Excuse my ignorance, but surely it must be possible to make a cockpit door that cannot be opened by man force? There was a demonstration by a Dutch chemical company (DSM) that could make lite weight panels that are able to absorb bullets....
DSM’s Dyneema fiber in first FAA-certified bulletproof cockpit door
Video cameras will be able to give the pilots information whats going on in the cabin, so they can inform the ground about the situation.
Of course it wouldn't stop the hijackers to threaten pax or inflight personal, but it would take away the threat and attraction to use the a/c as a projectile.
At the other hand terrorists will always find another targets like nuclear power-station or refinery, but that's a complete different discussion I'll leave to the political experts......
Cheers
Acap.TOSK
