fastened at impact, I believe
Mr. Optimistic commented earlier that the PF may have been preparing to get out of his seat to let the captain in. I think this is a reasonable conclusion to draw at the moment - I can see something like that occurring, but of course there is no evidence thus far to support this suggestion.