Friday, February 13, 2009

History of U.S. National Airspace System

"Learning should be fun. If you don't have fun in aviation then you don't learn, and when learning stops, you die." ~ Pete Campbell, FAA

Why did different kinds of airspace become necessary? How did the National Airspace System develop?

The history of aviation is fascinating! Go ahead... enjoy it!

The Federal airway system began in 1927, when the Department of Commerce acquired the transcontinental airway from the U.S. Postal Service. All airspace was uncontrolled, there were no real provisions for instrument flying, and very few airplanes. The only navaids defining that first airway were lighted beacons, and those, along with the airfields along the route, were what the Department of Commerce took over.

As more airplanes began to fly in and out of major cities, traffic became more of a problem. Sometimes observers on the ground helped coordinate takeoffs and landings, and the first radio-equipped control tower opened in 1930 in Cleveland.

Jimmy Doolittle demonstrated the first completely blind flight in September of 1929, and by 1933 the science of instrument flying had developed to the point that the Bureau of Air Commerce offered an instrument rating for pilots.

The first air traffic control facility was formed by four airlines in 1935 to coordinate their traffic around Newark, New Jersey. American, Eastern, TWA, and United worked together to provide separation for instrument traffic. When the government took over air traffic control a few months later, there were two additional centers, in Chicago, Illinois and Cleveland, Ohio.

The Civil Aeronautics Administration (CAA) was created in 1938, but did not take over the control of air traffic at airports until 1941. They took over control of the airways in 1942. With no radar, no transponders, and few radio navaids, it must have been quite a challenge to keep airplanes safely separated, especially with thousands of military airplanes swarming through the system in support of the war effort. The system of IFR flight plans, airways, position reports, and clearances is the basis of today’s ATC system.

To keep IFR traffic under ATC control all the way to the ground, a five-mile cylinder of controlled airspace was created around IFR airports, reaching up to the overlying airway. This “control zone” restricted VFR flights during bad weather, allowing the IFR flights to make their approaches without fear of collision. To allow IFR letdowns to begin farther from the airport, transition areas were created. These extended controlled airspace a few miles outward from the control zone, but still excluded the airspace below 700 feet AGL.

After World War II, the number of commercial and private flights grew even more. All IFR trips were flown along the airways, and except for the airways and the area immediately surrounding airports equipped for IFR arrivals and departures, the rest of the nation’s airspace was uncontrolled.

Since most general aviation airplanes were not equipped for IFR flying, there was little conflict between VFR and IFR traffic. The simple rules for cruising altitudes kept VFR and IFR traffic separated by at least 500 feet, and all pilots kept their eyes open in VMC to avoid other traffic. Airways were eight miles wide and linked radio range stations. There were no restrictions on the use of controlled airspace by VFR pilots, so long as visual separation was maintained.

As aircraft speeds increased and jets began to enter service, the capabilities of the old air traffic system were rapidly outgrown. A United DC-7 and a TWA Super Constellation collided over the Grand Canyon in 1956, emphasizing the inadequacy of the system.

The CAA became the FAA in 1958, when it was separated from the Department of Commerce to become an independent agency. By then, VORs were replacing the old four-course radio ranges, and radar was in use at major airports. The new VOR airways were called Victor airways, to distinguish them from the colored airways that linked four-course radio ranges.

At busier airports, both VFR and IFR traffic came under ATC control. Although most airliners had radios, most of the traffic still consisted of general aviation airplanes, and most of them didn’t have radios. The control towers could use radio or light gun signals to control traffic.

In the 1960s and 1970s, ATC radar expanded to cover most of the continental U.S., giving controllers a real-time view of the traffic. The FAA felt that any traffic above 18,000 feet should be under positive control, that is, monitored and directed by ATC, so pilots were required to be instrument rated and on IFR flight plans. This Positive Control Airspace was renamed Class A airspace in 1993.

The omnidirectional nature of VOR signals allowed pilots unprecedented freedom to create their own radio navigation routes, independent of the established airways. As navigation equipment such as RNAV, Loran, and INS came into use, pilots often abandoned the published airways to navigate directly from point to point. In the late 1970s, almost all the uncontrolled airspace in between the standard low-altitude airways was changed to Class E, leaving only the 1,200 feet just above the ground as Class G.

As this was happening, traffic at the busiest airports had become almost unmanageable. With jetliners streaming into the airports at hundreds of knots, the controllers needed to organize and sequence them at greater distances from the airports.

The old airport traffic areas and control zones were expanded and regulated. First, airplanes without radios were made unwelcome, then airplanes without transponders, then those without altitude-encoding transponders. The upside-down wedding cake arrangement at major terminals allowed airline traffic to descend below 10,000 feet AGL as much as 25 or 30 miles from the airport, without the concerns of seeing and avoiding VFR traffic that might not be under ATC control. Initially called Terminal Control Areas or TCAs, they became Class B airspace in 1993.

Airports with less traffic were designated Airport Radar Service Areas (ARSAs), and Terminal Radar Service Areas (TRSAs). ARSAs became known as Class C when the airspace was renamed in 1993, but TRSAs remain as a vestige of the old nomenclature.

With the adoption of satellite-based navigation, and the advent of digital datalinks between aircraft, the stage is set for another major step forward.

Resource from AOPA and Jeppesen.

1 comment:

  1. Excellent, and very well written.

    ReplyDelete