Various research has shown that aircraft could save fuel while flying in close formation. Hence a huger traffic simulation should benefit from the introduction of formation flight. Nevertheless the number of possible combination in a huger scenario puts limits on the scenario size as well as on the size of a single formation. This paper presents a method to select possible members for a formation from all aircraft in a scenario. Up to 5 of these possible members the most beneficial formation will be selected. In the process all aircraft in a scenario will be assigned to formations until no more benefits could be achieved. This paper will describe the selection method in detail as well as the optimization method for each formation. The boundary condition of the optimization process will be discussed as well. Finally this paper will show the results of these processes in a transatlantic scenario which include 2776 flights over 48 hours. The results will show the fuel savings in the scenario and the effect of different conditions on the formation (only same type of aircraft in one formation) and on the aircraft timing (takeoff time could be adjusted or remain fixed). Thus this paper could give estimation on how much fuel formation flight could save in today air traffic system.