Originally Posted by CONF iture
What I do see at AP disconnect time is a selected negative vertical speed of 5000ft/min and a vertical speed going in that direction (the quality of the given traces is somehow poor ...)
I also do see a pitch at zero (that's 3 degrees below the usual cruise pitch)
If you are referring to the 'zipper' trace on page 111, I don't see any effect of that on the airplane's behavior. If you look at page 42 you'll see the AP maintaining altitude with very small variations of V/S due to the turbulence that is more explicit in the trace of normal acceleration. I explain the pitch of zero (that's 3 degrees below the cruise pitch in still air) by the autopilot pitching the airplane down to maintain altitude in a somewhat stronger upcurrent that commences at 2h10 (*). Pitching the airplane three degrees down in still air would change the FPA three degrees down, i.e. result in V/S=-1000 fpm. To prevent the airplane from accelerating on the downward flight path, the thrust must be reduced. The A/THR appears to do just that, although it doesn't quite reach the level required for that FPA (page 113).
If what I do see is reliable (?) the initial action of the PF to pull is totally justified.
That may be so, I was only answering a question.
HN39, how would you justify the AP/FD vertical mode trace is not published ?
I have no opinion on that, except that its omission in the interim report probably means that the trace does not provide significant information.
(*) That 'somewhat stronger' updraft may well be responsible also for the rolling motion and for the very particular type of ice particles (crystalline structure, particle size, density) that clogged the pitots of AF447.