Flight deck doors are always kept locked inflight on American carriers, this is FAA mandated.
A determined hijacker would find a locked door no obstacle due to its design.
The answer pletharoe (if there is an answer) is to identify any potential threat before it gets close to the airplane. Unfortunately I think its a case of if the person/persons are resourceful enough there can never be a 100% guarantee.
Asshole! Try a better idea Excellent moderation Scroggs
[ 12 September 2001: Message edited by: SOHCAHTOA ]