Several causes of delay are deterring the air transportation system from being efficient. System capacity reductions are the major cause of delay. However, there are also some other causes, directly imputable to airlines, which may produce, in combination with the capacity reductions, undesired downstream effects. Therefore, with the purpose of containing delays and disruptions in flight schedules, it is important to grant flexibility to flight operations. This paper presents a mathematical formulation that allows to determine the degree of flexibility given to flights by identifying through a set of temporal intervals, called time windows, those flights that have a larger impact on the air traffic system performances. A time window is a period of time during which a certain phase of the flight (e.g., take off, landing and entry into a sector) has to be executed. The size of the time windows is variable as it reflects system's capacity constraints. The set of time windows, which maximizes the total width of the time windows, provides airline operators and air traffic control authorities with the largest degree of flexibility to perform their operations. Several formulations of the models are presented, which vary in the way of formulating the use of system capacity. However, by means of a computational analysis, we show that the solution of the time window model is insensitive to the formulation used for the capacity constraints.