Originally Posted by
Machinbird
Quote:
Originally Posted by
BOAC 
Henra,
If it had been Alt 1 law yes. Unfortunately this was in Alt 2 law. Nothing is leveling the wings in Alt 2 law but the pilot. There are indications that there was either a wing heavy condition or turbulence causing roll inputs to be required to avoid an extreme roll attitude.
Agreed we do not know how violent the right bank tendency was.
Looking at the fact that even deep, deep in the stall, at AoA > 45° the roll attitude always appeared to be relatively straight and controllable (for that AoA). This does not point to a dramatical static imbalance. Nor does it point towards a tendancy of the aircraft to roll dramatically in general.
Therefore we will have to wait for more details to judge if aileron input was required to correct this or if it would have been rather a case for rudder trim or even waiting, riding out the turbulence.
We simply do not know the rate and duration of the beginning roll attitude.
On the other hand I agree that a cross control effect from correcting roll attitude is a prime suspect for triggering the initial Nose Up command.
(The subsequent NU's are different, I mentioned already my somewhat favourite theory of somatogravic illusion of falling when reducing the RoC from 7k fpm to 700 fpm close to the stall speed.)
What could have caused the roll excursion:
- Fuel imbalance:
Not very likely, would likely have caused severe Wing Drop after the stall, putting the Aircraft probably either Upside Down or even more likely in a spin as consequence of continuous Left aileron at high AoA. Moreover we have no indication of a pump failure or some such.
- Rudder Trim:
Cannot be completely ruled out. Although there is no mention of Pilots correctiong rudder trim. Would only make sense in combination with asymmetric thrust. Apart from this it is still unclear to me, what would cause this.
- Asymmetric Thrust:
AT should prevent that. Even after disconnect, behaviour should be rather symmetric. I would consider it not extremely likely.
- Aileron mistrim by AP to correct for wind shear, lateral air movement:
Could be a potential reason but it would be quite a concidence if the conditions would just change at the same time as the AP quits.
Ányway amount of roll should be small in this case (< 5° bank)
- Turbulence / Up-/Downdraft:
Looking at the general environment where AF447 was traveling and the relative unlikelyhood of the previously mentioned causes this is probably by far the most likely reason.
Did I forget a reasonable case?
If it was indeed the consequence of turbulence I would assume that even when not touching the SS for 20s, you would still find the aircraft in a reasonable attitude. Not perfectly S/L but probably also not in > 30° bank (Yes Alt2 would not prevent exceding this, I conclude this rather from the general behaviour of the AC in roll- might be wrong though).
With Autotrim trimming for 1g you would even not start entering a spiral dive by doing so. But we will have to wait the next BEA report for details which would confirm or contradict this assumption.