Thomas Coupling wrote:
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 instinctively 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.
I'm far from an expert on the 135, but this sounds plausible to me. Or, at any rate, something along these lines - ie, something goes wrong, tired pilot makes mistake, then he tries to sort it out but too late. In other words, it's beginning to look as though this tragic accident was due to a combination of unexpected malfunction combined with human error.
I realise this probably leaves us with an almost infinite number of possible scenarios, limited only by our collective knowledge of helicopter aerodynamics and human factors. Well, we have several months....