This paper defines a set of temporal intervals,
called time windows, which are defined prior to flight departure
and constitute milestones to be met during the flight execution.
The size of the time windows is variable as it reflects all known
constraints, such as punctuality at destination, runway capacities
or congested en-route areas that the flight will cross. Once a
time window is defined, all the air traffic actors are committed
to guarantee that flight operations, e.g. enter an airspace sector,
depart from or arrive at an airport, are executed within the
time window. We propose a two-step approach based on a mixed
integer programming formulation. The first step determines
a set of time windows such that the overall cost of delay is
minimized. Then in the second step we choose the set of optimal
time windows which also maximizes the overall time window
size. In such a way, we provide to all air traffic stakeholders
the largest degree of flexibility to perform their operations
under the constraint that the minimum achievable delay is kept
constant. We also gain information on the critical flights of the
system: if the optimal width of a time window is equal to its
minimum available value, any disruption that may cause the
flight not to meet it may produce undesired downstream effects.
Our preliminary computational experience based on small-scale
random instances confirms that the flexibility granted to flights
increases with the capacity while the system delay simultaneously
decreases. We also show that when there is no congestion a
non negligible share of small size time windows may exist, thus
indicating the existence of bottlenecks and critical flights.
