Folks,
As one who pre 9/11 enjoyed one or two flight deck visits thanks to my Pilots Licence (albeit gliders), there is an obvious point that seems to have been missed in this discussion.
You can add the most impregnable door in the world to the flight deck and you can introduce whatever procedure you like for food / toilet visits but there is one integral weakness.
If a stewardess or a passenger is being held at knifepoint outside the door with the threat that they will have their throat slit or whatever unless the door is opened, how many crew would be mentally prepared to deal with the consequences of not complying with their request? Who can honestly say that the frantic screams and pleadings of a hostage could be ignored, especially if it is a child? How many deaths would it take before that door is unlocked?
Personally I feel that the emphasis should be placed on screening before the passengers board the aircraft, not the door to the cockpit.
'BLW'