ORIGINAL: SMK-at-work
Monkeys Brain suggested above that they might be useful for air units - can't do any harm......
To be fair Ben Hur may be right [;)]
When I put corps HQ on or beside airport with planes I didn't noticed any improve in supply but TOAW have some fuzzy logic for this and also planes do get maximum supply even on under supplied source (I think that I read this)
EDIT: well maybe they can also shoot some planes if airf. is attacked.

