Virtually impenetrable cockpit doors were introduced on passenger planes worldwide after the September 11 terrorist attacks.

They are designed to control who goes in and who comes out of the cockpit - and that, from within.

If necessary, cabin crew can activate a special code to gain access to the cockpit.

In 2002 - one year after the terrorist hijackings - Airbus released a video explaining how the new cockpit doors work.

An Airbus spokesperson interviewed by DW would not comment on the particular cockpit door fitted on the Germanwings Airbus "A320" which crashed in the French Alps on Tuesday, due to the ongoing investigations.

Electronic lock with access code

The Airbus video shows a regular passenger aircraft door system consisting of five main components, including a secure and bullet-proof door leaf with an electronic triple bolt.

The door also has a peephole for the pilots.

And video cameras in the cabin now allow the pilots in the cockpit to monitor the passenger and door area.

The pilots can operate the door with a lever on a control panel.

There are three settings: norm, lock and unlock.

There is also a keyboard panel in the cabin area with a green and red light, as well as a telephone for the crew to talk to the pilots.

The cockpit has an emergency buzzer and pressure sensor that responds in the event of a pressure loss in the cabin.

Protection against intruders

Usually, both pilots remain in the cockpit and the door is locked. It can't be opened manually from outside, because there is no handle. The control lever setting is "norm" - which means the three additional door bolts are not activated.

Any flight attendant who wants to access the cockpit must first ring the bell on the control panel.

The pilot then has to move the switch to "unlock" for the person to enter. As soon as the door is closed again, the setting reverts to "norm."

Anybody trying to enter the cockpit without using the telephone - either by knocking or hammering on the door - would raise the pilots' suspicions.

In such a case, they would switch to "lock" - effectively activating the additional three bolts. An intruder would need more than raw force to break open the door.

Access to the pilots in emergency situations

But there can be situations in which the door has to be opened by cabin personnel - for instance, if they have to administer first aid to a pilot. This could arise in the event of gas poisoning, rendering both pilots unconscious.

The cabin crew has an access code for such emergency situations.

Entering the code triggers an alarm in the cockpit and the word "open" appears in bright letters next to the door switch.

If the pilots don't react within 30 seconds, the door opens automatically, giving crew five seconds to get inside, and then administer first aid.

If the pilots suspect they are being hijacked and want to prevent anyone from getting inside the cockpit, they can still activate the "lock" position.

This would bolt the door irrevocably for five minutes, according to the 2002 Airbus video.

Aviation expert Tim van Beveren told DW this period has been extended to 20 minutes. The lockout scenario applies, for instance, if terrorists have forced cabin crew to give them the access code.

There is another scenario in which the door opens without the pilot's involvement: if a sensor in the cabin ceiling registers a loss of pressure in the cockpit.

But even then the same rule applies: if the door is switched to lock mode, it remains locked for twenty minutes.