The possibility that missing Malaysia Airlines flight MH370 became a ghost plane flying on autopilot above the Indian Ocean with an unconscious crew has raised new questions in one of aviation's greatest mysteries.
No trace of the Boeing 777 has been found since MH370 vanished on March 8 during a flight from Kuala Lumpur to Beijing with 239 passengers and crew aboard, including New Zealanders Paul Weeks and Ximin Wang.
Somehow, the airliner diverted sharply from its planned route and crashed into the remote Indian Ocean off the coast of Western Australia. Theories have ranged from wild conspiracies to an elaborate suicide by one of the flight crew.
The latest hypothesis has come from a team of international experts assembled by the Australian Transport Safety Bureau to determine the most promising area for what will be a long and exhaustive underwater search. A key part of this was understanding what went on in MH370's cockpit, leading to an assumption that hypoxia - oxygen deprivation caused by the loss of air pressure - may have overcome the pilots. Supporting evidence included the loss of radio communications and the fact that the airliner flew for hours on a steady cruise altitude without large changes in direction.
Gallery: MH370: 'Off the map'
"Given these observations, the final stages of the unresponsive crew/hypoxia event type appeared to best fit the available evidence for the final period of MH370's flight when it was heading in a generally southerly direction," the report said.
It is also believed that at some stage the autopilot had been engaged deliberately, allowing MH370 to continue flying until its fuel ran out.
Transport Safety Bureau head Martin Dolan told reporters it was "highly, highly likely" that MH370 had been set on autopilot before it dived into the ocean: "We would generally expect that if the autopilot is operational ... it is because it has been switched on."
Under the hypoxia scenario the airliner probably flew for seven hours and 38 minutes before fuel ran out, first in one engine and then minutes later in the other, finally causing both to flame out. The airliner's electrical systems would have shut down, sending it into the ocean in a spiral that could have carried it for more than 180km before impact.
This scenario is not universally accepted. Shortly after MH370 vanished Malaysian investigators said they believed someone had deliberately turned off its communications systems and diverted it well west of its planned flight path.
This week Britain's Sunday Times reported investigators considered the airliner's captain, Zaharie Shah, as a prime suspect, and speculation remains that suicide by a crew member caused the crash.
The Transport Safety Bureau report said its assumption had been made only to help define a search area and was not intended to infringe on Malaysia's role in identifying the cause of the airliner's disappearance.
Dolan declined to speculate on the possibility that the setting of the autopilot system pointed to a deliberate plan to crash the airliner.
The truth is that no one has any real idea of what happened in the cockpit. The focus is now on trying to find out where MH370 went down, following the failure of a massive international air-sea search.
The starting point was the airliner's disappearance from radar screens about an hour after leaving Kuala Lumpur, when it was about 220km off Kota Bharu on the east coast of Malaysia. Its final communication was received almost seven hours later.
The bureau's report says the airliner at first flew according to its flight plan then turned right, cutting data from its secondary radar. The primary radar showed it tracked along the Malacca Strait before heading across the Indian Ocean.
In defining a new search area the team of Australian, British and American experts focused on where the aircraft turned off its northwest heading, its performance limits, and analysis of satellite communications data. All were subject to "uncertainties".
A small variation in the frequency of burst transmissions, for example, could shift MH370's final flight path by as much as 1000km. Burst transmissions are signals sent automatically to satellites allowing experts to calculate an aircraft's position by timing the delay between sending and receipt.
The report says continuing refinement of satellite data analysis could change priorities and shift the focus of the search.
But using the latest analysis and matching it to the airliner's airspeed and altitude experts could work out how much fuel remained when the last transmission was received. Further signals were probably lost when fuel ran out and electrical systems shut down.
The team also used data from radio communications, the airliner's aircraft communication addressing and reporting system and the Inmar-sat and Satcom satellite networks.
The result is still a vast area of the Indian Ocean, extending along an arc 650km long and 93km wide covering about 60,000sq km, almost all of it unmapped. The surface of the zone had previously been searched without success.
The hunt is now going underwater. The ocean floor is being mapped by the Chinese survey ship Zhu Kezhen and the Australian-contracted vessel Fugro Equator.
It will be followed by a painstaking underwater search using specialist equipment that could take as long as a year - and it might change focus as new data emerges.