@klykke89 I was confused too. The rulebook clearly states that planes can fly over enemy territories as if they were neutral, and that AAA guns can only shoot at planes when the territory the AAA gun is in is being attacked.
However I have seen some house rules that actually allows AAA guns to fire at enemy planes that fly over the territory they are in.
I think that is cool, but I also think that gives the AAA gun much more power and value and that they should cost more than 5 IPC if that is the case.