possibility of systematic discrimination against certain
classes of aircraft operators (e.g., general aviation)
when it comes to runway access. In a dynamic
environment, this may even result in a compromise
of safety, if some aircraft are indefinitely relegated to
the end of the queue as new aircraft show up to land.
These observations have led many investigators to
study the runway-sequencing problem with the objective
of increasing operating efficiency while ensuring
that all airport users are treated equitably. Dear
(1976) and Dear and Sherif (1991) developed the concept
of constrained position shifting (CPS), i.e., of a
limit in the number of positions by which an aircraft
can deviate from its FCFSposition in a queue. For
instance, an aircraft in the 16th position in a FCFS
queue would have to land in one of the positions
14–18 if the specified maximum position shift (MPS) is 2.
Through many numerical examples and for several
reasonable objective functions, Dear (1976) showed
that by setting MPSto a small number, such as two
or three, one can obtain most of the benefits of an
unconstrained optimized system (e.g., 60%–80% of
the potential improvements). This finding motivated
several researchers (e.g., Psaraftis 1980, Venkatakrishnan
et al. 1992, Bianco et al. 2001) to investigate
a number of increasingly complex and realistic versions
of the sequencing problem. Two advanced terminal
airspace automation systems, CTASand COMPAS,
that have been implemented in the United States
and in Germany, respectively, incorporate sequencing
algorithms based on CPS(Erzberger 1995). However,
this feature of CTASand of COMPAShas not
been activated, primarily because of concerns about a
potential increase in controller workload.
Gilbo (1993) and Hall (1999) have gone beyond the
sequencing of arrivals only by considering how available
capacity can best be allocated in a dynamic way
between landings and takeoffs to account for the distinct
peaking patterns in the arrival and departure
streams at airports over the course of a day. Pujet
et al. (1999) have further examined the issue of optimizing
the number of aircraft taxiing out during periods
of congestion, based on the empirical observation
that departure rates at major airports seem to decrease
when the number of active aircraft on the taxiway
system exceeds a certain airport-specific threshold.
Although still at the theoretical stage, some of these
promising ideas will eventually find their way into
practice.
4.2. Air Traffic Flow Management
The most advanced OR work on aviation infrastructure
to date is undoubtedly associated with air traf-
fic flow management (ATFM). ATFM took on major
importance in the United States and Europe during
the 1980s, when rapid traffic growth made it necessary
to adopt a more strategic perspective on ATM.
Rather than addressing congestion through local measures
(e.g., by holding arriving aircraft in the airspace
near delay-prone airports) the goal of ATFM is to prevent
local system overloading by dynamically adjusting
the flows of aircraft on a national or regional
basis. It develops flow plans that attempt to dynamically
match traffic demand with available capacity
over longer time horizons, typically extending from
3–12 hours in the future. The prototypical application
of ATFM is in ground holding, i.e., in intentionally
delaying an aircraft’s takeoff for a specified amount
of time to avoid airborne delays and excessive controller
workload later on. Other ATFM tactics include
rerouting of aircraft and metering (controlling the rate)
of traffic flows through specified spatial boundaries
in airspace.
An important difference in the nature of the ATFM
problem in the United States and in Europe should
also be noted. In the United States, ATFM is primarily
driven by airport capacity constraints, whereas in
Europe en route airspace acts as the principal “bottleneck.”
Europe’s Central Flow Management Unit,
located in Brussels, currently determines (heuristically)
ground delays to ensure that no en route sector
capacity constraints are violated. This difference
may, however, become moot in the near future due
to continuing progress in increasing en route airspace
capacity in Europe.
OR model development related to ATFM can be
viewed as going through two distinct stages. The
first stage involved problem definition and development
of large-scale mathematical optimization models
of an aggregate scope. Attwool (1977) was the
first to cast ATFM issues in mathematical terms, while
Transportation Science/Vol. 37, No. 4, November 2003