What is this Free Flight site about?

    Free Flight is an Air Traffic Management (ATM) concept whereby airspace users are allowed more freedom in selecting routes. This could mean they are no longer restricted to airways but are allowed to fly direct routes from origin to destination at their optimal altitudes, using favorable winds and avoiding hazards themselves. By no longer following airways it could, however, become harder for Air Traffic Control (ATC) to monitor and control the traffic. To prevent this and to provide true self-optimization, airborne separation has been proposed as a part of Free Flight. There have been many potential ATM concepts in the past which have aimed at direct routing, but airborne separation, even under IFR conditions, is a unique feature of the Free Flight concept.

    Several questions arise when thinking about realizing Free Flight: What on-board equipment is needed to avoid traffic? Will it result in chaos? Will the pilots be able to cope with the extra task? Is it safe? Will pilots feel safe? What happens in extremely busy traffic situations?

    Though the study originally aimed at answering mainly the human factors questions, the feasibility of a conceptual design of Free Flight has been studied as well. As a result, initial answers have been found to the questions posed above (as well as some others). This site intends to give a brief overview of the NLR work in this area. It consists of both simulator studies (both flightdeck and ATC) and off-line studies. For answers to specific questions, please contact us. (see link on the right side)

    Conceptual Design

    The RTCA definition of Free Flight describes a range of concepts with varying responsibilities on the ground and in the air. A more precise definition is not yet available. As a part of the 1997 Phase I study NLR has defined a concept in more detail. This concept used and tested at the NLR is based on several assumptions:

    • Strategic separation control remains on the ground and is centrally organized. This means that aircraft are allowed Free Flight, once their departure and arrival times are checked and approved by Traffic Flow Management. This could also include checking of en-route choke points (e.g. due to bad weather).
    • Tactical separation control is delegated as much as possible to the aircraft (equipage permitting). This means that aircraft are responsible for their own separation.
    • Automatic Conflict detection and Resolution Advisories are part of the Airborne Separation Assurance System (ASAS). These are based on the Modified Voltage Potential algorithms. Normally they require all aircraft to manoeuvre out of conflict and do not use priority rules. Lookahead time used for these systems is 5 minutes.
    • A Traffic Display is also part of the ASAS.
    • No intent knowledge is used. Instead a rule of the road is added, stating that no maneuvres should be initiated that result in a predicted conflict within 5 minutes. To this purpose a PREDictive conflict detection system is added to ASAS (PREDASAS).
    • To ensure co-operative behaviour some form of Air Traffic arbitration is foreseen, whereby uncooperative behaviour is penalized. No detailed implementation of such a system has been proposed yet.
    • Unequipped aircraft are separated from equipped aircraft by a vertical division of airspace. This means that unequipped aircraft are not allowed above a certain Flight Level, whereas equipped aircraft can fly "Free" above that Flight Level and controlled beneath that Flight Level. This corresponds to the "Flight Level" option studied during the Phase II experiment.

    Human Factors of Free Flight

    Although the advent of Free Flight assumes certain enabling technologies (e.g., ADS-B capability, airborne traffic displays, and advanced ground-based conflict probes), Free Flight would represent as much an operational, as a technological, evolution. Under Free Flight, the roles of the pilot and controller would be drastically redefined. It is for this reason that attention is being focused on the role of the human components of the air traffic system, both in the air (pilots) and on the ground (ATC). Some of the major human factors issues surrounding Free Flight include the following:

    • Can controllers be expected to perform a tactical monitoring and separation assurance role?
    • What information will air and ground exchange? Will they withhold any information?
    • What are the workload implications of information uncertainty?
    • What happens when equipment fails? Can controllers serve as backups to automated conflict probe / resolution functions?
    • Will underloading /overloading present problems (e.g., in terminal areas)?
    • Will memory demands or situation awareness decrements present problems?
    • What are the best ways to design displays and algorithms, so as to facilitate information sharing between air and ground?
    • Are there behavioural bases for defining intervention strategies, airspace structures, resolution time horizons, etc?
    • How should Traffic Flow Management (TFM) handle potential "gaming"of arrival intent information?
    • Will pilots / controllers accept the concept of Free Flight?
    • How should future personnel be selected, and trained?

    To understand how profoundly the change to Free Flight could influence controller workload and monitoring, consider the following simple diagrams, that depict the principles of controlled flight (i.e., using a current day navaid-driven route structure) and Free Flight in the en route phase. Under controlled flight, there are a limited number of areas where conflicts are likely to occur. Indeed, the historical reasons behind the current-day fixed route structure have to do more with human limitations than with technical or procedural concerns. Under Free Flight, on the other hand, assuring separation of the same number of aircraft now seems a daunting task for the air traffic controller.

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