International air transportation is fundamental to the integration of global economies. The ability to connect far flung cities depends on an elaborate system of airline interconnections. Geographers have long had an interest in describing and modeling the networks of links and the resultant levels of accessibility and their importance to global systems. Concepts such as hubs and gateways are in everyday use and indicate places that have developed special infrastructure to permit handling of long distance interactions.

The purpose of this research project is to develop a linkage between the spatial organization of air networks and the possible implications of that structure for the environment. The project will examine the costs and benefits in variants of such arrangements: patterns that are often referred to as hub and spoke systems (which channel flows between major hubs) and point to point systems (which provide more convenient direct linkages). At base, the contrast between heavy flows between hubs and more direct but necessarily thinner flows between point to point pairs, poses a complex set of trade-offs. The purpose of this research project is to develop measures of the relative merits of alternative network arrangements. The project will examine the tension between efficient flows from a transport cost point of view, and the potential for such flows to impose rather heavy environmental impacts. The project will examine the fuel efficiency of various aircraft range and size configurations. The topic has ramifications for the location of hubs, and ultimately may have implications for the best way to select an air passenger fleet to accommodate the preferred patterns of interactions. The project aims to synthesize two contrasting views, one from the energy consumption of individual aircraft and the other from the overall network viewpoint. Research to date has tended to come at this problem from one side or the other and the research will develop an increased understanding of the interdependence between systems and their elements. To achieve the project's objectives, innovative linkages between operational and network efficiency points of view will be developed and tested in models of a simulated network.

Because of the penetration of air transportation into everyday life, this project has several societal implications. The impacts of the project will range from educational ones to heightened awareness of the prevalence of economies of scope and scale in transportation. Specifically the results will provide a more complete understanding of the environmental and economic costs of air passenger interaction. The project will also bring the examination of fuel use and emissions more centrally into models of the operation and planning of air transport systems. The research will contribute to explanations of the ways that a hub serves as a focal point for interactions, and for models of diffusion which have implications for a range of application from improving airline security to understanding disease epidemiology. These topics have proven quite difficult to pin down quantitatively, and a transport network analysis will provide the tools to measure these elements in a spatial context. The project is likely to influence the way networks are assessed and measured.

Project Report

Hub airports are an important aspect of national and international air transportation. Flows between regions (international/intercontinental) require main exit and entry points, referred to as gateways. Hubs and gateways serve an important role as consolidation points through which flows are channeled between cities. Research shows that the use of hubs implies greater passenger miles and higher fuel consumption than direct flights; however, such networks have the advantage of a smaller number of consolidated flows. In summary, the bundling of flows, while adding passenger miles, reduces the number of necessary operations. The research examined the pros and cons of hub-oriented spatial movement. Without hubs, there is no way that pairs of cities at distant locations could generate a sufficient volume of flow to warrant dedicated service (e.g. Columbus, Ohio and Warsaw, Poland). Yet, with hubs, it is possible to make connections to form a reasonably efficient path between these remote city pairs. Thus, hubs are vital to ensure connectivity. The tradeoff for increased connectivity is that flows are routed through indirect paths with a resultant increase in passenger miles. Looking beyond the inconvenience to the passenger, the greater concern is the added fuel needed to move the same number of people. When considering total fuel consumption, a relevant observation is that extreme long range flights (whether through hubs or not) have a fuel burn penalty -- this is because aircraft on an extended range flight need more fuel simply to lift the weight of the fuel needed for the entire trip. As a result, when networks are organized with a set of medium range connections (e.g. through a convenient intermediate location) they save (avoid) the premium on the fuel burn needed for extended range operations. Because hubs allow two shorter trips instead of one long one, there is a trend towards hub connecting global networks, using cities such as Frankfort, or Dubai. [See Chart 1.] To be clear, even these hub connections can involve quite long range flights. These are essential to provide the kind of global connectivity that passengers need for business and personal travel. In addition to the fuel issue, the number and density of flight operations have serious consequences for airport congestion, and atmospheric emissions. On these factors, there is not such a clear answer from the research. A basic expectation is that there are many short range flows, and relatively fewer long flows; however, the research has found that the aggregate share of fuel burn and passenger miles accounted for by the longer trips is about equal to the number of miles that are consumed in the much more prevalent short range flows. Aircraft are manufactured to have an ideal operating range, and networks that have a large proportion of their travel links in an optimal range will be favorably advantaged. The research found that there is an optimal point in terms of fuel consumption around 1500 - 2000 nautical miles where the combination of aircraft size, the density of passenger seating configuration, and the fuel needed for the distance all align favorably to produce a relatively economical fuel demand per seat nautical mile. [See Chart 2.] In addition to a consideration of the tradeoff between fuel burn and passenger miles, the research has highlighted and confirmed the decision by air carriers to switch to more efficient aircraft. Using models derived from this research program it is possible to calculate the fuel efficiency for the aircraft and make optimal adjustments. This work confirms the obvious result that new technologically superior aircraft dominate in terms of fuel and payload. However, the research also offers insight into less intuitive actions, such as retiring some regional jets. Although these aircraft would seem to be good from the point of view of their small size and small fuel need, their actual fuel burn per seat nautical mile is not especially good. Thus, our models and data show that airlines should move instead to the latest models of mid-size jets. In part this is also a function of fact that the best technology advances have been focused, for example, on mid-sized aircraft like the Boeing 737-800 which provides an ideal combination of seating and fuel consumption for the continental scale US. Another example that shows the complex tradeoffs is the fact that air freight places a premium on timing and speed of delivery. In that case a single non-stop leg on a newer technology jet such as the Boeing 777 freighter has advantages in terms of service. Technical versions of these reports are available from

National Science Foundation (NSF)
Division of Behavioral and Cognitive Sciences (BCS)
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Thomas J. Baerwald
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Ohio State University
United States
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