I'm sure when getting their type rating for Airbus FBW aircraft pilots learn how to fly in alternate and direct law. After that how much opportunity do they get to practice this skill?
On Air France 447 and Indonesia AirAsia flight 8501 the A/P switched off unexpectedly and the control law degraded to alternate 2. It appears in both situations that the PF had trouble stabilizing the aircraft roll attitude. It's difficult to determine whether this contributed to the pitch up and eventual stall but the original pitch up command may have been inadvertent while struggling to level the wings.
Under normal law, which pilots are accostomed to flying, the FAC calculates the amount of aileron deflection required to turn the amount of lateral sidestick deflection into a rate of roll with a maximum of 15° per second. In alternate 2 law the computer cannot make these calculations so it becomes direct stick-to-aileron control.
This can mean that the aircraft responds very differently than what the pilot is used to. Sometimes the difference may be slight but at high speed and high altitude the same stick deflections can result in a significantly higher roll rate.
Additionally the stick does not provide tactile feedback as to how much force is required to move the control surfaces. With conventional flight controls "roll or pitch rate would more directly correlate with the force applied on the control wheel, and not the amount of deflection required."1
These things combined can result in the pilot over-controlling. The control inputs can get out of phase with aircraft movements resulting in pilot induced oscillation in the roll axis. This occurred in both of the above accidents.
It must be terribly disconcerting for a pilot when the A/P disconnects and they are handed control of an aircraft they fly every day, but doesn't respond the way they are used to. I'm wondering how much practice they get at flying in this configuration, either in a simulator or in the aircraft itself. Do most airlines flying FBW aircraft require this regularly? Does Airbus - or anyone else - recommend any amount of further training or practice? Have these recommendations changed in light of the accidents?
1Bill Palmer, Understanding Air France 447