SKY MARSHALLS
It would be all too easy for unarmed terrorists to overcome an Air Marshall and then use his fire arm to take control of the ac, provided they can spot him/her.
Any fire arm in the cabin is extremely dangerous
I would have thought that, as many of the pilots in charge of civilian airliners will be ex-military, we could arm them instead. At least they are behind the secure cockpit door.
If terrorists new that they would be facing armed resistance from the cockpit, if they tried to take control, they might think twice (or more) about attempting it.