We should stay on solid ground. Therefore I am going to steer clear of any (so far) mechanical faults as mentioned in the AAIB prelim from now on.
The usual caveat: can a current operator confirm the feasability of this please as it's been a while since I drove the old girl? However my current experience on heavies should transfer across hopefully.
The pilot is coming to the end of his task having spent some considerable time doing the same old, same old. I recall getting bored/tired towards the end of some monotonous sorties and my mind wandering.
At some challenging stage in his flight he experienced a potentially benign run down or torque fluctuation caused by one of the ECU's stalling/surging (as witnessed by outsiders). "Normally" one determines the duff engine by comparing Nr movement with the suspect engine Ng. The duff engine drives the Nr out of its governed range thus enabling the pilot to select the correct engine for shut down. Perhaps this time the wrong engine was shut down whilst tired/confused. This left the duff engine driving the Nr and if that engine was providing less than nominal output the Nr will droop or even worse. He may have instinctlively raised the collective to maintain his cruise height anyway (or tried to stop his increasing descent) and the Nr would continue to decay - ever closer towards its point of no return (what is it in the 135? 82%?). He sees this and tries in vain to salvage the decaying Nr by chopping the remaining engine and lowering the collective but too late, he sees the looming deck and pulls for all its worth to salvage some of the RoD. By then there is very little Nr left, but what is left is washed out by that final application of lever. RoD continues unchecked and the cab hits hard with no engines or little minimal Nr.
