The cockpit door is not a cause.
I will start by saying that I know this is a thread drift but here we go. The cockpit door was absolutely the cause of that accident as it was with the Helios 737 and possibly MH370. If the PIC had been able to reenter the flight deck then the Germanwings accident
would not have happened. The hardened cockpit door was a political response to a security issue that EL Al had found a solution for in the 70's. In Australia the Office of Transport Security mandated that HCD were installed on aircraft of less than 50 seats. They asked CASA for advise and they just said not our problem. The end result is that a pilot has to leave the control seat to unlock the door. The focus is still on the mental health of pilots instead of an engineering solution to allowing the pilots access to a toilet without being locked out of the flight deck.