Entering the cockpit
How did the 9/11 hijackers enter the cockpit?
No-one has yet offered a clear explanation, however there are possible hints.
The Boston Globe talked of a standard key that would open any cockpit (though we're not sure if that was the case for both United and American Airlines flights):
And Michael Sweeney, husband of Flight 11 flight attendant Amy Sweeny, told the FBI that he believed the two attendants stabbed on Flight 11 were those who carried cockpit keys.
Coincidence? Or had the hijackers learned this through observing flight procedures, and so launched these attacks in order to obtain the keys?