route traffic control center (ARTCC) manage the flow of traffic along the airways between the TRACON areas. Overall flow of aircraft across the entire United States is managed by the Air Traffic Control System Command Center in Herndon, Virginia. Separate elements of the air traffic control system are also represented by oceanic control for overseas flights and by military controllers when military aircraft are flying within special-use airspace.

SAFETY AND EFFICIENCY

The stated goal of the air traffic control system is to accomplish the safe, efficient flow of traffic from origin to destination. The joint goals of safety and efficiency are accomplished by controllers through an intricate series of procedures, judgments, plans, decisions, communications, and coordinated activities. The communications and coordinations between the pilot and the controller are most familiar to the public. However, every bit as critical are the coordinations that take place within and between the air traffic control facilities themselves. Controllers must hand off aircraft as they pass from one controller's sector of responsibility to another. This handoff communication is sometimes done within a facility and sometimes between them. Also, hierarchical communications flow from the most global, national perspectives to more regional and local ones. That is, the Air Traffic Control System Command Center in Virginia considers national weather patterns and traffic needs each day and establishes national traffic patterns. The constraints established there are passed downward and outward throughout the system. Hour by hour, traffic patterns are monitored in the en route systems and may be used to identify bottlenecks, which in turn may give rise to specific instructions to hold aircraft from proceeding from one sector to the next.

The two goals of safety and efficiency are to some extent partially contradictory, and each is subject to tremendous pressures. We describe each in detail below.

Safety is ensured, in large part, by guaranteeing minimum separation between aircraft, a separation defined by altitude and lateral dimensions, creating a sort of "hockey puck" of space around each aircraft. These dimensions have different values in different regions of the airspace. The pressures for safety obviously come from the traveling community and are increased by reports of very rare midair collisions (Wiener, 1989) and somewhat less rare near-midair collisions (Office of Technology Assessment, 1988).

Of course, to ensure total safety, aircraft would never fly; to ensure a greater safety level than we have today, separations between aircraft would be greater than is currently the practice. However, that would compromise the second goal: efficiency. Two forces put strong pressures on the system for efficient flow: consumers and pilots. The traveling public, whose wishes are generally expressed by airline management, is understandably impatient with overbooked



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