Fall 2009 - H. Milton Stewart School of Industrial & Systems ...

ON THE MOVE
Transportation Logistics at ISyE
Airline Optimization in ISyE
By Ellis Johnson and George Nemhauser
We first entered the field of airline optimization
about twenty years ago with a project on fleet
assignment for Delta Airlines. There is an
amusing story about how this work for Delta
began. At the time, Delta was using a new crew scheduling
system that was based on the polynomial time linear
programming algorithm developed by Narendra Karmarkar
at AT&T Bell Labs and implemented in the KORBX
optimization system, an eight processor Alliant computer.
The system reputedly cost $8.9 million and, not surprisingly,
only two such systems were ever sold. Delta bought one of
them to solve crew scheduling. The important point to note
is that crew scheduling is an integer programming problem,
which is hard to solve, and KORBX could only solve linear
problems, which are much easier relatively. So, the AT&T
folks could only provide a heuristic for converting linear
solutions to integer solutions.
We had some new ideas for solving crew scheduling
problems; however, our attempts to contact Delta fell on deaf
ears until Mike Thomas, then school chair, had a brilliant
idea. Ron Allen, an ISyE graduate, class of 1964, had recently
been appointed CEO of Delta. Thomas arranged a dinner at
which Allen would be honored, and we would get a chance
to suggest to him that we could help Delta with scheduling.
The evening went well except for the end. Atlanta had just
started car emission checks, and because of the low fee paid
to the stations that could perform them, there were very
long lines. A black market emerged for the emission stickers
that were attached to license plates.
When we left the Alumni House at the end of the dinner,
we discovered that each of the cars that previously held
these stickers were missing the part of their license plate
where the sticker had been. Allen’s car was one of them.
Nevertheless, from Allen we got an inroad to Delta’s IT
group. Because of their work with KORBX, they were not
interested at the time in help with crew scheduling. But they
decided to work with us on plane scheduling, which is called
fleet assignment in the industry.
Given a schedule that lists the time, origin, and destination
of all flights, fleet assignment addresses the question of what
type of aircraft should be assigned to each flight. The answer
is driven by demand and the network structure of all of the
flights. Large planes should be assigned to high-demand legs.
However, if a large plane is assigned to a flight from A to B,
then there should be a flight from B that departs soon after
the A-to-B flight that also has large demand and so on. The
optimization model minimizes flying costs and the costs of
lost demand. Cindy Barnhart, then an assistant professor in
our group, now a professor and associate dean at MIT and a
leading international expert in airline research, got her start
on the Delta project. We successfully completed this project
and then went on to do research sponsored by almost all of
The Alumni Magazine for the Stewart School of ISyE Fall 2009 • 13