Machinbird
I've seen no evidence in the record that AF447 suffered from Dutch Roll. The A330 aircraft has an independent BYDU unit to provide yaw damping. Normally yaw damping alone is sufficient to prevent Dutch roll, although one jet I flew had "stability augmentation" on all 3 axes (and flew the same way a drunk walks when it was switched off.)
Iīm with you on all you wrote concerning PIO and itīs possible influence. I had my share of it once during airrefueling on a KC-135 in pitch dark night just prior contact flying from the backseat during instructor training.
And i can confirm your last sentence ......."and flew the same way a drunk walks when it was switched off" .......as we probably think about the very same aircraft.
Some reference to the Backup Yaw Damper Unit:
BACK UP YAW DAMPER UNIT
The Back up Yaw Damper Unit (BYDU) provides yaw damping in case of yaw
damper servo actuators deactivated, through an independant unit including its own electrical generation on Blue and/or Yellow hydraulic. It computes a yaw damping order similar to lateral alternate law (without turn coordination).
No "turn coordination" and hence no sideslip compensation most probably was contributing to the aparent problems in lateral control in the phase from AP dropout/ALT2 Law to stall entry and later on in the stalled situation. The FDR on Page 106 of BEA IR3 shows the graphs for Sideslip, and on page 112 itīs together with aileron input.