Germanwings Q&A: Why would the cockpit door have been locked and would there have been no way to get in?

One pilot on the Germanwings plane that crashed on Monday had left the flight deck shortly it began its fatal descent, according to reports said to be based on the "black box" – or cockpit voice recorder – found at the crash site. According to the New York Times and the AFP news agency, he made increasingly frantic efforts to re-enter, but was unable to do so before the Airbus A320 crashed in the French Alps.

Is this version of events correct?

We don’t know. At present the BEA, the French Office of Investigations and Analyses for Aviation Safety has not directly revealed any details of the data on the cockpit voice recorder. The reports that have emerged are based on what are said to be leaks from officials.

Frank Taylor, the former director of the Cranfield Aviation Safety Centre, told the BBC: "My first reaction was not to believe it because it’s totally unprofessional, probably illegal, for any organisation, any person outside the official investigators – that’s the BEA – to issue any statement at all.

"But having read the details that have been leaked, they do indeed look authentic, they sound convincing."

How secure is the flight deck door?

Very secure. Until the events of 11 September 2001, most passenger aircraft had a fairly flimsy door between the cockpit and the cabin. That one or more passengers could attack the pilots and take over the aircraft was not considered plausible, partly because of a misplaced sense of safety bestowed by airport security checks. The 9/11 hijackers, armed with blades that they had taken through security, were able easily to access the flight deck and kill the pilots to take control.

Is there no way for authorised crew to re-enter the flight deck?

Yes. The scenario has been considered that, if one pilot has left the cockpit, the remaining pilot may become incapacitated. There is a standard key-code system for entry. But there is an override switch – effectively a deadlock – that can be engaged from the inside, meaning the door cannot be opened from the outside for a pre-set length of time between five and 20 minutes.

Is there a standard procedure when one pilot leaves the flight deck?

It varies from one airline to another. On some, it has long been the case that there must be two people on the flight deck at all times. When one pilot leaves the cockpit, a member of cabin crew is instructed to remain on the flight deck until he or she returns. But in many airlines it is considered normal for only one pilot to be on the cockpit. Up until the disappearance of MH370, Malaysia Airlines was among them – but the procedure was changed after the jet disappeared. The Independent is seeking clarification from Germanwings about its policy.

The key standard operating procedure for the flight door is concerned not with locking but with the risks around its opening – with a cabin-crew member deployed to stand between the cabin and the flight deck, and the door being opened for as little as time as possible.

If the events are as described, what could explain them?

Two of the most plausible theories are that the pilot remaining on the flight deck was incapacitated, for example by illness; or that the aircraft was deliberately steered on a collision course in an act of suicide and mass murder. Both possibilities raise more questions.

If the pilot was incapacitated, how did the descent commence? One explanation could be that he suddenly took ill and took the first step to instigate an emergency landing – starting to lose altitude – but was unable to continue with the other necessary steps, involving turning the aircraft back towards Marseille-Provence airport. It could be that the normal keycode system failed at the same time.

If it was an intentional act, it would not fit the pattern of previous pilot-suicide events – in which the aircraft nose is directed towards ground level at high speed.

Could the pilot have collapsed and slumped over the controls in such a manner to be able to guide the plane down?

Most unlikely. While Boeing installs the more traditional yoke or control column on the flight deck, with Airbus jets each pilot has a side-stick to left or right (similar to a computer gaming joystick) to control movement. It is difficult to envisage circumstances in which it was involuntarily operated to initiate the descent path that flight 9525 followed.

Has anything similar happened before?

Because we are still in the realms of speculation, it is impossible to say. But in November 2013 a LAM Mozambique Airlines jet crashed in Namibia in what was, according to the preliminary report, a deliberate act by the captain. The flight data recorder indicates that he set the flight management system to a point below sea level, while the cockpit voice recorder includes the sound of the first officer hammering on the door.

What happens next?

Things could start moving very quickly. If the reports are false, the BEA is likely to act to discredit them. If the organisation does not refute the report, it will come under pressure to release full details from the cockpit voice recorder.

It may also be that Germanwings, its parent Lufthansa and other airlines make changes to their standard operating procedure to remove the possibility of one pilot being along on the flight deck. But this morning The Independent asked Germanwings about its policy, and was told: “It is far too early to draw conclusions about adjusting existing processes and proceedings. We will make such an examination in any case as soon as we have confirmed facts available.”