The specific cause of this accident seems something of a mystery, ie why did the pilot descend in IMC into high ground. There was no apparent reason to. He had the equipment to know his position with the iPad moving map, the height of the terrain in his path, and his altitude. There was nothing wrong with the aircraft and no reason eg icing to force him down. And he had engaged autopilot modes to enable stable flight in IMC.
The task before him was pretty simple really. Had he just continued on track at the 3000ft he achieved he could have got to his destination. So what went wrong?
Well, we know he was sloppy. His log book was not remotely up to date and he well overloaded a limited power aircraft. We know too that the weather was going to mean he had virtually no chance of achieving this flight according maintaining VMC, as his licence required him too. So what went through his mind pre flight ref weather? One scenario is that he just didn't look carefully enough at the met info. Seems a bit strange to me that the visited pages were rainfall, not TAFs and METARs. I guess it is possible he was clueless enough not to be able to obtain and understand a general weather picture, but after over 3000hrs VFR experience that seems very unlikely.
Another scenario is that he did know the weather was likely to be poor en route but that he was happy (ish) to continue IMC, because he had an autopilot and knew how to push the buttons. And indeed he'd done this before. The AAIB report surprisingly didn't consider this possibility. Though if this was the case why did he descend at all?
If he intended to maintain VMC why climb as high as 3000ft, into an increasing headwind, so early - ie 20 miles before he needed to for terrain clearance? Possibly to get a smoother ride - it was probably fairly bumpy. But I'm not sure this pilot would have thought this way. And if he wanted to maintain VMC, why not descend earlier and faster to maintain VMC, or simply turn back. Two very easy actions.
Another point - I was surprised by the lack of any reference in the report to any RT comms. He was squawking 1177 which would indicate he was talking to London Information. If so, I wonder what he said to them?
The rate of descent from the peak of 3000ft is very interesting and significant to me - it is minimal, at 25ft/min, and is maintained for 15mins. Who ever descends at such a fine rate - and immediately after climbing? Even in a cruise descent to a destination you might choose a VS of say 200ft/min to optimise speed, but not 25ft/min. And I can't believe the cloudbase he was keeping under was reducing so slowly and progressively.
So here's a scenario. The pilot climbs to 3000ft having selected a climb on VS. At 3000ft he turns down the VS to level, but doesn't select ALT. But actually he's selected not 0, but 25ft/min down. He makes both mistakes because he's sloppy. And he doesn't check the altimeter for the same reason.
Now he trundles on at 115kts, passing through and over bits of cloud, until eventually he comes to much more solid cloud ahead. At this point he decides to descend and keep under it. He knows highest ground is 2300ft but as he's at 3000ft he can descend 700ft before he's going to hit anything. And he can see patches of ground well below through gaps in the cloud.
However of course he's not descending from the 3000ft he thinks he's at, but 2700ft. And that 300ft makes a fatal difference. Any thoughts?
An alternative is that his rate of descent rapidly increases when he encounters the forecast mountain wave conditions. Maybe he hasn't deliberately descended faster at all - after all, 185ft/min is still strangely slow to be an effort to maintain VMC - and that this extra modest descent is from downdrafts.
And then finally there is the slightly mysterious reference to "other substances" being found in the pilot's body. Which clearly has the potential for judgment to be even more flawed.