Hi Rudderrudderrat,
Originally Posted by Rudderrudderrat
Is the reason that AB FBW don't have the option for the pilots to deliberately select Direct Law (unlike B777) because the allowed CofG can be so far aft and the subsequent handling characteristics?
CG set at max envelope aft should not be an issue for handling in direct law if one is carefull about not over controlling (elevators are really powerfull at high altitude). In any circumstance, with full aft CG, (beside some related FCMC faults) Airbus state that no forward fuel transfer is needed (turbulences, whatever...). Nonetheless, direct pitch is loosing autotrim and, during normal operation at cruise, there is a constant displacement of the CG due to fuel transfers (by +/- 0.5%) . Hence in direct law, one would have to retrim constantly in pitch or disable the fuel transfer function.
Originally Posted by Rudderrudderrat
I agree that rudder should be used to trim out any roll tendency - maybe if the aircraft had been perfectly in trim at the time of AP disconnect, then the PF could have just left the controls centred and continued at FL 350.
Right. They had just completed a virage before AP disconnected. It's then possible that ailerons were still adjusting in autoflight when AP kicked off and PF had to correct this roll tendency. It certainly played its part as the attention of the PF was then attracted by this correction of the roll rate rather than keeping his pitch in range. As the roll axis was direct, it was much more sensitive than previously and he certainly over-controlled for some time. This could also happen if pitch would be translated directly into direct law, especially at this altitude where the aircraft was flying.