bearfoil, quote:
"I had always thought the purpose of Aft Trim Tank, or even "geographic" changes to enhance aft trim were intended to add Drag, "Lift" to the tail such that the AoA of the wing could be reduced, and lower Fuel Burn. It is a slight Overall reduction in Drag, but on long haul can save buckets of dough. Was I laboring under a misconception?"
Don't think so, although you don't want to "add drag", and the tailplane never produces positive lift on a public-transport aircraft. (Note I said TAILplane: the HS on a Rutan canard does.) Less negative lift from the HS is the objective, enabling less lift from the wing, and less drag from both. As I was trying to say to DW, I know of no evidence to suggest that the cruise THS setting on AF447 was anything other than routine prior to the UAS event.
bearfoil, quote:
"So now we know the Airbus does use PITCH to control SPEED ? And does it do this with Autotrim (PITCH) with the Throttles in "manual" (A/THR disconnect)."
Presume you mean pitch TRIM? Aerodynamically, Airbuses are conventional aeroplanes with elevators mounted on a trimable horizontal stabiliser
– very like a B767, but with FBW controls. When the AP is disengaged, autotrim is still available. It acts a bit like the CWS (control-wheel steering) on a DC10, for example. Like any such aeroplane, 1G flight with a reducing airspeed (and constant thrust) can only be maintained by up-elevator and/or trimming in the nose-up sense. If not, the nose drops and the aircraft starts to follow a curved trajectory at less than 1G. When this results in a steep enough descent, the speed starts to rise.
The Airbus FBW, on the other hand, maintains 1G without any sidestick displacement until
– in the case of Pitch-Alternate Law – the wing runs out of lift. (In Normal Law, as witness Habsheim, the nose is lowered to prevent a stall – even with full back-stick.)
bearfoil, quote:
"Could the a/c ever have autotrim without Autothrottle?"
Most of my hand flying on the A320 was without A/THR. Never had to use manual pitch-trim except for training purposes.
DozyWannabe, quote:
"With autoflight out, the only thing that the autotrim could respond to (if the system was indeed behaving as designed) was pilot input."
Non monsieur! I respectfully suggest that you
read what I carefully explained to you at Jun14/1157z, particularly the last paragraph, as well as my note to Bear herewith.
Err, by the way, agree with your comments re the BEA.