Donald O. Weitzman TRW Information Services Division
Free flight (FF) is a flight operating capability under instrument flight rules (IFR) in which aircraft will have the freedom to select their route and speed in real time. FF will allow pilots to select optimum routes and altitudes and control their own schedules through the application of communications, navigation, and surveillance technologies and the establishment of air traffic management procedures that maximize flexibility while guaranteeing safe separation.
The major difference between today's direct route (or off-airways) clearance and FF will be the pilot's ability to operate the flight without specific route, speed, or altitude clearances. FF would apply enroute unless air traffic becomes so congested that FAA restrictions must be imposed. For example, when aircraft approach congested airspace, where airport capacity is limited, when unauthorized entry into restricted airspace is imminent, when potential maneuvers may interfere with other aircraft, and when safety appears to be compromised, air traffic control (ATC) will again be imposed.
Currently, air traffic control is based on a highly centralized, ground-based, human intensive system. This system is graphically illustrated in Figure 1. To carry out the control task the controller has a variety of information presented to him/her, the most important parts being the flight plans for the aircraft under his/her control and the present traffic situation as shown by radar. On the basis of the present position and altitude as seen by radar, the controller will mentally predict where the aircraft will be in the future and whether two or more aircraft will violate the predetermined minimum safe separation standard (5 nautical miles).
FF calls for a more highly automated, technology-intensive system. In FF, aircraft will operate in an environment that minimizes ground-based control while making full use of cockpit technologies, as shown in Figure 2. The Global Positioning System (GPS) provides precise position, velocity and time estimates for all GPS equipped aircraft while Automated Dependence Surveillance (ADS) automatically broadcasts position, velocity and time data from one aircraft to another and to ground-based controllers using digital data communication (data link). Furthermore, the cockpit display of traffic information (CDTI) makes viable the division of responsibility between ground and air for maintaining safe separation among aircraft. Accordingly, only position and short-term intent information will be provided to the ATC controller who will perform monitoring functions and serve as an emergency backup in the event of technical system failure. This short-term intent information will also be provided to nearby affected aircraft. The ATC controller will only intervene to resolve conflicts that he/she is able to predict; that is, short-term restrictions will be imposed only when two or more aircraft are in contention for the same airspace. In normal situations, aircraft maneuvering will be unrestricted. Therefore, a greater emphasis will be placed on recovery from incidents than on their prediction. FF will be reactive rather than proactive. This design philosophy would seem to eliminate the human component on the ground from active, ongoing participatory roles in the system. Similar design philosophies have been studied and have received strong criticism from the human factors community ( Perrow, 1984; Endsley & Kris, 1995; Billings, 1996; Weitzman, 1997).
Questions regarding FF are generally directed toward the roles that technology will play rather than