Things get uglier when the 2nd engine cuts out...
1) TAC will (would?) drop out when it detects that both engines are out.
2) When the engines stop supplying AC power and hydraulic pressure, the hydraulic flight controls (including the TAC-adjusted rudder) will remain locked in place.
3) The Autopilot will drop out (disengage) with the total loss of AC power.
>> This creates a race condition in which the exact sequence of events will determine the flight control settings locked in after power is lost.
The suggestion has been made that the Autopilot would drop out first, with the rudder trip from TAC still fully applied. This would lead to an unfavorable condition while power is out. And, when power returns, the autopilot remains disengaged, so these flight control positions will be maintained.
-- This is what leads to the expectation of a very short glide period. The the plane will begin turning right at 2nd engine flame out, and continue that turn even when power is restored from the APC and/or RAT.
This is most likely the 'spiral decent' mentioned in the June 26th ATSB report.
That's not quite correct. The aircraft would have lost AC power when the second engine failed, triggering a series of events:
1. The flight control system would revert to secondary mode and the TAC would immediately drop out as a result of that reversion.
2. The RAT would immediately deploy and begin supplying electrical power via the standby electrical system and hydraulic power to the primary flight controls via the centre hydraulic system.
3. The APU would attempt to start automatically.
There would be several power transfers while all this was taking place, but the left and centre flight control power supply assemblies would remain powered throughout, via the main battery.
I don't believe the flight controls would remain 'locked in place' as you suggest. Some hydraulic power would be lost when both engines failed, but there would still be some hydraulic pressure and shortly thereafter the RAT would spin up and begin providing pressure to the centre system. With no further control inputs, the rudder would centre.
There is a theory that a spiral descent would be triggered by the extra drag from the RAT, which deploys from the RHS of the rear fuselage, behind the wing.