Rescue helicopters fly over the mountainside near Seyne-les-Alpes, French Alps. Photo / AP
Aviation experts are focusing on the possibility that key sensors on the Airbus A320 could have iced up, causing the aircraft to descend rapidly.
Another theory is that the crew could have been incapacitated by a sudden loss of pressure.
Flight 4U 9525 from Barcelona to Dusseldorf had reached a cruising altitude of 38,000ft (11,500m) with 150 passengers and crew on board. The Airbus A320 began an unexplained descent before dropping off radar screens. No distress call was issued and nobody on the Germanwings flight survived the crash near the ski resort of Barcelonnette in southeastern France.
There appear to be parallels with an incident last November involving a Lufthansa Airbus A321, which dropped more than 3000 feet a minute, with the crew regaining control only when they turned off the onboard computers.
In common with other modern aircraft, the Airbus A320 is a "fly-by-wire" aircraft, meaning that instead of relying on hydraulics, much of the work is carried out by computer. It is considered safer and also makes the plane lighter and cheaper to operate.
But experts pointed out that a fault in the "angle of attack" sensors - which measure the position of the wings relative to air flow - could cause serious problems.
Bob Mann, an American aviation consultant, said: "If it [the computer] thinks a plane is about to stall, it will cause the nose to pitch down."
The steady path of the plane could indicate that it was being controlled by computer.
This could also have indicated that the crew had lost consciousness, said Nick Brough, an aviation consultant based in Italy.
"The aircraft appears to have gone into a descent lasting eight minutes, at a more or less constant velocity, until hitting terrain.
"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.
The black box recorders will be vital in the investigation.
Much has already been made of the fact that the sensors fitted to this type of Airbus were covered by a wider Emergency Airworthiness Directive issued in December by the European Aviation Safety Agency (EASA). This came after they were thought to have contributed to a Lufthansa aircraft briefly falling into an uncontrollable dive over Spain. In that case pilots managed to regain control at 28,000ft.
James Healy-Pratt, a specialist aviation lawyer, said the sensors would "almost certainly" be part of the French investigation.
The crashed Airbus A320 is a generally safe aircraft type and a bird strike or simultaneous engine failure is thought to be very unlikely, according to Phil Giles, formerly with the UK's Air Accidents Investigation Branch.
"What's remarkable about this incident is the measure of control it appears there was as the plane descended from its cruising altitude to impact. From the data available it appears it flew at a steady speed of around 400 knots and its descent of 4000 feet per minute is not extreme and may not have even been picked up on by all the passengers if the airplane remained pressurised."
According to Giles, in a "loss-of-control situation" the rate of descent normally "rockets" while the speed "really ramps up" before impact. This does not appear to have been the case with Flight 4U 9525 and investigators will now be looking at whether the automatic pilot intervened.
Aviation expert Neil Hansford says the most crucial aspect is "that at no stage did the captain or first officer put out a mayday signal".
Early indications are that the pilots may not have been "in control of the environment", Hansford said.
"If it was a total and catastrophic explosion, the debris would be over a much bigger field ... if it was a loss of power it would have glided further." The 24-year-old plane had undergone its last routine check on Monday.
Australian aviation security expert Desmond Ross said the crash would again raise the issue of the need for live streaming of flight data.
"The issue's been discussed over the last year since MH370 went missing," he said, referring to the Malaysia Airlines plane that disappeared en route from Kuala Lumpur to Beijing last year.
What does the lack of a distress call mean?
"The crew did not put out a Mayday call. It was air traffic control that decided to declare the plane in distress after losing contact with the crew," said civil aviation authorities. "It was the combination of the loss of radio contact and the descent." A former investigator with France's Bureau of Investigation and Analysis for the Security of Civil Aviation said: "The lack of a Mayday call opens up all possibilities."
What are the main theories?
"For the moment, it could be a technical problem, a non-technical problem, a poor reaction by the crew to an emergency situation as was the case with the AF447 flight from Rio to Paris," said the aviation expert, referring to the Air France plane that crashed into the Atlantic in June 2009 after stalling in midair.
Can we rule out a terrorist attack?
"If a plane explodes in midair, the debris are spread across several kilometres as was the case with the Malaysia Airlines plane that was shot down over Ukraine." That does not rule out the possibility that the plane was hijacked by people on board prior to the crash. That will only be ruled out by inspection of the wreckage and the black box recorder.
A collision?
No other plane - whether civilian or military - has been listed as missing.
- Telegraph Group Ltd, Independent, AAP