Aircraft maintenance by spare parts availability
Add a feature that changes the maintenance cost of the aircraft/family by the availability of spare parts. spare parts can be calculated by either the actual number of aircraft operated in the game or the number of aircraft operated in the real world. Less common aircrafts will have higher maintenance costs because spare parts are rare and hard to obtain. I think Interjet's SSJ100 had this problem. On the contrary, common aircrafts such as A320/B737 can have the opposite effect.