if a plane is an AA plane, you shouldn't need a script 2 make it target air units. It should use AA missiles automatically.
Alternatively, Sefe wrote a 'guided SAM' script that will allow u to fire any missile at a moving airborne target and hit it (assumin it doesn't dodge *very* quickly). This won't actually have the *plane* fire the missile, but u can camCreate it from the plane's position (usin atan2 to make sure the plane is pointin towards the target, o'course) so it *looks* like the plane fired it - there's no difference to the player, after all.