Hey Isaioux2x, put the plane (Su25/A10/whatever) on the taxiway adjacent to the main runway (make sure the plane is facing in the same direction as the arrows on the taxiway).
The AI should taxi onto the runway and take off. However, I think if you leave the AI to it's own devices it just takes off and keeps going so you might wanna give it some waypoints...
This only works on some of the runways. Others, like the one on the island to the North of Nogova, just don't work...
That said, I think if you give a plane a big enough space anywhere and give it a move waypoint, it will take off and go to the waypoint. Doesn't have to be a runway...