AF 447 Thread No. 12

[DozyWannabe]

Originally Posted by _Phoenix_

More over, FBW algorithm does not meet the Federal Aviation Regulations Part 25 for static longitudinal stability

Not in the strictest sense. In Normal Law, the aircraft has neutral longitudinal stability. Outside of Normal Law, the EFCS is configured to provide a form of longitudinal stability in concert with low energy warnings.
http://easa.europa.eu/system/files/d...Airbus_340.pdf
http://www.caa.govt.nz/aircraft/Type..._A320_A321.pdf
http://www.caa.co.uk/aandocsindex/22733/22733000000.pdf

There were special dispensations based on certification tests, which all regulators (including the FAA) approved.

There's a nice summary of the Airbus EFCS here: http://www.davi.ws/avionics/TheAvion...ook_Cap_12.pdf

Remember that the trim did not move of its own volition - it moved because the pilot input was commanding it to move. In fact, had Robert looked to his right during the minute the trim was rolling back in response to Bonin's sidestick, he may not have been able to see the latter's sidestick, but he'd definitely have been able to see the trim wheel moving. There is a trim position indicator on the wheel which would have told them their trim position had they looked.

Originally Posted by Machinbird

This suggests that his inner tension was being reflected as an inadvertent aft stick pressure. (Never mind, Dozy, if it takes a fair amount of force to do this, we are talking about a guy who is really on edge already and has just fallen into an unfamiliar flight condition) The adrenaline is kicking in.

That's one possibility, as is Franzl's suggestion regarding 'tunnel vision' on roll - but as always I'd urge caution on terms of reading too much into things. The fact is that we cannot know for certain what was going through his head, and that fact is as immutable as it is frustrating.

But it sure does suggest [inadvertent back pressure].

Again, it's possible - but as I said before though, his explicit reference to being "at maxi nose-up for some time" implies otherwise.

So what we're left with includes these possibilities (please add any I've either forgotten or don't know):
If inadvertent:

• He may have had his seat/armrest positioned badly, and induced a pitch component to the input
• He may have been "tensing up" from adrenaline with the same response

If deliberate:

• He may have been trying to follow the intermittent FDs
• He may have not heard and/or realised the consequences of Robert's "Alternate Law" callout, and assumed the hard protections were still there
• He may have fixated on the risk of excessive speed or overspeed from the outset, and been trying to mitigate that risk by using pitch to slow the aircraft

My personal opinion, for what it's worth, is that the last point (i.e that it was intentional and he was fixating on excess/overspeed risk) is the most plausible - though again, I should add the caveat that it's an educated guess and certainly not a done deal. So why do I think it most likely?

1. The "crazy speed" comment, followed by his attempt to deploy speedbrakes, is the most obvious indication that he's concerned about overspeed
2. This happens about a minute and a half into the sequence, but if you look at his comments prior to this, there's a distinct impression that he's been concerned about overspeed for some time ("We don't have impression of speed", "TOGA", "I'm in TOGA eh")
3. He's been sounding nervous about transiting the weather and potential turbulence since before Capt. Dubois went to rest - IIRC standard procedure in turbulence is to reduce thrust and slow down to reduce risk of airframe stress
4. He's likely aware that the THR LK function has been disabled with the TLs in CLB, which goes contrary to the principle of reducing thrust and slowing down
5. Thrust is then set to TOGA, which goes further contrary to that practice
6. He's a glider pilot, so how do you reduce speed? Pitch up.
7. Possibly due to concentrating on damping the roll, he seems to be unaware of the climb he began with the pitch up, and the stall regime begins to creep up just as he gets the roll under control
8. The SW sounds, but he's still in the "overspeed" mindset (similar to the case of the Birgenair 301 Captain), and either does not hear or disregards it
9. The stall condition becomes fully developed and the aircraft begins to descend. When the descent becomes established, Bonin's stick soon becomes slammed against the backstop for almost a minute
10. Nevertheless, the aircraft does not climb - and furthermore, the roll control is lost again
11. This lack of control response is indicative of stall, but similar unusual control responses can also be indicative of overspeed
12. If Bonin did not notice the extent of the pre-stall climb (which he does not mention on the CVR), but he's aware that they've been at higher than normal thrust settings for nearly two minutes (which he refers to twice on the CVR) - he's already set up his mental model with an expectation of overspeed

From this point on, the lack of cohesion on the flight deck becomes terminal. Bonin clearly thinks they're in overspeed, but Robert, in choosing to wait for the Captain, is second-guessing his own SA by the time he returns. Neither Robert nor Dubois offer an alternative explanation for the situation to Bonin, so in the back of his mind he may still be thinking "overspeed" and reacting accordingly.

This is all supposition on my part, but it seems like a fairly logical progression.

@Winnerhofer - there is no useful "car" analogy to that situation. ABS works by using sensors to detect zero wheel rotation when braking is applied at any speed.