Sidestick, Elevator, and THS
Quote from
PJ2:
Your previous post on the THS on thread #4, here, explains the THS operation very well and this one connects that explanation with how the THS likely functioned with AF 447. I think its a reasonable explanation of what occurred to the THS and how, after the initial pitch-up.
Thanks for your kind acknowledgement of my recent beginners' guide to "Primary and Secondary Pitch Control" on the previous AF447 thread, which also attempts to describe very briefly how the basic concepts are applied on Airbus FBW, and their relevance to AF447.
The key to the THS movements are, of course, how much elevator the EFCS is using: you explained that here before I ever did.
In a couple of follow-up posts, including the one you have quoted in your post, I tried to develop the argument to describe part of what may have been happening with pitch control as the aircraft reached its apogee, and immediately after:
http://www.pprune.org/tech-log/45465...ml#post6559334
http://www.pprune.org/tech-log/45465...ml#post6560444
Finally, I offered a partial explanation of the problems the PF seems to have been having with roll and pitch control using the sidestick, and how the EFCS may have interpreted what it thought he wanted in pitch.
http://www.pprune.org/tech-log/45465...ml#post6561309
Quite how the aeroplane eventually established itself in – as others have commented – a remarkably stable descent at AoA > +40 and pitch ~ +15 remains mysterious, partly because this is uncharted territory in terms of the aerodynamic performance of the fuselage and THS, as well as the wing. The resulting pitch-moments are therefore unclear. In that context, I recommend a look at Meikleour's contributions, the first of which is here, which relate to a simulator exercise many of us have not done.
The other unclear factor is how the C* pitch-function of the EFCS would treat the invalidation of CAS (IAS) data, when determining the crossover from g-control to pitch-control. As I understand it, that crossover is normally a gradual transition as the airspeed falls below a certain figure in routine flight. Understanding how the UAS affected this will be the other key step in solving the relationship between sidestick position, elevator/THS position, and the achieved trajectory.
As these pieces are on the previous thread, the links provided will enable you to reach them more easily, and I look forward to any comments or objections. If none should be forthcoming, I shall infer that even the most erudite of our contributors find nothing to disagree with in my proposals... 