here is a cheaper easier way to prevent a person form taking over a plan. it may be old school but some times low tech is better... a metal wall dividing the pilots from the passengers. give them a separate "door" to the out side. (not a door to the cabin) Thus making it impossible for them to seize the plane. they might be able to take a hostage at most, but at least a steerable missile(the plane) would be not be in there control.