• For multi-stop itineraries, the arrival time of a flight at an intermediate stop must be less than the departing time of the next flight from that very same intermediate stop.
If these three constraints are satisfied for at least 1 passenger, then the capacity of the system is at least 1.
Data Collection: To collect the required information you can visit the sites of the respective airlines and grab all the itineraries which satisfy the above constraints. For instance, the timetable for AA is available here: https://www.aa.com/travelInformation/flights/schedule. Just plug in the source and destination cities to get all the flights between them (non-stop and multi-stop, along with aircraft type). Knowing the aircraft type is critical, because these will form the edge weights (capacities) between two nodes (for example if AA is operating the Airbus A320neo between two cities/nodes, then the capacity of that edge will be 165). You can look up Wikipedia for the specifications (maximum seating capability) of the aircraft. Once you have collected the data, you need to compute the capacity of the NAS for that day.
Hint: To accomplish this task, you might be thinking in similar lines with the process of “constructing a directed graph with all the cities represented as nodes and directed edges from a node A to B if there is a flight travelling from city A to B”. However, such a graph only captures the spatial information (flights between cities) and not the temporal information (time of the flights). Thus, such an approach is incomplete and is not going to capture the entire picture. The graph you create should capture the temporal aspect of the task as well, in order to accurately capture the capacity of the NAS. For further simplification, you can round the flight times (departure and arrival) to the nearest hour.
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