The change in ownership is what gets tracked with the "homeFields" array - it says who owned what field when the terrain loaded. As long as you fly from a field that was originally on your "soil" you can fly whatever you want.
Keep going Dok ... I think that HT missed the point that the "home" array would be intially loaded with all the airfields that each country owned when the map was initalized.
homeFieldsRook[] = [A1,A4,A8,...]
homeFieldsBishop[] = [A34,A66,A99,...]
homeFieldsKinights[] = [A22,A44,A7,...]
When sitting in the tower, you would look at the "home" country of the pilot and at the airfield number. If the airfield is not in the "home" array, disable the planes if there is an imbalance.
This would, in effect, be the same, if not better than "distance from HQ".
SWEET !!!