Excellent responses, guys.
The aircraft(FCU) would see it as asymmetry and attempt to shut down flap movement by closing the Flap Bypass valve. However the power for driving the valve to the close position comes from the 28volt DC BUS 3 which in the case above will be UNPOWERED so the valve cannot close therefore the flap drive unit will continue to drive to it's commanded position until mechanical followup nulls the PDU in the selected position (i.e whatever position the input actuator is in)
Thanks, Spanners. This seems to be a pivotal point.
I just wish I understood why my notes say:
If two (flap lever) sensors have failed, flaps will only work in Alternate mode
Didn't QF land in BKK on standby power? Surely a discussion would have come up on a flapless landing if they had done one.
During the QF Bangkok water-in-the-MEC incident, one person on the PPRuNe forum said that the leading edges were (already) deployed early for the approach into BKK, so it wouldn't have been a completely flapless landing. The same person said, however, that the flaps were deployed in Alternate mode (very unlikely if the main elec busses were dead!).
Many thanks....
Rgds.
NSEU