Originally Posted by
CONF iture
Question is why the elevators did the opposite of the sidestick displacement ?
That is the question that all of you should try to find an answer for ... but don't worry both BEA + Airbus have taken great care to avoid the question in the first place ...
Now who's being misleading? The answer to that question is summarised right there in the report - the behaviour of the aircraft was consistent with High AoA Protection mode. I suspect that the problem is not that you don't know the answer so much as you don't *like* the answer.
I must admit that despite initial reservations, creating this thread has given me an opportunity to understand the situation in much more detail, and everything new I've seen seems to back up the BEA's summary.
Correct me if I'm wrong, but you seem to be operating on the assumption that pulling up when in High AoA Protection will command the flight control systems to achieve and hold Alpha Max (as it is displayed on the graph) almost immediately based on a snapshot of the aircraft's orientation and configuration at that precise point. If so, this assumption appears to be in error.
For starters, the FCTM material earlier in the thread seems to flatly contradict this assumption - the wording states either that Alpha Max "may" be achieved, or doesn't refer to Alpha Max at all ("a maximum AoA" isn't the same thing). What they state is that the system will maintain a setting which will provide maximum lift based on the current status of the aircraft.
As OG correctly points out, in order for a digital system to work reliably in real-time there needs to be a degree of "filtering" of the data. I'd be very surprised if the systems did not, in addition to this filtering, observe trends over time (as they would have to when, for example, checking that the A/THR disconnect switches were held down for a certain period of time).
Presuming that the FCTMs are correct, what pulling back on the sidestick will do when in High AoA Protection mode is command the systems to configure the flight surfaces to provide maximum lift based on the data over a certain period of time. Airbus spent the best part of a decade, with their experimental FBW A300 and the first A320s off the line, collecting and refining the data used to define the aircraft's behaviour.
As I was saying when I first donned my Speculation Helmet, the systems would have been fed with data indicating that the aircraft was slowing down. Because the laws of physics demand that there must be a delay between the movement of a flight surface and the aircraft responding to that movement, as well as other uncontrollable external factors such as wind speed and direction, it follows that the pitch attitude commanded in a situation where the aircraft is slowing down will need to be somewhat shy of what it is at that precise moment - otherwise it risks encroaching on approach to stall, which would defeat the whole purpose of the protection.
Therefore, as pulling back in that mode cannot be assumed to give Alpha Max (or, at least, not immediately), what that sidestick command actually "tells" the flight control system is "give me the best AoA you can". So, going back to the beginning, the elevators were briefly commanded nose-down
because the systems were trying to maintain the optimum (not necessarily maximum) AoA, and that was what was required to maintain it. And if I were to hazard a guess, given the period in which the elevator position was noted, the reason it was required would have been to counter the pitch-up moment from the engines as they spooled up and started producing significant thrust.