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.