Aviation experts are focusing on the possibility that key sensors on the Germanwings Airbus A320 could have iced up, causing the aircraft to descend rapidly.
Another theory being advanced in the immediate aftermath of the crash was that the crew could have been incapacitated by a sudden loss of pressure.
Even though it will take some time to determine exactly what caused the Airbus to crash, there appear to be parallels with an incident involving an Airbus A321, operated by Lufthansa, last November.
It led to the European Aviation Safety Agency issuing an emergency directive setting out what should be done should the 'angle-of-attack' sensors freeze. In that case the aircraft dropped 1,000 metres a minute with the crew only regaining control of the aircraft when they turned off the onboard computers.
In 2012, Airbus began replacing icing-prevention devices on nearly 700 jets because regulators found they may actually increase ice build-up on the angle-of-attack sensors.
In common with other modern aircraft, the Airbus A320 is a 'Fly-by-wire' plane. It is considered safer and also makes the plane lighter, cheaper to operate and reduces the pilot's workload.
But experts pointed out that a fault in the sensors feeding information to the computer could cause serious problems.
Bob Mann, an American aviation consultant, highlighted the angle-of-attack sensors used on the A320.
"If it thinks a plane is about to stall, it will cause the nose to pitch down," he said. "From the flight radar it looks like a nine-minute descent at a constant 400 knots."
The steady path of the plane could indicate that its path was being controlled by computer.
This could also indicate that the crew had lost consciousness, said Nick Brough, an aviation consultant based in Italy.
"The aircraft appears to have gone into a decent lasting eight minutes, at a more or less constant velocity, until hitting terrain," said Mr Brough.
"Without intending to speculate, if it is true that the crew made no attempts to make radio contact, they may have been suddenly incapacitated. At this stage oxygen starvation cannot be ruled out, as in the Helios Airways accident near Athens in 2005.
"Without oxygen, you lose consciousness very quickly - hence the onboard safety announcement that tells you to put the mask on your own face first and only then on babies and children."
James Healy-Pratt, a London-based aviation lawyer and qualified pilot, believed both theories could be valid. The freezing up of the sensors, causing the plane to drop suddenly remains a "compelling theory" for the AirAsia crash over the Java Sea just after Christmas, which also involved an Airbus A320, he said.
But in the latest accident the possibility of the crew having been rendered unconscious also had to be considered. "Crew incapacitation cannot be ruled out, and there has been speculation over the possibility of an explosive decompression at altitude," he said. (© Daily Telegraph, London)