[St_Thomas_Aquinas]

These days are the cockpit doors so strong they could not be kicked open?

Yes. The doors were strengthened after 911.

But they're a problem when you have a suicidal Mohammedan pilot.

It could have been a heart attack, but I can't think of a reason why one pilot would lock out the other.

[CivilWarBrewing]

I think the cockpit locks AUTOMATICALLY. I still don't see why a pilot would exit the cockpit rather than just page someone via his intercom.

Perhaps that pilot had to take a dump and waited for the aircraft to reach cruising altitude before doing so. That's a likely scenario.