What a terrifying idea! Once they're on board a firearm will do you no good at all. At least some of the Sept 11th guys were intent on suicide. I don't know if you have any experince of weapons but you can be rushed very easily even when armed. If the gun is safely stored and unloaded as it would have to be you would not have time to react. Locking the door and heading for the closest runway is the answer if the unthinkable happens but stopping them at source is the only sure way.