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 FCFS position 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 MPS to 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, CTAS and 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 CTAS and of COMPAS has 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 traffic 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