National Airspace System

The National Airspace System (NAS) is the airspace, navigation facilities and airports of the United States along with their associated information, services, rules, regulations, policies, procedures, personnel and equipment. It includes components shared jointly with the military. It is one of the most complex aviation systems in the world and services air travel in the United States and over large portions of the world's oceans.

A flight through the NAS typically begins and ends at an airport which may be controlled (by a tower) or uncontrolled. On departure, the aircraft is in one of five of the six classes of airspace administered by the Federal Aviation Administration (FAA), and different flight rules apply to each class. Depending on the class of airspace and flight conditions, communication with controllers may or may not be required. Operation of each flight is always the responsibility of the pilot in command, but air traffic controllers give instructions for sequencing and safety as needed. When a controlled flight is airborne, control passes from the tower controller who authorized the takeoff, if the airport is controlled. The next step is typically Terminal Radar Approach Control or TRACON which may be identified as "approach" or "departure". Between the sectors administered by TRACONs are 20 contiguous areas of US airspace above 18,000 feet, each managed by an Air Route Traffic Control Center (ARTCC) typically referred to on the radio as "Center". A flight is handed off from one Center to another until it descends near its destination, when control is transferred to the TRACON serving the destination, and ultimately to the tower controller serving the airport. Some airports have no TRACON around them, so control goes directly to or from a Center, and some flights are low enough and short enough that control is kept within one or more TRACONs without ever being passed to Center. As of February 2015^[update] the NAS is transitioning to a new system known as NextGen, which applies non-radar surveillance of aircraft equipped with GPS satellite-based navigation systems continuously reporting their location. Aircraft also receive the broadcast location of others nearby, which improves safety. The system also allows pilots to use more precise and efficient landing paths, saving time and fuel. NextGen is being phased in piece by piece.

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