I had fairly well concluded that the most likely cause was a fire disrupting the electrical and control systems, when CNN now say the sharp left turn was pre-programmed 12 minutes before sign off from Malaysian Air Traffic control, which was followed fairly quickly by that left turn.
CNN claim to have this from an US official, from data sent back before the reporting systems went off. It is hard to know what to make of it: obviously there are large economic interests that much prefer blame to lie with the pilots rather than the aircraft. But if it is true then the move was not a response to an emergency. (CNN went on to say the pilot could have programmed in the course change as a contingency in case of an emergency. That made no sense to me at all – does it to anyone else?)
I still find it extremely unlikely that the plane landed or crashed on land I cannot believe it could evade military detection as it flew over a highly militarized region. Somewhere there is debris on the ocean. There have been previous pilot suicides that took the plane with them; but the long detour first seems very strange and I do not believe is precedented. However if the CNN information on pre-programming is correct, and given it was the co-pilot who signed off to air traffic control, it is hard to look beyond the pilots as those responsible for whatever did happen. In fact, on consideration, the most improbable thing is that information CNN are reporting from the US official.