After 11 September, 2001, airlines and authorities needed to ensure that no unauthorised persons could get into the cockpit. This resulted in the creation the cockpit doors we have on aircraft today. No more did we see the cockpit door left open on flights or people wandering in and out or passengers visiting. The doors themselves are virtually impenetrable.
These may not have been topical until this week when the copilot of Germanwings 9525, Andreas Lubitz reportedly locked Captain Patrick Sondenheime out of the cockpit before deliberately flying the Airbus 320 into a French mountain.
This video (which is a little cheesy in places) shows how the modern cockpit door system works and by association, shows how the Germanwings captain was locked out.
In this system, the pilots operate the door with a switch on their control panel. It has three settings: norm, lock and unlock.
There are three possible options during normal flight operations:
1. Authorised access with the pilots allowing someone into the cockpit (switch is set to unlock)
2. Preventing unauthorised access (switch is set to lock activating additional locks in then door)
3. Entering in an emergency should the pilots be incapacitated (switch is left in norm)
In the Germanwings situation, the co-pilot Lubitz triggered the switch which locked the cockpit door. Captain Patrick Sondenheimer kept trying to get his offsider to let him in before reportedly resorting using a fire axe to try and get into the cockpit.
The impenetrable cockpit meant the copilot was able to crash the plane.