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Old 4th Aug 2011, 22:04
  #1557 (permalink)  
xcitation
 
Join Date: Jun 2009
Location: California
Age: 51
Posts: 203
Originally Posted by airtren
I've asked the question myself, and have contemplated several speculative explanatory scenarios, due to the limited official information at the time. I've posted one of the scenarios a week, or two, before the BEA report was made available. The posted scenario was that at AP disconnect, the AP was in the process of responding to a drop in altutude, and roll due to heavier turbulence, which was interrupted by the AP disconnect. So the PF reaction was to continue the AP's action with NU, and roll.

The BEA Report made available recently brings more information.
The graphs at the end of the document seem to provide several concurring elements to support the scenario I've considered:

1. there is a slight altitude drop on the Altitude graph, right before the AP disconnect - see page 108 of the English version of the report. The drop is very small, and it's hard to tell how much it is - maybe around 100 ft or so - as the graphs scale corresponds to 35000 ft or so, but Zooming IN, helps seeing it a bit better, within the limits of the graph resolution, and scale.
The altitude drop coincides with a reduction in thrust - see the N1 Actual Eng1% and Eng2 % graphs - and increased roll - see Roll graph.

2. the Normal Acceleration graph, which if I understand correctlty is a reflection of the Turbulence, shows increased up/down air turbulence activity right at that time.

So, I think, turbulence, drop in altitude (with reduced thrust) and roll, were the factors that the PF was responding too with Roll and NU.

I have a vague recollection of seeing posts tangentially, or directly mentioning the same thing, but can't recall exactly which ones.
Very important point. So...

AP disconnects
a/c at 0 deg pitch => will descend
pilot stick back 3/4 causing nose up pitch 11 deg.
climb rate goes up.
after 4 seconds first stall warning
after 15 seconds altitude has not changed (downdraft/reduced thrust?)

So PF was correct to stick back but he over does it.
Stall alarm sounds, he eases off on the stick pushes it fwd briefly, alarm stops.
Then he resumes stick back to maintain altitude.
Now the THS starts helping PF to pitch nose up.
Now he is gaining altitude with help from THS.

Report #3, Page 111, Longitudinal parameters

Notice the insidious influence the auto THS (cyan) has on elevator position. At some points he is stick forwards (red line, 02:12:17) yet elevators stay around -30 deg (purple). Clearly this would add to the confusion of inputs.

Last edited by xcitation; 4th Aug 2011 at 22:29.
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