1 - A320 depressurization occurs. Pilots set plane on a descent to safe "cabin altitude" (1000, 1500 or max 2000m depending on airplane).
2 - Pilot leaves cockpit to check/attend to emergency/request/crew/etc in cabin and closes cockpit door behind him (safety regulations mandate door must be closed at all times, and can only open from the inside)
3 - For some reason, co-pilot is unresponsive (unconscious, malfunctioning oxygen mask, not using it, etc) and cannot open door. He has no change in breathing to the end, so alive, but clearly not seeing mountains/hearing alarms. Even if on purpose he'd be breathing heavy while facing death.
4 - Sadly they were going over a ridge with >2000m and the plane was set to fly lower than it. With no one to correct it (very easily considering good visibility at the time and plane warnings), it flew itself into the ground.
A complicated set of circumstances turned a "simple" emergency into a tragedy, leaving many questions still unanswered:
- what made the pilots dive the plane, was it depressurization?
- why did the pilot leave the cockpit and the co-pilot alone (was he?)
- what happened to the co-pilot that he could not (in clear visibility) correct the plane trajectory or open door?
- and most importantly, what can be made to prevent this from ever happening again?
