The United States airspace system, or National Airspace System (NAS), is a massive network of airports, equipment, regulations, procedures, services, technical advances, and manpower that serves to protect and control the airspace of the country. The military also operates under the NAS procedures. America’s airspace is defined as any point in the air above the country and extends twelve nautical miles off of the coasts of the lower 48 states. The United States chose the classification system for the NAS based on an international system created originally by the International Civil Aviation Organization. There are six main classifications of airspace in the system: Classes A through G, excluding F. These can either be controlled, under the guidance of an air-traffic controller, or uncontrolled.
Class A airspace is defined as any airspace over 18,000 feet above sea level, and in this class all aircraft must fly under the instrument flight rules (IFR). Class B airspace surrounds the nation’s busiest airports, and often has two or more circular zones leading up from the ground to about 10,000 feet. The class contains all instrument procedures once an aircraft enters the space, and air traffic control clearance (ATC) is necessary for an aircraft to fly in the area. Class C airspace surrounds smaller airports with an operational control tower, and also has several zones that surround the airport, rising to a height of 4,000 feet. Aircraft in class C must make communications with ATC in order to proceed. Class D airspace also surrounds small airports with a control tower, and extends 2,500 feet above ground. Two-way communications with ATC are necessary. Class E is defined as controlled airspace that doesn’t fit the qualifications of the first four categories, and contains airspace from 14,500 feet up to 18,000 feet in altitude, unless specifically defined at a lower level. Class E is used for airports that don’t have a control tower. The final zone, Class G airspace, is uncontrolled by ATC, but still subject to weather restrictions. Often, restricted areas exist as well, protecting aircraft from unseen hazards or airspace designated for the military.
Weather is one of the most important factors in determining if it is safe to fly. Aviation publications like the Airport Facility Directory (AFD) and other briefs give pilots the information they need about the weather on their routes. Ultimately, the Federal Aviation Regulations (FARs) are responsible for pilots’ rules and regulations, while the Airmen’s Information Manual defines the NAS in more depth. In the NAS, ATC has control over most aircraft in the air and on the ground. This strict control is necessary for the safety of the airspace and citizens.
To travel the airways safely, pilots using IFR use the Federal Airways system, which determines preset routes that the aircraft can navigate to their destinations. Charts, GPS, and other navigational systems give pilots the tools to navigate this airspace, while the Terminal Procedures Publication (TPP) guides pilots in the airport area. There are regulations for departure procedures, Standard Terminal Arrival Routes, and arrival procedures. The National Airspace System is able to maintain strong control over the United States airspace and protect the country more easily through its organized structure.