hell yeah it should be. If the passengers on the Sept. 11th flight had guns to shoot the terrorists with, we'd be saved 3000+ lives. Of course this would be up to the airport's decisions, but it should be legal considering the lives of the passengers is at stake.
Wouldn't it be quite easy to make the plane crash by shooting through the walls or into the cabin or something?
I think they've actually done studies that prove that is bunk.
Yes. What happens is that there will be cabin pressure loss (as well as hearing of those near the hole due to the sonic wind speed), but it won't crash or break apart in the air. A pilot can limit the cabin pressure loss by decending to a lower alttitude and slowing down, but the plane would still need to land as soon as it could in order ensure to limit the effects of exposure for those in the cabin.
Now that is assuming the bullet his the nothing while travelling through the walls. Since there is limited space on a plane, if the gunman were to fire up or down, they risk hitting electrical wires or fuel lines which would either cause the loss of control of the plane or start a fire in the void space. If either of those two issues occur, then you can pretty much kiss the plane goodbye.
As far as if the passengers on the airplane had guns, yes, they could have stopped the terrorists . . . assuming the terrorists didn't bring their own guns aboard the plane. However, I'm not sure if I would want a terrified passenger firing a gun within the cabin in the direction of the cockpit.
Good argument to only allow rubber bullets(since they don't pierce, but they do hurt like hell so you could knock a terrorist down with one and physically restrain the guy) if you are going to allow guns on planes.
