A320 Yaw-Roll Coupling - in practice
Quote from GMDS:
To get better please tell me: If you approach with a constant small bank angle and then you kick in the rudder and a small roll will ensues that was not demanded. Does the AB roll back to what was demanded or does the new rudder induced roll prevail?
[Unquote]
No, she will not roll back (that is to say, not automatically).
Yes, the bank-angle resulting from the displacement will become the new datum, and the FBW will try to maintain it.
These displacements are normal during de-crab, in my experience. Whether they are due to coincident gusts, is arguable. I have seen this happen on crosswinds from moderate to strong; smooth and gusty. That is why it soon became my practice on the A320 to pre-empt the undesired roll by "putting the wing down" slightly, during de-crab, i.e., inducing a slight sideslip.
I agree with PBL [above] in at least one respect:
this aeroplane was not only conducting an approach in a MEAN crosswind-component that was at or above limits; it was also affected by at least 2 gusts that, in my opinion, were outside the gust limit.
In my 14 years' experience, the roll-yaw coupling was not something to be relied on.
The take-off, which – prior to lift-off – is conducted in Roll-Direct Law (if memory serves), is also worthy of comment. On rotation, prior to lift-off, the upwind wing invariably rises, even though the pilot is easing off the usual slight downwind rudder. [You would expect the upwind movement of the rudder, at the latter point, to counteract the tendency of the upwind wing to rise, but it is not enough, in practice.] The solution, again, is to pre-empt it with aileron; just like a conventional aeroplane
Hope this helps.
Last edited by Chris Scott; 7th Mar 2008 at 11:44.
Reason: Correction and clarification of the take-off case.