One pilot locked out: Germanwings crash theories intensify

Could one of the pilots have become incapacitated, the other locked out?
Sources close to the investigation into the crash of the Germanwings Airbus 320 from Barcelona to Duesseldorf in a remote corner of the French Alps on Tuesday morning say that one of the two pilots was locked out of the cockpit and was trying desperately to get back in.
As relatives of the 150 passengers and crew who were killed on flight 4U 9525 visit the site today, it is being reported that a full analysis could take weeks or months. The extreme terrain means the recovery of bodies will also take a long time.
The cockpit voice recorder was recovered quickly and is still being analysed, but the flight data recorder has not yet been found amid the pulverised wreckage.
It is not clear if it was the captain or the first officer who was locked out, but the New York Times quotes an unnamed investigator as saying: “The guy outside is knocking lightly on the door, and there is no answer. And then he hits the door stronger, and no answer. There is never an answer. You can hear he is trying to smash the door down.”
An alarm warning that the aircraft is close to the ground can be heard moments before impact. The frantic situation contrasts with “very smooth, very cool” conversation between the pilots during the early part of the flight.
The BBC reports that the so-far unnamed co-pilot had 630 hours of flying experience and joined Germanwings in September 2013, while the captain had flown more than 6,000 hours and had been with the carrier since May 2014, having flown previously for Lufthansa and Condor.
Travel news site eTN speculates that one pilot could have taken a restroom break, while the other could have suffered an unexpected medical condition, experienced a situation or sudden loss of cabin pressure, or have fallen asleep. Then the other pilot could not re-enter.
In the US, pilots are required to have a flight attendant in the cockpit if one of them is on a restroom or meal break, to open the cockpit door if the remaining pilot becomes incapacitated. But this does not appear to be policy at Lufthansa, eTN says.
New York Times / BBC / eTN
[pictured: Crash site, March 24; wikimedia]