[m348e912]

I'm not the first to suggest this, but I think "fly at your own risk" airlines would be popular with some people. Keep the cockpit door reinforced, and maintain a gentleman's agreement among travellers on what to do if a passenger threatens a flight. Airport security is now reduced to 10 seconds.

[daveoc64]

As noted elsewhere, that approach doesn't stop someone flying the plane into a building.

[m348e912]

My understanding is that flight security protocols and cockpit hardening introduced after 9/11 made it significantly harder to replicate what happened that day.