2.2. Fleet Assignment With the flight schedule determined, the fleet assignment problem is to find the cost-minimizing assignment of aircraft types to legs in the flight network. Fleeting costs are comprised of: (1) Operating costs: Specified for each flight leg— aircraft type pair, representing the cost of flying that flight leg with that aircraft type. (2) Spill costs: Measuring the revenue lost when passenger demand for a flight leg exceeds the assigned aircraft’s seating capacity. One of the earliest research efforts on airline scheduling was that of Ferguson and Dantzig (1956a). They developed a linear programming model to allocate aircraft to roundtrip routes to minimize operating plus spill costs. In a follow-up paper (Ferguson and Dantzig 1956a), they expanded their approach to handle uncertain demands. These first models, while applied to simplified flight networks and tiny problems by today’s standards, identified key problem attributes and launched decades of research on fleet assignment. As a result, some 30 years later, researchers developed the capabilities to solve fleeting problems representative, both in complexity and size, of those truly faced by airlines. Abara (1989) and Hane et al. (1995) formulated the fleet assignment problem as a multicommodity network flow problem with side constraints. The underlying network (depicted in Figure 1) has: (1) Nodes: Representing the times and locations of flight leg departures and arrivals. (2) Flight arcs and ground arcs: Each flight arc corresponds to a flight leg with its arrival time adjusted to include the minimum amount of time required on the ground for disembarking and embarking passengers, unloading and loading baggage, and refueling. Ground arcs represent idle aircraft on the ground between flights. The objective is to flow commodities, that is, aircraft types, through the network feasibly and with minimum cost. Side constraints enforce the following requirements and restrictions: (1) Cover: Each flight leg is assigned to exactly one aircraft type.