Free Flight

Free Flight

36TH ANNUAL CONFERENCE, Taipei, Taiwan, 17-21 March 1997

WP No. 93

Free Flight


The 35th conference charged SC1 with the responsibility of monitoring developments of the rapidly evolving concept of “Free Flight”.


Three basic philosophies are being developed all with a similar objective :

a) RTCA proposal tasked by the US FAA;

b) EATMS concept being formulated within EATCHIP (EUROPE);

c) Independent study being promoted by RMB and Associates (US).

These first two concepts have as their core elements. the need for automation in ATC systems, dynamic separation standards, automated aircraft systems with modern satellite navigational capabilities capable of being dowinlinked directly to a controller workstation equipped with direct controller pilot communications capability.

A recent (January 95) report by the US RTCA Select Committee on Free Flight defined Free Flight as:

“A safe and efficient flight operating capability under 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 unauthorised 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 move towards free flight”.

The European view of “free-flight” and the speed with which it might be introduced is substantially different to that of the USA. This is not least because Europe has not only the world’s densest air-traffic Environment but also a highly developed ground-based infrastructure which is already capable of performing many of the ATM functions required for at least the initial stages of’ FANS implementation.

Eurocontrol the European Commission and the European Space Agency are working together to develop the global-navigation satellite system (EGNOS), and the group is maintaining contacts with the FAA to ensure compatibility.

The RMB proposal has as its core element current generation conflict prediction/resolution tools (BDACS & PRAT) which cannot consistently predict accurately where aircraft are going to be in the future.

A enhanced conflict probe with greater proven accuracy needs to be developed before this proposed concept can he relied on for separation, in traffic areas where dynamic density continues at high levels.

FANS, CNS/ATM Free Flight: the names change, but the story remains the same. Aviation is moving away from the reliance on ground based systems which has marked its first century, and into an era of satellite based communication and navigation; from ground radar’s to passive surveillance; and from air traffic control to air traffic management In a. mature concept, control and separation functions may be shared between the ground and the cockpit.

Development of Free Flight will be an evolutionary one, with methods used to utilise the proposed benefits differing according to airspace design and capability.

Transition must be an integral part of any ATM conceptual thinking and not an afterthought. Such transition will be benefits driven, and that in the reality of a cost conscious business, some worthwhile benefits which would be of potential benefit to controllers, will not go ahead on the grounds of an apparent lack of cost effectiveness. This must be avoided, particularly if identified safety benefits are lost.

Training requirements for controllers who are placed in the situation of transition from basic procedural control to that of an advanced sophisticated ADS/Free Flight environment, or from a fully automated radar environment to ADS/FF structure, should have the appropriate ratings and training prior to being allowed to control in an operational capacity. As it has been recognised that the human will remain at the center of the control process, it follows that the training of the controller must encompass all elements of the task to be accomplished.

Design features must encompass user requirements, state boundaries, aerodrome location and existing route structure. i.e.: A planned framework of’ operational concept possibilities All affected parties must strive for a common aim: to optimise the available capacity and at the same time avoid overloads so that safety standards are not compromised. System design should be based on the controller being at the centre of the control process. Safeguards must be put in place to ensure that the controller remains active rather than passive monitor of’ any automated system Imperative in the design,, and capability functionality, is an ability for accurate prediction of system degradation and default condition to allow the controller to readily manage the traffic utilising an alternative traffic management procedure.

Capability requirements shall include traffic density, aircraft capability and ground support mechanisms The expected increases in capacity will only be achieved where the workload of the controller is constrained within manageable proportion.


RTCA Task force 3 in its’ tabled report contained some 37 recommendations for near term actions in 1997 arid 1998, which was used by the FAA in the preparation of the FAA FY 1997 budget for presentation to congress. A further 10 recommendations were included for medium term (1998 – 2000) and long term action (2001 and beyond).

The US FAA is proposing to launch a full scale Free Flight demonstration/evaluation in the Hawaii/Alaska regions of the US starting in 1999. The project titled “Halaska.” could involve up to 2000 aircraft.

Human Factors

The fundamental tool required for the implementation of Free Flight is an automated conflict probe. Other automation aids or decision support systems are required to assist the controller in facilitating free Flight, these could include controller workload monitors, sequencing and spacing tools, dynamic density predictors and the like.

An automated decision support system outputs some form of problem resolution. Because the controller will still have responsibility for the outcome, he/she must have the authority to override machine output, in principle, i.e., to filter the computer’s recommendation or decision. However, If the controller is to filter machine solutions, he/she must be able to comprehend the air traffic situation and have an adequate understanding of the automated decision process. If not, then the controller is likely to be placed in a very stressful situation where the only sensible strategy is to “opt-out” rather than risk overriding the automation.

The question is how good will controllers be at discriminating correct from Incorrect machine solutions when the dynamic control of traffic in FF will be moving further and further away from the controller. It must be recognised that in giving up the interplay between knowledge and understanding and its regular practical exercise, we are departing from the only conditions we know for successful human performance.

Monitoring aids must support the controller, but care must be exercised in their use, because certain routine actions are required by the human in order to maintain a mental picture of the operating environment – this supports the identification of errors by controllers and the technical systems themselves.

Skies should be managed as expressways rather than bottlenecks. We do not want multiple solutions determined by geographical region, but rather one solution that ideally can be applied globally. States therefore need to endorse a strategy for ATC system harmonisation and integration.

The development of Free Flight concepts should not be motivated only by immediate commercial considerations. It we do lake this course, we will defeat the purpose of global implementation of a seamless system – The industry will inevitably be driven by commercial considerations, but we should not forget that our objective is global. if we start fragmenting the system at the outset, then we will defeat the purpose for which the system has been established. We need a structured incremental approach to development, but not the one leading to fragmentation of the system which is envisaged as the global one. Such a structured process will enable a participative approach, contributing to the building of a consensus. We need to secure the benefits as early as possible, but not to the detriment of the worldwide enhancement of the air navigation services. We must avoid the danger of adopting an overly conservative approach, which presumes too much of the current concepts to be inevitable and immutable.

IFATCA is not opposed to the basic concepts of FREE FLIGHT. It does however, consider, that what ATC needs to be provided with, in order to increase its’ efficiency and capacity) is not a technology driven proposal, but an Air Traffic Management (ATM) system which is based on flexible use airspace, on an area concept, as opposed to a segregated system, and on one with sufficient airport capacity to meet the demands of both the airspace users – and A’I’C.

Free Flight will evolve as a function of available technologies, procedural changes, aviation community requirements, and increase in airspace system capacity.

The RTCA report emphasised the need for collaboration within the user community. It is imperative that IFATCA maintain a pro-active role in the evolutionary process of “Free Flight” and actively seek representation on all bodies associated with this project development and at all levels within.

The benefits to the whole civil aviation community should be our prime goal for the future.

Last Update: September 28, 2020  

March 4, 2020   457   Jean-Francois Lepage    1997    

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