I have 6 freight plains stuck at the airport, because all the spots are taken by my passenger planes.
Usually when you send a freight plane for take off, you can only send it to a freight destination. A passenger plane however, you can send anywhere, which means they block the spot for a freight plane (Still with me? ).
So how about showing freight and/or passenger icons above (or wherever) the destination airport (once you press take off), so you can see the difference.