Locked out pilot tried to 'bash down cockpit door' before Germanwings flight crashed

One of the pilots of the Germanwings Airbus A320 that crashed into the French Alps on Tuesday was locked outside the cockpit and tried to bash the door down, investigators have said.

As investigators ponder why the plane with 150 people on board plunged for eight minutes before slamming into a mountain in mostly clear skies, evidence gathered from the flight recorder indicates one pilot was not in the cockpit at the time of the cash, according to the New York Times.

A senior military official working with the investigation suggested there was a "very smooth, very cool" discussion between the two pilots in the early stages of the flight as it travelled from Barcelona in Spain to Dusseldorf in Germany.

Audio from the black box flight recorder suggests that one of the pilots exited the cockpit during the flight but was never able to get back in despite apparently bashing on the door.

"The guy outside is knocking lightly on the door and there is no answer," the investigator said.

Read more: Photos released of battered black box in Germanwings Airbus 320 crash

"And then he hits the door stronger and no answer. There is never an answer.

"You can hear he is trying to smash the door down."

The startling news has shed some light on the mystery surrounding the plane's rapid decent but it has also thrown up more questions.

Investigators have "no idea" why the pilot left the cockpit said the official, who spoke to the newspaper anonymously as the investigation is ongoing.

"But what is sure is that at the very end of the flight, the other pilot is alone and does not open the door," the official said.

The initial data gathered from the Airbus A320 suggests its descent from about 38,000 feet occurred gradually over a 10-minute period which indicates the jet was not significantly damaged.

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At no point during the descent did the plane issue a distress call or set-off any emergency warnings to ground control.

France's Bureau of Investigation, charged with investigating the crash that occurred on a mountainside northeast of Nice, has declined to comment.

The bureau's spokeswoman Martine de Bono said investigators were continuing to analyse the plane’s flight recorder.

"As soon as we have accurate information we intend to hold a press conference."