Why not put a sealed door on the cabin, and carry Nitrous Oxide on board that can be pumped into the ventilation system or some other form of knock out gas? It would keep everybody out of it, including the terrorists until the pilots could land the plane.
Of course you could argue, maybe the terrorists would carry gas masks in their carry on luggage and access it. So as soon as the plane gets ready to taxi have the compartments lock electronically. Have better imaging systems at the xray machines as well.
In addition, a couple police on board, and tasers for the flight attendants located in password protected boxes should make it hard as hell.
How much additional cost there? A lot for the gas, obviously. A few thousand for a sealed cabin. For the electronic locks? A few thousand per aircraft. The Tasers are cheap. Having 2 armed officers on board? Maybe $300 per flight? Wow, an two or three dollars per ticket on large passenger aircraft. The smaller aircraft are much less wanted targets. They have short ranges, few hostages in comparison, and they can do much less damage.
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