air transport facilities and manage the country's air traffic control system. Many airlines operate on a "hub and spoke" model. This system gives the predominant airline in a given airport a strong competitive position as it feeds passengers to and from the hub, maximizing the number of passengers on each flight. Examples of airline hubs include United Airlines at Houston, Newark Liberty, Chicago-O'Hare, Denver and San Francisco; Delta Air Lines at Atlanta, Salt Lake City, Detroit, Minneapolis, and Seattle; and American Airlines at Charlotte, Chicago-O'Hare, Dallas/Fort Worth, Miami, Philadelphia and Phoenix. Air cargo Air cargo comprises a large number of daily flights in the United States and are operated by private parcel companies such as FedEx and United Parcel Service. These organizations operate some of the largest fleets in the world. Most air cargo moved by these organizations is time sensitive overnight and 2nd day parcels. The U.S. Postal Service also moves much of its letters and time sensitive parcels via air, but on regularly scheduled passenger flights. At one time the U.S. Mail charged a premium for letters sent by airmail, but no longer does so except for overnight express mail. Other large American cargo airlines include Atlas Air, Polar Air Cargo, Kalitta Air and National. Delays Airline delays have been the subject of some controversy, prompting a GAO audit and Congressional debate in 2007–08. Roughly one in four passengers experienced a passenger trip delay in 2007 and the average duration of delay experienced by these passengers was 1 hour 54 minutes. 24% of flights were delayed and 2% were cancelled entirely. Overall, passengers were delayed 320 million hours in 2007 with an estimated deadweight economic loss of up to $41 billion. An average of 40% of passenger aircraft delays in the U.S. originated in the New York metropolitan area, some in the area and others due to cascading effects. One-third of aircraft in the national airspace system move through the New York area at some point during a typical day. To deal with delays, takeoff and landing scheduling caps have been imposed on certain urban airports at various times since 1968, including Washington Reagan National, Chicago O'Hare, and the three New York airports. Other short-term measures have been taken, including minor procedural changes, use of military airspace on peak travel days, and appointment of a "New York Airspace Czar" (Director for the New York Area Program Integration Office).