How to use "AA" Shells?
 

So some of my vehicles have AA shells. I use them and fire them at a plane and they just Whiz right past the plane and explode around a km away. How to make them airburst near the plane instead of them just flying past and exploding km's away?

Welcome aboard!
 

Those shells are flak shells. They are supposed to blow up near the planes and cause collateral damage. You don't need to hit the plane dead on, justnear them.

Germans never invented a proximity fuse. Idea of aa fire is to create a barrage about 2 km away and prevent the aeroplane from making it's attack run.