Remember the concept of airspace? You could use that to add a third dimension to civ. An air unit can act like any other unit, but it can only occupy airspace, unless it is landed. They will have a certain nunber of spaces that they can go from their airbase before having to turn back. If they don't, they will have to find another landing space, or crash. Combat will reduce the amount of MPs left.
The only way that they can interact with land or sea units is if they bomb them. Flak, during its turn, will have the option to fire at a nearby aircraft, depending on its firing range. When an aircraft attacks a stack that includes a flak unit, the flak guns will automatically fire on it. Depending on the size, defense, and speed of the aircraft and the quality and number of flak guns, the plane can either be shot down, damaged, or untouched.
The only way that they can interact with land or sea units is if they bomb them. Flak, during its turn, will have the option to fire at a nearby aircraft, depending on its firing range. When an aircraft attacks a stack that includes a flak unit, the flak guns will automatically fire on it. Depending on the size, defense, and speed of the aircraft and the quality and number of flak guns, the plane can either be shot down, damaged, or untouched.