One of the few narratives that has come across as plausible is this one, by Chris Goodfellow, a U.S. pilot. His thought is that a fire had been gradually taking hold on the aircraft, damaging electrical systems, and it was detected just at the point at which communication was being transferred to Vietnam. The aircraft was turned towards the nearest suitable airport, but the pilots were then busy dealing with the smoke on board. Eventually, they were overcome by smoke or fumes and died, but the fire died out, and the aeroplane flew on until it ran out of fuel and crashed.
This hypothesis has the advantage of at least sounding like the sort of thing that a pilot might do, and also repaints the pilots of MH370 as heroes. However, it has not received complete acceptance. Two rather sarcastic responses can be found here and here.
I wrote my own response when I first saw Goodfellow's theory being circulated, as people were asking me about my opinions, and I wasn't convinced. My thoughts were that it was a possibility, and I wouldn't rule it out yet (after all, we are still very short of plausible scenarios). But there were several reasons that I wasn't convinced.
- The aeroplane was actually not that heavy (contra his assertions). 239 people on board an aircraft that will comfortably carry over 300 - the payload could be increased by 10 tonnes or more. Eight hours fuel on board an aircraft (B777-200ER) that has an absolute range of 18 hours - the fuel load could be increased by probably 40 tonnes. So it was probably at least 50 tonnes below its maximum takeoff weight. Neither was it that hot when it left Kuala Lumpur - it was midnight local, the temperature was probably around the mid-20s. So it was not a "hot night" or a "heavy aircraft" (indeed, if it were that heavy, then it would not have been able to get up to 45,000 feet later on). These details aren't necessary for the scenario - however, it does paint a picture of a scenario that has been constructed to fit Goodfellow's ideas, rather than the facts.
- Key events need to take place at just the right time. The fire knocked out ACARS early in the flight apparently - an event that went unremarked by the pilots. (Incidentally, it's not hard to switch off ACARS - it may be as simple as selecting a different radio frequency). The smoke suddenly appeared and demanded a response from the pilots just at the point that they were handed over to Vietnam. It's possible, but requires a surprising series of coincidences.
- Most electrical fire "power down" procedures don't leave you with no radio at all. Pilots would be on oxygen; the oxygen masks have microphones in them; and emergency electrical procedures will leave at least one pilot flying and operating a radio whilst the other seeks to deal with the problem on board. Some sort of distress call would have been a priority with the survival of the aircraft in jeopardy, even if "communicate" is third on the list of priorities after "aviate and navigate".
- Finally (though not exhaustively) where did the aeroplane go? No trace of it has yet been found.
There are a couple of key bits of information that would help to make sense of the incident. Things like: what was the actual fuel load (and hence range)? The pilots can use ACARS to get information during the flight, and also get information before they arrive on the aeroplane; presumably, this information is recorded; what did they ask for? And other similar questions. I have little doubt that, if there are people who take seriously some of the more alarming theories circulating, they will have asked these questions already. Of course, I may be wrong - I only have the same information as everybody else.