the human operator's experience and judgment will be needed to cope with such conditions.

The implementation of automation in complex human-machine systems can follow a number of design philosophies. One that has received considerable recent interest in the human factors community is the concept of human-centered automation. As we mentioned earlier in this report, human-centered automation is defined as "automation designed to work cooperatively with human operators in pursuit of stated objectives" (Billings, 1991:7). This design approach is discussed in more detail toward the end of this chapter.

Over a decade of human-factors research on cockpit automation has shown that automation can have subtle and sometimes unanticipated effects on human performance (Wiener, 1988). In a recent report (Federal Aviation Administration, 1996a) the impact of cockpit automation on flight deck crew performance has been documented in some detail. Similar effects have been noted in other domains in which advanced automation has been introduced, including medical systems, ground transportation, process control, and maritime systems (Parasuraman and Mouloua, 1996). Understanding these effects is important for ensuring the successful implementation of new forms of automation, although not all such influences of automation on human performance will apply to the air traffic control environment. The nature of the relationships between controllers and ground control systems on one hand, and pilots and aircraft systems on the other, will also change in as yet unknown ways. For example, at one extreme, control of the flight path and maintenance of separation could be achieved by automated systems on the ground, data-linked to flight deck computers. At the other extreme, as in some of the concepts involved in free flight, all responsibility for maintaining separation could rest with the pilot and on-board traffic display and collision avoidance systems (Planzer, 1995). Whether or not the most advanced automated tools are implemented, however, it is likely that the nature of the controller's tasks will change dramatically. At the same time, future air traffic control will require much greater levels of communication and integration between ground and airborne resources.

In this chapter, we focus on four aspects of automation in air traffic control. We first describe the different forms and levels of automation that can be implemented in human-machine systems in general. Second, we describe the functional characteristics of several examples of air traffic control automation, covering past, current, and new systems slated for implementation in the immediate future. We do not discuss automation concepts that are still in the research and development stage, such as free flight, or the national route program, which will provide indications of air traffic control capabilities and requirements relevant for free flight considerations. Third, we discuss a variety of important human factors issues related to automation in general, with a view to drawing implications for air traffic control (see also Hopkin, 1995). Recent empirical investigations and human factors analyses of automation have been predicated on the view