RULES WRITTEN IN BLOOD. Remember Arthur Hailey's Runway Zero-Eight? A former military pilot is passenger on a plane with an incapacitated flight crew. He saves the day. Now reflect on that recent crash of a Cypriot airliner in Greece. The pilots were incapacitated. Pilots have a separate oxygen system from the main cabin. We don't know whether the oxygen masks dropped down in the cabin, or if passengers reacted promptly enough if they did. (In Airport, one of Hailey's characters reflects that the passenger briefing ought to go something like "If you see a mask in front of you, stick your nose in it and ask questions later.")
But under the new flight deck (the railroader in me wants to say "cab") security rules, a heroic passenger that managed to get on oxygen would not be able to get to the cockpit to assist an incapacitated pilot. Here's a challenging uncertainty problem. There is some positive probability that bad guys will get on the flight deck and fly the plane into a building. There is also some positive probability that the pilots will be incapacitated by an in-flight mechanical failure. What is the best balance of those two risks?