OVERVIEW: The Free Flight Concept
According to the RTCA's 1995 Select Committee on Free Flight report, the current Air Traffic Management (ATM) system is plagued by insufficient capacity, limited access, and excessive operating restrictions. This has resulted in increased delays, higher operating costs, and diminished system efficiency. One possible solution to these problems might lie in an entirely new vision of ATM, known as Free Flight (FF). The RTCA report defines FF as:
"a safe and efficient flight operating capability under instrument flight rules (IFR) in which the operators have the freedom to select their path and speed in real time. Air traffic restrictions are only imposed to ensure separation, to preclude exceeding airport capacity, to prevent unauthorized flight through special use airspace, and to ensure safety of flight. Restrictions are limited in extent and duration to correct the identified problem. Any activity which removes restrictions represents a step towards free flight...."
Unlike today's air traffic system, in which ground-based personnel ("Air Traffic Controllers," or ATCos) control aircraft, a future FF system of air traffic might shift some or all of the responsibility for route selection and separation to airborne systems. Under mature FF, aircraft outside of terminal areas would generally be free to fly their preferred routes, and self-separate, with minimal intervention from air traffic control (ATC). Although some sort of flight plan would still be available, it would be used to assist flow management (the strategic control of air traffic), but not used as a basis for separation.
As shown in this figure, the FF concept envisions two "zones" surrounding each FF aircraft: an inner Protected Zone and an outer Alert Zone. The Protected Zone represents the minimum distance that two FF-equipped aircraft can attain. The size of the protected zone would vary inversely with the accuracy of position data, the communication rate, and the performance of intent monitoring systems. The Alert Zone represents the projection of the Protected Zone forward in time. So, for instance, increases in aircraft manoeuvrability result in a larger future swath in space. Whereas the inviolate protected zone (which would ensure separation) would be of fixed dimensions, the size of the outer alert zone would vary with aircraft speed, performance, and CNS/ATM capabilities.
There are many research topics that need to be addressed as part of the evolution to FF. Some of these general topics include the following:
- What are the likely safety and cost benefits of FF?
- How can seamless operations be achieved? Can interoperability from the perspective of both the pilot (who must operate in various airspaces) and the controller (who must accommodate mixed equipage aircraft) be assured?
- How can we transition from current to a future ATM system?
- What are the minimal technical, infrastructure, and procedural requirements for FF?
- How will the roles of air and ground operators be redefined?
- How should airspace transitions (e.g., into the terminal area) be handled?
- What new rules-of-the-air will be defined for FF?
- What new procedures will be defined for delegating separation assurance to the pilot?
- How do airspace factors (e.g., traffic density, or complexity) effect the feasibility of FF?
Although FF will shift separation authority from ground to air, most experts agree that (certainly in the near-term) the controller on the ground will not be completely removed from the loop. But what will the role of the "Air Traffic Controller" be in the future, as we transition to FF? Will controllers still be responsible for tactical separation? Will they intervene only in case of system failures? Or will their job become more strategic (e.g., traffic flow management)? These questions have implications for the displays, tools, and procedures that will be used to carry out ATM in the future.
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