Cockpit doors / access
The atrocious acts of terrorism would not have happened if it had been impossible to access the flight deck. All the security measures you could dream up to search people at airports will not stop dangerous people boarding aircraft. I believe the passenger who nearly downed the British Airways Nairobi flight was completely unarmed. What you can do is deny them access to the flight deck. This would be the most effective single measure to prevent this disgusting act occurring ever again.
If banks can have security doors to prevent the theft of a few thousand pounds or dollars, surely we should have the same facility to prevent the theft of thousands of innocent lives.
It is time to reconsider the training that we receive in response to unlawful intervention of flights.
Commercial aircraft should not be used as weapons of mass destruction.