Do your teams planes AND the enemy planes have to start on the ground? You could set one or both of them to "flying" in the special option for the unit.
Otherwise, you will need to be aware that airplanes can only take off and land from one specific airfield on any given map, usually the airfield heading in a North/South direction, if your map has more than one.
With a fair bit of scripting, I'm sure it would be possible to take off/land at the other airfield, but is that really the road you want to go down?
To get AI units to take off, place them on tarmac running alongside the runway, facing south, and give them a move waypoint anywhere on the map. The planes will automatically taxi to the runway, and take off (heading North). If they are part of a group, AI planes will automatically circle the airfield until all of the group are in the air. They will then all proceed to the next waypoint.