The Basics

Interceptors attack, escorts defend. If their are more interceptors than escorts, each extra interceptor attacks a bomber. (make sure to roll those interceptors separately)

There are also rules for Tactical Air Combat. Basically, aircraft fight for 1 round of combat before other units join. (casualties have to be other aircraft) It makes combat favor those with the larger airforce. Germany also has a chance against Russia by being able to kill the fighters off without having to kill all the infantry first (and giving the fighters a multitude of rolls)

Max's rules are probably the best rules for their price. ($8-10 US) It is a booklet of 20 optional rules that you can pick and choose from. I'll have to check when I get home, but I believe it was the first expansion marketed. There are no map modifications, and only a few unit placement change options. There is also an expanded tech chart based on a 2d6 roll.

(I'm not affiliated with Stratamax in any way, nor will I make any money if you buy it, I just really like the expansion)