Traffic Control in Crowded Skies

In February of this year President Eisenhower called EDWARD P. CURTIS,vice president of Eastman Kodak Company, to be his special assistant for aviation facilities planning. “Modern aircraft,” wrote the President to Mr. Curtis, “can be operated in the numbers required by the national defense and the civilian economy only if airports, navigation aids, air traffic control devices and communications systems are suitable for their needs. . . . To delay the formulation of a comprehensive plan is to invite further congestion of the airspace, needless hazard, economic loss, inconvenience to users, and possible impairment of the national security.”


The airplane has become the prime mover of our population in its circulation over distances beyond 200 miles. There has been no serious lack of foresight in our seizing upon the opportunities presented by the air. We have developed 158,000 miles of airways over continental United States, and our aircraft population has grown from 29,000 in 1936 to 90,000 today. By 1975 we expect the U.S. aircraft population to increase to 125,000.

As these aircraft become more versatile, productive, and dependable, they are flown more each year. While in 1936 there were 5 million take-offs and landings at the nation’s airports, there are now 65 million, and 115 million are forecast for 1975.

During peak hours on busy days in 1956 there were 270 aircraft simultaneously airborne in the Los Angeles area (which generates the largest volume of general aviation in the world). It is estimated that by 1975 this number will have grown to 730.

At the present time, our aviation facilities are hard pressed to manage, within the bounds of prudence, even the fair-weather flow of air traffic. They are wholly incapable of meeting more than a fraction of the far heavier demands upon our airspace which the national strategy and the market are certain to impose by 1975.

Actually there is no shortage of airspace for any demand that can now be foreseen. Our problem is an obsolescent traffic control system which was never designed to cope with the complex mixture of civil and military traffic that now fills the air. So intense is the pressure, especially around the major metropolitan centers, that a drastic rationing of airspace will soon be inescapable unless we install a more efficient system of traffic control.

Today, even in clear weather, when traffic congestion reaches the magnitude indicated by our statistics or when the speed of converging aircraft reaches that of a 45-caliber bullet, complete reliance on a pilot’s ability to see and avoid a collision is questionable. A safe system now and in the future requires continuous traffic control regardless of visibility condition.

By 1975 the airport capacity of the country must be doubled, and the number of aircraft under traffic control will increase by ten times.


1. We are convinced that aircraft depending solely on pilot vision and alertness to avoid collision cannot safely share the same segments of airspace as those aircraft which are kept separate by air traffic control. The plan envisions, therefore, division of the airspace into zones.

2. First, all airspace above a designated altitude will be set aside for controlled separation at all times, because visibility and speeds at higher altitudes are such as to make “see-and-be-seen” flight unsafe.

3. Below this upper zone, a further division of airspace will be made. Funnels and cylinders of airspace are reserved for controlled separation of aircraft traveling from the upper zone to airports. Separate traffic patterns, final approach lanes, and separate runways will, whenever practical, be provided for high and low performance aircraft either at the same airport or adjacent to it. Highways of controlled airspace connecting the funnels and cylinders are reserved for controlled separation of low-flying traffic.

4. Airspace outside the highways, funnels, and cylinders is reserved for aircraft capable of visual collision avoidance.

5. For flight in see-and-be-seen airspace, aircraft must meet accepted minimum cockpit visibility requirements. They must be flown below a designated airspeed and carry a barometric altimeter to determine altitude boundaries of controlled airspace. It may also be necessary for see-and-beseen aircraft to carry a simple electronic device to detect the edges of controlled airspace.

6. The paths required by the users vary with type of aircraft, length of flight, and weather and wind conditions. For short-distance flights between population centers, fixed paths will be used.

7. Transcontinental flights, on the other hand, will have flexible paths to take advantage of the most favorable flying conditions. These flights commonly take place at high altitudes where they do not interfere with traffic on the fixed paths.

8. Direct flights between minor population centers arc a small percentage of the total traffic and tend to spread out over the whole country. Thus, in any particular area, the density of this type of traffic will be very low, and separation can be provided without undue complication.

9. For flights inbound into terminal areas, descending paths that converge on the airport arc required. Also, inbound aircraft must maneuver so that they will be lined up and ready for the final approach at the correct altitude and airspeed. The path an aircraft follows during this maneuvering process will combine descent with horizontal path stretching in such a way that aircraft reach their destination in a sequence with other aircraft landing at the same terminal. Ascending paths will be provided for outbound flights.


Control of air traffic will be carried out by a central ground authority by continuous prearrangement of airspace reservation for each aircraft, as contrasted to free flow of traffic with the control system intervening occasionally to resolve potential conflicts. The airspace will be divided up further into blocks of airspace defined electronically by a navigation system, and used as a basis of defining airspace reservation.

The air situation will be determined by the air traffic control system by two independent methods: reporting of position by the aircraft, and a three-dimensional ground surveillance system. The position data from these two sources will be correlated by air traffic controllers to determine flight progress and to assign clear airspace in the process of air traffic control.

Communication will be essentially automatic between all elements of the system, thus eliminating the delays encountered today because of the delays in voice communication. Voice communication, however, will remain the basic emergency means of communication throughout the system in case of equipment failure, and for those operators who do not, because of the expense involved, install coöperating automatic communication equipment.

Control will be decentralized, with each controller having a display of the airspace under his jurisdiction tailored to fit the geometry of his airspace. Data will be concentrated and organized to suit the functions of each controller. The human controller should — and can — retain his vital role as the decision-making element of the system.

But the processing, storage, communication, and display of data used to make the decisions will be thoroughly mechanized. Additionally, automatic devices will be used to carry out calculations to facilitate scheduling and to expedite the flow of traffic.