Nightingale said: Maybe some of us are doing the captain an injustice. The cargo information is hazy. (Latest reports say the Malaysians are declining to provide the manifest to the Australians without a formal written request.) Let us not forget the plane was flying to Beijing. Maybe there was something "sensitive" on board for the Chinese government and the Malaysians need to clear this with China before releasing the information. Maybe one or more passengers had got wind of this cargo and tried to hijack the plane to make demands against release of the cargo. The captain and FO were forced to turn off the signals but managed to turn south before they were killed. Then there was no-one left to fly the plane so it continued on auto pilot until it crashed into the Indian Ocean. Well it is no crazier than any of the other theories being put forward here"
I think you are spot-on ..... in response to a catastrophic fire caused by e.g. the Li-Ion batteries being carried as cargo. If these were in the forward cargo they could have affected electrical busses in close proximity resulting in loss of ACARS and transponder as well as radios.
It would be perfectly normal under such circumstances to don O2 masks and select heading back towards the nearest suitable airport (KUL or Phuket?) then FLCH to descend the aircraft to FL120 while trying to communicate (unsuccessfully) and avoid breathing acrid fumes. Oxygen bottles could have overheated from a fire and blown out so no O2 supply for the crew.
The aircraft turns onto the selected heading and descends to FL120 at the selected speed. It levels off and continues flying on the same heading at FL120 and maintaining e.g 280 knots until it runs out of fuel and glides at 280 knots into the ocean.
Do the maths using the FPM at FL120 and 280 knots or thereabouts but I wish these so-called "experts" would leave the crew and their poor families out of it unless they have any concrete evidence otherwise.
(Remember LI-Ion batteries carried as cargo brought down a brand new 747-400 in Dubai in 2010. The crew were completely incapacitated and unable to fly the aircraft in daylight and in contact with ATC)