act because of air traffic control separation requirements.
For example, in the case of a pair of intersecting
runways, the location of the intersection relative
to the points where takeoffs are initiated or where
landing aircraft touch down greatly affects the combined
capacity of the two runways. Similarly, in the
case of two parallel runways, the capacity depends on
the distance between the centerlines of the runways.
Approximate capacity analyses for airports with two
parallel or intersecting runways are quite straightforward.
Multirunway analytical capacity models also
provide good approximate estimates of true capacity
in cases involving three or more active runways, as
long as the runway configurations can be “decomposed”
into semi-independent parts, each consisting
of one or two runways. Such models have proved
extremely valuable in airport planning, as well as in
assessing the impacts of proposed procedural or technological
changes on airport capacity.
Another topic of intensive study has been the estimation,
through the use of queueing models, of the
delays caused by the lack of sufficient runway capacity.
This is a problem that poses a serious challenge to
operations researchers: The closed-form results developed
in the voluminous literature of classical steadystate
queueing theory are largely nonapplicable—at
least when it comes to the really interesting cases. The
reason is that airport queues are, in general, strongly
nonstationary. The demand rates and, in changing
weather conditions, the service rates at most major
airports vary strongly over the course of a typical
day. Moreover, the demand rates may exceed capacity
( > 1), possibly for extended periods of time, most
often when weather conditions are less than optimal.
This has motivated the development of numerical
approaches to the problem of computing airport
delays analytically. In another landmark paper,
Koopman (1972) argued—and showed through examples
drawn from New York’s Kennedy and LaGuardia
Airports, at the time among the world’s busiest—that
the queueing behavior of an airport with k “runway
equivalents” (i.e., k nearly independent servers) can
be bounded by the characteristics of the M(t)/M(t)/k
and the M(t)/D(t)/k queueing models, each providing
“worst-case” and “best-case” estimates, respectively.
Note that this allows for dynamic changes in the service
rates, as well as in the demand rates.
Extending the work of Koopman (1972), the
M t/Ekt/k system was proposed by Kivestu (1976)
as a model that could be used to directly compute
approximate queueing statistics for airports—
rather than separately solving the M(t)/M(t)/k and
M(t)/D(t)/k models and then somehow interpolating
their results. (Note that negative exponential service
times (M and constant service times (D) are simply
special cases of the Erlang (Ek) family, with k = 1
and k = , respectively.) Kivestu (1976) noted that k
should be determined from the relationship ES
/S = √
k, where ES
and S denote the expected value
and the standard deviation of the service times and
can be estimated from field data. He also developed
a powerful numerical approximation scheme
that computes the (time varying) state probabilities
for the M t/Ekt/k system efficiently. Malone (1995)
has demonstrated the accuracy and practicality of
Kivestu’s (1976) approach and developed additional
efficient approximation methods, well suited to the
analysis of dynamic airfield queues. Fan and Odoni
(2002) provide a description of the application of
Kivestu’s (1976) model to a study of the gridlock conditions
that prevailed at LaGuardia Airport in 2000
and early 2001.
Additional (numerical) analytical models for computing
airport delays have been developed over the
last few years. Peterson et al. (1995) and Daniel
(1995) describe two different models for computing
delays at hub airports, which are characterized by
sharp “banks” or “waves” of arrivals and departures.
Hansen (2002) has used a deterministic model, based
on the notion of cumulative diagrams, to compute
delay externalities at Los Angeles International Airport.
Finally, Long et al. (1999) and Malone (1995)
present two dynamic queueing network models and
their application to the study of congestion in the
National Airspace System. Ingolfsson et al. (2002)
offer a comprehensive survey and comparison of several
alternative approaches to the analysis of nonstationary
queueing systems.
Many of the best features of some of the analytical
capacity and delay models just described have
been integrated recently in a number of new software