The New York Times reports an investigator believes one of the Germanwings pilots was locked out of the cockpit prior to the crash. He is heard on the voice recorder slamming into the door and calling out to the other pilot, who does not respond. No Mayday radio call goes out.
I can think of at least two primary scenarios involving those facts which would explain the crash. Scenario 1 has several alternatives, depending on the cause of incapacitation. Here they are:
Scenario 1: Pilot A needs to use the restroom; pilot B locks the door behind him to prevent unauthorized access. Pilot A fully expects B to unlock the door when A knocks and identifies himself. At this point something incapacitates B so he is unconscious or dead - an aortic rupture, a heart attack, a seizure, cockpit depressurization - and B cannot unlock the door. B slumps forward against the wheel putting the plane's nose down leading to a crash.
Scenario 2: Pilot A needs to use the restroom; pilot B locks the door behind him to prevent unauthorized access. Pilot B has decided to commit suicide and executes a long, shallow power dive. Pilot A feels the nose inappropriately tilt down and tries to reenter the cockpit. Ignoring A's shouts and banging, pilot B holds his course and crashes.
I'm sure there are other possibilities.
