Germanwings: What are the main lines of inquiry?

Sign warning of low flying aircraft near the spot where a Germanwings airbus crashed, 24 March 2015

Air investigators are trying to piece together the mystery of Germanwings flight 4U 9525, which crashed in the French Alps. Answers may not come quickly, but what are the main lines of inquiry?

The plane, an Airbus 320, made a controlled descent for a period of eight minutes before it flew into a mountainside killing all 150 people on board. Two things stand out - why there was apparently no distress call and why the plane began a long descent towards the Alps.

Investigators will focus on three aspects, says a crash expert who does not wish to be named - the flight recorder or black box, the debris on the ground, and the radar traces.

The damaged flight recorder

The radar traces have yielded the story so far - the location and speed of the plane and its loss of altitude. For instance, its long descent appears to rule out a huge explosion. But this only gives an outline.

The flight recorder is made up of two elements - a data recorder logging reams of information from the plane's operating systems, and a voice recorder capturing any sound in the cockpit.

Only one of these boxes has been found so far - the voice recorder. They are made to withstand a huge impact. Investigators - who listen to the device over four speakers to recreate the sound of the four cockpit microphones - say they have been able to retrieve usable sounds and voices but have not been able to draw any conclusions yet.

Germanwings plane takes off from Dusseldorf airport, 25 March 2015 A Germanwings plane takes off from Duesseldorf airport, March 25 2015

The first big clue is whether the pilots are talking. If they were silent in the build-up to the crash that could point to some sort of decompression of the plane. Planes are pressurised as otherwise people would not have enough oxygen when flying anywhere near cruising height. Should the plane depressurise quickly, crew and passengers could find themselves unable to breathe properly and disorientated because of hypoxia. This eventually leads to unconsciousness.

Pilots respond to a sudden depressurisation by bringing the plane quickly down to a safe altitude, perhaps in the region of 10,000ft. Loss of pressure typically triggers oxygen masks to fall from the ceiling of the cabin for passengers to breathe from. Pilots have oxygen masks at their sides. Oxygen masks do not fall from the ceiling in the cockpit. Instead an alarm will sound when the plane decompresses to an equivalent of about 10,000 feet.

So in any scenario where rapid decompression was to blame for an accident there would have to be some sort of problem that stopped the crew using their oxygen supplies properly. "What we don't know is what happened when they crossed the coast of France to start their descent," says aviation expert Sean Maffett.

Plane wreckage Debris from the wreckage of Flight 4U 9525

If the pilots were talking, the voice recorder might contain panicked conversations pointing to another eventuality - a fire, some other mechanical failing or some kind of incident in the cockpit. It would need to explain two things - that there was apparently no communication from the cockpit to air traffic control during the plane's last minutes and the steady, controlled descent into a mountainside.

If the data recorder was to be found that would provide a more comprehensive picture. The pressurisation of the plane would be recorded. And it would answer the question of whether any of the plane's electronic systems failed. Most commentators believe this would be unlikely as there are several back-ups in place. There has already been speculation that the pilots may have been tricked into descending by sensors freezing up.

Flightradar web page A web page from flightradar24 with the exact point where the radar signal was lost

Collecting the wreckage could tell investigators about mechanical failings. Investigators will gather as much as they can find from around the crash site. "You leave everything there exactly where it is," says the crash expert. "You photograph it. In the meantime someone is analysing the flight recorder, using that to decide which areas to look at." The fragments will eventually be taken to a hangar and assembled in the way that they were found, or to reflect how the plane broke up. In the absence of compelling information from the flight recorder, damage to the plane's structure or parts might yield a clue as to what caused it to crash.

Former BA pilot Eric Moody says the descent was not a normal course but neither was it dramatic enough to suggest a desperate attempt to re-pressurise the plane. The plane appears to have remained on autopilot, unusual if pilots were attempting to rapidly descend, he says. "It makes me think they did the initial part of a rapid descent but then something else intervened and they were incapacitated. They had no input into the autopilot."

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Pilots train for just such an eventuality, says Maffett. But it is intense. "It's a moment of high drama, it's terrifying." It's not clear how many of the three pilots were on the flight deck at the time, he notes.

Some oxygen masks allow pilots to talk while wearing them, says Moody. Different airlines use different systems - it's not clear what kind the Germanwings plane was equipped with. But even if the Mayday call could not be spoken, pilots can raise the alarm by switching the plane's transponder to its emergency setting.

Much of the mystery centres on why in broad daylight, in apparently good visibility, the plane flew lower and lower into the mountains. "The pilots would have seen the aircraft doing the wrong thing out of the window immediately," says the crash expert.

It would make the idea of faulty speed sensors unlikely, he says - if the pilots were conscious they could see out of the window that the plane was descending. Until more clues are discovered, the cause of the crash will remain swathed in mystery.

Pupils stand in front of candles and flowers that have been left outside the Joseph-Koenig college in Haltern am See, Germany, 24 March 2015. Candles outside the Joseph-Koenig college, 16 of whose students died in the crash

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