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The resulting Free Flight concept is the outcome of those efforts. Both the RTCA working groups and their counterparts at the FAA have provided a phased timeline for the introduction of those efforts. One legitimate criticism that can be levied against the available literature on Free Flight is that the concept is described from an outcome-oriented viewpoint, not from a more traditional description of equipment and procedural implementation details. This has left the concept of Free Flight open to a wide variety of interpretation. At its core, Free Flight is
“ ... 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 imposed to ensure separation,
to preclude exceeding airport capacity, to prevent unauthorized flight through
Special Use Airspace (SUA), 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 move toward free flight.”1
This provides a fairly clear goal, but the exact implementation details are lacking. This raises several risks that must be considered when weighing the utility of the free flight solution against other options.
Some envision Free Flight as ultimately enabling aircraft to “maneuver at will.” This is perhaps what has fueled notions of autonomous aircraft that are capable of self-separation, and conjures up visions of aircraft maneuvering wherever and whenever they want without any coordination or negotiation with other stakeholders. We believe it is important to dispel this notion of Free Flight. In its purest form, Free Flight means that both separation and traffic flow constraints are provided to individual aircraft in terms of discrete time/position states. For example, an aircraft is provided with a Required-Time-of-Arrival at a specified approach fix, or with a time window during which it can’t be at a particular position and altitude. It is left to the aircraft, the airline, or some combination to satisfy the time/position state by manipulating the path and speed of the aircraft. If the time/position state can be met, then the aircraft can “maneuver at will” in the process. We believe that such an “open-loop” ATM system is untenable, at least for high density airspace. The reason is this: each aircraft is a potential constraint on other individual aircraft and traffic flow as a whole, so a perturbation in one aircraft’s route without feedforward to predict the
1 Final Report of RTCA Task Force 3: Free Flight Implementation. (Section 3.1.1, pg. 23)
consequences on other aircraft paths, or overall traffic flow, could create an unstable, chaotic state. This suggests, unlike the connotation of the phrase “maneuvering at will,” that the strategic aspect of airborne flight planning, especially prediction and the coordination and sharing of information, plans, and goals among a variety of shareholders, may need to increase, rather than decrease, in Free Flight. This doesn’t imply less flexibility; it means that greater flexibility may require greater planning, coordination and collaboration.
　
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