The Air rules are a little complicated, so here is a brief summary and forgive me if you know this already.
- To be able to attack an enemy aircraft that is launching a ground assault, you can only use those of your Fighters and Fighter-Bombers that are already on "CAP".
- Fighters and F-Bs that are CAPing enemy aircraft move after the enemy has finished moving, so as Markonz says you have the opportunity to fly over the battlefield and position the CAPing aircraft so that they are out of the arc of any defensive AA fire (either from the aircraft or the ground).
- Then each aircraft formation is shot at before it gets to shoot or attack, so your CAPing aircraft get shot at first, then any that remain shoot at the enemy aircraft and finally any remaining enemy aircraft complete their ground assault mission.
- At some later point in the turn, you have the option of launching your fighters and F-Bs on an "Intercept" mission. This operates in exactly the same way as for "CAP", except that obviously it does not stop the enemy ground-assault. However, Ork Fly-boyz get a +2 to their action test to perform an "Interception" mission so it may be a better option in some circumstances.
Finally, don't forget that the 2008 errata have given Fighters and FBs on "CAP" and "Intercept" a +1 to their shooting (which makes Nightwings horribly effective in CAP), and that Fighters and F-Bs can "jink" when shot at (getting a 4+ save but preventing that formation from shooting if it has not already done so).
