Evidence from the Germanwings cockpit voice recorder indicates that the co-pilot locked the captain out of the cockpit. Here is how the aircraft crew is supposed to lock and unlock the cockpit door, based on a video by Airbus, the manufacturer of the plane. 

1. The lock

The cockpit door locks by default when it is closed.

2. The intercom

If a crew member wants to enter the cockpit, he or she must first call the pilot using an intercom.

3. The codepad

The crew member then presses the pound key on the codepad, setting off a buzzer inside the cockpit.

4. In the cockpit

On the cockpit pedestal next to the pilot is a door-control toggle. The pilot sets the toggle to the "unlock" position to let the crew member in.

5. The door toggle

If the pilot is incapacitated, the door lock can be overridden using a special emergency code on the keypad. However, the pilot, or someone else present in the cockpit, can prevent access by moving the door toggle to the "lock" position, which disables emergency access for five minutes.

New York Times