Getting AAA to fire

[Imp]

As for LOS; if my AAA is in the field, and there is a row of trees at the end of the field and the a/c come in in the hex row behind the trees is the game playing that the a/c are behind and below the trees? I fear this is what is happening, and if so that means his attacking a/c are flying at about 30 feet

It all depends how close you are to the trees LOS obstacle.
If the obstacle is a reasonable distance away they will see the plane & fire.
The closer it is the less of the sky they can see.
Typical example if you are walking down a street you cannot see a lot of sky because the houses block it, the closer you move the less you can see.
Same in a field 500m from a copse of trees you can see a lot more sky than if you were 150m from it.
Also remember you have to track the aircraft to get a chance to hit it a breif view of it & you would miss