A/THR compensation for declining airspeeds
Thank you both for these answers.
In the AF 447 case, the A/THR was engaged till the long sequence of problems began at 02:10Z (by its disengagement, with the AP, etc...).
The procedure "severe turbulence" was not applied (A/THR off, fixing N1).
Since the problems were already ongoing at 02:10Z when the A/THR disengaged (an air data module had been probably voted out leading to an ADR disagreement a few minutes later), the airspeeds have probably rolled back just before the A/THR disengagement and I try to know whether the A/THR could have compensated this airspeed drop (by augmenting the thrust/a gentle nose down) just before it was disengaged. I would need to have an idea of the time constant of the A/THR control loop (the control loop seems to be a PID).
If the A/THR did augment the thrust before being disengaged at 02:10Z, it probably did it just before, in the few seconds before, so the crew might have been distracted from this by the long sequence of ECAM fault reports ? Well... before speculating about this possibility, I should try to confirm/infirm that the A/THR augmenting the engine thrust due to corrupted airspeeds is possible in the conditions of interest (AF 447 penetrating a MSC around ~02:00Z).
I found an occurrence of an A/THR being involved into an incident with an A340 in Cayenne but in approach (not in cruise):
"Le vent de face atteint un pic à 17 h 45 min 22 puis diminue très rapidement (11) ; la vitesse air décroît. A 17 h 45 min 26, l’auto-poussée commande une augmentation de poussée mais la vitesse air continue de décroître du fait de la diminution continue du vent et du temps d’accélération des réacteurs. La poussée atteint 40 % une seconde et demie après et 58 % trois secondes après le début d’accélération."
the surface winds were blowing strongly when they suddendly calmed, as seen from the plane, the airspeeds decreased and the A/THR reacted to this by augmenting the thrust to 40% in 1.5 sec., 58% in 3 sec. (the lagging is due to the engines, not to the A/THR: it seems able to respond quickly).