Airline hubs are airports that an airline uses as a transfer point to get passengers to their intended destination. It is part of a hub and spoke model, as opposed to the Point to Point model, where travelers moving between airports not served by direct flights change planes en route to their destinations.
Many airlines also use focus cities, which have a good catchment area and function much the same as hubs, but on a smaller scale and may also function as feeders to main hubs. Some airlines also use the term secondary hubs for large focus cities. Some airlines may use only a single hub, while other airlines use multiple hubs. Hubs are used for both passenger flights as well as cargo flights.
A hub in the middle of a route is more effective than at either end as connecting traffic more easily fills the plane - passengers prefer a one-stop (two-leg) route over a two-stop (three-leg) route. The FAA uses the term airline hub based on number of commercial passengers in the FAA airport categories, re-evaluated every year. Airlines often have their headquarters in a major hub, whether on the same campus as the hub airport (for example, Delta Air Lines has its headquarters at Hartsfield-Jackson Atlanta International Airport, a Delta hub), or otherwise in the city served by the hub (for example, United Airlines has its headquarters in Chicago, which is served by O'Hare International Airport, a United hub).
A dominant airline will take competitive measures to defend its preferred position at a hub airport. This can stifle competition, for example Pro Air's battle with Northwest when it briefly flew out of Detroit City Airport: Northwest was able to out compete the short-lived discount carrier by matching its fares and offering more frequent flights.
New entrants, such as Spirit Airlines at Detroit Metropolitan Wayne County Airport and Dallas-Fort Worth International Airport, or AirTran at Hartsfield-Jackson Atlanta International Airport, allege to have been the target of exclusionary practices by the dominant carrier, like in Cincinnati/Northern Kentucky International Airport or all Delta Air Lines hubs.
Airports where a single airline's share of flights is at or above 70 percent can be called fortress hub. For example, in 2010 US Airways occupied 85 (plus 1 shared with Lufthansa) out of 97 total gates and accounted for approximately 90% of passenger traffic at Charlotte Douglas International Airport. The existence of fortress hubs makes possible the use of airline booking ploys such as "hidden city" ticketing.
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