Air Navigation and Air Traffic Control


Just as automobiles use roads to travel from city to city and ships use sealanes to cross the ocean, airplanes depend on airways to reach their destinations. Thousands of these invisible paths crisscross the nation's airspace. They are highways of the sky, traveled daily by tens of thousands of airplanes, hundreds of thousands of passengers, and millions of dollars in cargo.

To assure that traffic proceeds safely along these aerial highways is the responsibility of FAA's air traffic and airway facilities services. The men and women of these organizations serve as air traffic controllers, flight service specialists, engineers, electronics technicians, and supporting personnel.

Today's sophisticated air navigation network has its roots in the 1920s, when pilots relied on scattered radio stations and rotating light beacons to hop from one landing field to the next. During periods of poor visibility, however, the usefulness of light beacons was severely limited. By the end of the decade, the Federal Government had introduced the first of many navigational aids that could serve the pilot day or night, fair weather or foul. This was the four-course radio range, a device that transmitted radio signals in four directions. The Government installed a network of these facilities to guide pilots to their destinations.

As aviation grew, more than four paths were needed to handle the navigational needs of air traffic, and the original radio range was replaced by the very-high frequency omnidirectional range (VOR), a device developed during World War II. VORs were deployed on the airways in large numbers after the war, and are still the chief air navigation aids on U.S. airways. Today's VOR uses sophisticated electronics, but operates on the same principle as its predecessors. It emits signals in the pattern of a huge wheel, with the station at the center and 360 spokes radiating from the hub. Each radial represents a radio course that a pilot can use to guide an airplane accurately along a desired track.

Navigational facilities also help a pilot descend from cruising altitude to land on an airport runway - a relatively small spot of the earth's surface - even under poor weather conditions. The Instrument Landing System (ILS) is the most widely used equipment in the world for making safe runway approaches in difficult weather. The FAA has deployed nearly 1,000 of these systems at airports across the United States.

An ILS sends out two radio beams to approaching aircraft. One beam, the localizer, gives the pilot left-right guidance; the other, the glide slope, gives the pilot the correct angle of descent to the runway Even when visibility from the approach end of the runway is only a few hundred feet, properly instrumented aircraft can now land with pinpoint accuracy.

In addition to accurate navigational aids, pilots need assistance in avoiding midair collisions. The air traffic control system is crucial to civil aviation, keeping airplanes safely separated from each other and regulating their flow into and out of airport terminal areas.

Under instrument flight rules, standard separation between two airplanes depends on a number of factors, including the size of the airplanes being separated and the kind of airspace they occupy Generally airplanes close to an airport are kept apart by at least three miles horizontally and 1,000 feet vertically When airplanes are flying between major terminal areas, standard separation is never less than five horizontal miles and 1,000 vertical feet.

Making this system work are the personnel who staff FAA control towers, terminal-area radar facilities, air route traffic control centers, and automated flight service stations. Each type of facility performs a different task. Tower and terminal-area controllers handle airplanes that are landing and taking off, taxiing on the ground, and flying in the vicinity of the airport. Center controllers handle airplanes en route from one terminal area to another, while flight service station specialists provide pilot briefings, in-flight communications, and other services.

A sophisticated array of radar, computer, communications, and electronic equipment helps air traffic personnel perform these missions. Highly trained electronics and environmental technicians work around the clock to keep this network operating at peak efficiency.

A new jewel in FAA's program for cutting-edge electronics is the Aircraft Situation Display, a color radar system that provides a bird's-eye view of selected air traffic anywhere in the country - on a national or local scale. For example, this device can show all airplanes currently in the air, anywhere in the country, that list Chicago's O'Hare International Airport as their destination -whether it's a passenger jet that just took off from Orlando or a corporate jet an hour out of Hartford. With this information, FAA air traffic specialists can predict if and where congestion and delays might occur, and take action to prevent them. Installed at en route centers across the country, Aircraft Situation Display gives added control over a system in which giant jets soar across the sky at speeds up to 550 miles an hour.

Denver Center Stapleton hangerAir traffic control began in 1935. The system then involved teletype machines, wall-sized blackboards, large table maps, and movable markers representing airplanes. While they nudged the little markers across the flat surface of a map, controllers estimated the positions of real-life airplanes moving through three-dimensional space.

Radar, developed during World War II, transformed the control of aircraft from an art to a science. It made possible the surveillance of traffic in the air, and was eventually adapted to the control of aircraft taxiing on the surface of large airports during low visibility Radar brought with it new techniques and procedures. Among the most useful was the requirement that aircraft above a certain altitude fly by instrument rather than visual flight rules, even if the pilot had unlimited visibility This helped to segregate aircraft flying by different procedures, and thus to reduce the risk of midair collision.

Advances such as these increased the capacity of the airspace system by allowing controllers to handle aircraft spaced more closely together. The procedures themselves remained labor intensive, however, requiring controllers to spend 75 percent of their time in voice communications and in such activities as preparing paper strips recording flight progress. By the early 196Os, it became clear that most of these functions could be automated.

Combined with radar, the computer became a bridge carrying air traffic control into the future. Signals transmitted by airplanes were received at control centers, digitized by computers, and displayed on radarscopes in the form of a "tag" accompanying the blip that marked a plane s location. The tag told the controller the aircraft's identifying call sign, its altitude, and its speed. This reduced paperwork and allowed controllers to concentrate on essential tasks. The result was another great increase in both safety and system capacity.

Radar systems at terminals and en route centers are upgraded periodically Controllers operating the more sophisticated types of equipment now receive computer-generated warnings if an aircraft descends below its minimum safe altitude, or if two aircraft are on a potential collision course. They can also take a "quick look" at weather conditions by calling up weather advisory information on their radarscopes. A backup radar system serves as a safety net if the main system fails.

The FAA monitors the airspace system with its own fleet of airplanes equipped with precision receivers, recorders, signal analyzers, and other devices. These flying laboratories assure the integrity of radar, communications, and navigational aids across the nation. They are part of an unending inspection program that helps to keep air traffic flowing smoothly - and safely -along the highways of the sky.