Air Traffic Control
By U.S. Centennial of Flight Commission
In the earliest days of aviation, so few aircraft were in the skies
that there was little need for ground-based control of aircraft. In
Europe, though, aircraft were often flown in different countries, and it
soon became apparent that some kind of standard rules were needed. In
1919, the International Commission for Air Navigation (ICAN) was created
to develop “General Rules for Air Traffic.” Its rules and procedures
were applied in most countries where aircraft operated.
The United States did not sign the ICAN Convention, but later developed
its own set of air traffic rules after passage of the Air Commerce Act of
1926. This legislation authorized the Department of Commerce to
“establish air traffic rules for the navigation, protection, and
identification of aircraft, including rules as to safe altitudes of flight
and rules for the prevention of collisions between vessels and
aircraft.” The first rules were brief and basic. For example, pilots
were told not to begin their takeoff until “there is no risk of
collision with landing aircraft and until preceding aircraft are clear of
As traffic increased, some airport operators realized that such general
rules were not enough to prevent collisions. They began to provide a form
of air traffic control (ATC) based on visual signals. The early
controllers stood on the field, waving flags to communicate with pilots.
Archie League was one of the system's first flagmen, beginning in the late
1920s at the airfield in St. Louis, Missouri.
As more aircraft were fitted for radio communication, radio-equipped
airport traffic control towers began to replace the flagmen. In 1930, the
first radio-equipped control tower in the United States began operating at
the Cleveland Municipal Airport. By 1932, almost all airline aircraft were
being equipped for radio-telephone communication, and about 20 radio
control towers were operating by 1935.
Further increases in flights created a need for ATC that was not just
confined to airport areas but also extended out along the airways. In
1935, the principal airlines using the Chicago, Cleveland, and Newark
airports agreed to coordinate the handling of airline traffic between
those cities. In December, the first Airway Traffic Control Center opened
at Newark, New Jersey. Additional centers at Chicago and Cleveland
followed in 1936.
The early en route controllers tracked the position of planes using
maps and blackboards and little boat-shaped weights that came to be called
“shrimp boats.” They had no direct radio link with aircraft but used
telephones to stay in touch with airline dispatchers, airway radio
operators, and airport traffic controllers. These individuals fed
information to the en route controllers and also relayed their
instructions to pilots.
In July 1936, en route ATC became a federal responsibility, and the
first appropriation of $175,000 was made. The Federal Government provided
“airway” traffic control service, but local government authorities
where the towers were located continued to operate those facilities.
In August 1941, Congress appropriated funds for the Civil Aeronautics
Administration (CAA) to construct and operate ATC towers, and soon the CAA
began taking over operations at the first of these towers, with their
number growing to 115 by 1944. In the postwar era, ATC at most airports
was eventually to become a permanent federal responsibility. In response
to wartime needs, the CAA also greatly expanded its en route air traffic
control system. Women, too, for the first time were trained as controllers
during the war, and, at their peak, represented well over 40 percent of
the controller workforce.
The postwar years saw the beginning of a revolutionary development in
ATC, the introduction of radar, a system that uses radio waves to detect
distant objects. Originally developed by the British for military defense,
this new technology allowed controllers to “see” the position of
aircraft tracked on video displays. In 1946, the CAA unveiled an
experimental radar-equipped tower for control of civil flights. By 1952,
the agency had begun its first routine use of radar for approach and
departure control. Four years later, it placed a large order for
long-range radars for use in en route ATC.
Beginning in 1950, the CAA began consolidating some airport traffic
control towers at smaller airports with airway communication stations, the
forerunners of today's flight service stations. By 1958, it ran 84 of
these combined station-towers, the last of which closed in 1981.
In 1960, the FAA began successful testing of a system under which
flights in certain “positive control” areas were required to carry a
radar beacon, called a transponder, that identified the aircraft and
helped to improve radar performance. Pilots in this airspace were also
required to fly on instruments regardless of the weather and to remain in
contact with controllers. Under these conditions, controllers were able to
reduce the separation between aircraft by as much as half the standard
Air Traffic Control
personnel in the Carrier ATC Center aboard the USS
DOD photo by Carolla Bennett
For many years, pilots had negotiated a complicated maze of airways.
In September 1964, the FAA instituted two layers of airways, one from
1,000 to 18,000 feet (305 to 5,486 meters) above ground and the second
from 18,000 to 45,000 feet (13,716 meters). It also standardized aircraft
instrument settings and navigation checkpoints to reduce the controllers'
Although experimental use of computers in ATC had begun as early as
1956, a determined drive to apply this technology began in the 1960s. To
modernize the National Airspace System, the FAA developed complex computer
systems that would replace the plastic markers for tracking aircraft.
Instead, controllers viewed information sent by aircraft transponders to
form alphanumeric symbols on a simulated three-dimensional radar screen.
By automating some routine tasks, the system allowed controllers to focus
on providing separation. These capabilities were introduced into the ATC
system during the ten years that began in 1965.
The FAA established a Central Flow Control Facility in April 1970, to
prevent clusters of congestion from disrupting the nationwide air traffic
flow. This type of ATC became increasingly sophisticated and important,
and in 1994, the FAA opened a new Air Traffic Control System Command
Center with advanced equipment.
In January 1982, the FAA unveiled the National Airspace System (NAS)
Plan. The plan called for modernized flight service stations, more
advanced systems for ATC, and improvements in ground-to-air surveillance
and communication. Better computers and software were developed, air route
traffic control centers were consolidated, and the number of flight
service stations reduced. New Doppler radars and better transponders
complemented automatic, radio broadcasts of surface and flight conditions.
The FAA recognized the need for further modernization of air traffic
control, and in July 1988, selected IBM to develop the new
multi-billion-dollar Advanced Automation System (AAS) for the Nation's en
route ATC centers. AAS would include controller workstations, called
"sector suites," that would incorporate new display,
communications and processing capabilities. The system would also include
new computer hardware and software to bring the air traffic control system
to higher levels of automation.
In December 1993, the FAA reviewed its order for the planned AAS. IBM
was far behind schedule and had major cost overruns. In 1994 the FAA
simplified its needs and picked new contractors. The revised modernization
program continued under various project names. Some elements met further
delays. In 1999, controllers began their first use of an early version of
the Standard Terminal Automation Replacement System, which included new
displays and capabilities for approach control facilities. During the
following year, FAA completed deployment of the Display System
Replacement, providing more efficient workstations for en route
In 1994, the concept of Free Flight was introduced. It might eventually
allow pilots to use onboard instruments and electronics to maintain a safe
distance between planes and to reduce their reliance on ground
controllers. Full implementation of this concept would involve technology
that made use of the Global Positioning System to help track the position
of aircraft. In 1998, the FAA and industry began applying some of the
early capabilities developed by the Free Flight program.
Current studies to upgrade ATC include the Communication, Navigation
and Surveillance for Air Traffic Management system that relies on the most
advanced aircraft transponder, a global navigation satellite system, and
ultra-precise radar. Tests are underway to design new cockpit displays
that will allow pilots to better control their aircraft by combining as
many as 32 types of information about traffic, weather, and hazards.
Note: This article was commissioned by and
first appeared on NASA's U.S. Centennial of Flight web site. It
appears here with permission. We gratefully acknowledge both the author