Hello HN39,
Quote:
“I've been trying to imagine the mechanism by which re-engagement of the autopilot at corrupted airspeed would cause a violent upset. The AP controls altitude, so the first assumption one needs to made is that the altitude at re-engagement differs from the s elected altitude. Let's assume it is lower. The AP will then command movement of the elevator control to initiate climb to capture the selected altitude. The AD suggests that the initially commanded movement of the elevator could be too abrupt when the real speed of the airplane is much greater than what the AP 'thinks' it is. [...]”
I was considering a similar idea, though not necessarily with the extra control authority associated with under-reading ASI that you suggest. The possibility of a high FD pitch-bar, and the AP following same, may be subject to two points:
(1) pilots are warned not to engage the AP unless and until the FD command bars are very close to neutral;
(2) (tentatively) does the FMGC not default from ALT (hold) mode to the basic HDG/VS mode (or TRK/FPA mode, if appropriate) in the event that the aircraft deviates a certain amount from the selected altitude? (Currently have no manuals to hand, and a 30Kb/s internet connection, so cannot research this.)
In the event of (2) coming into operation, it could even have resulted from a relatively small excursion below the selected altitude. As the PF adjusted the VS selector-knob to a positive value, ALT* mode would take over sooner or later, possibly causing a pitch-up initially. Many of us have reason to remember the great authority of ALT* mode (altitude-capture mode) on the A330 and other Airbuses, and the potential problems associated with that in the past. No doubt Airbus addressed this issue in the 1990s. However, if the speed was already at or near the low-speed buffet, for whatever reason, any pitch-up would be... unfortunate. I appreciate, of course, that like me you may be pursuing a scenario in which under-reading of IAS leads the speed to be higher than normal. (Did the intakes freeze before the drain holes, etcetera...)