Dynamic and Flexible ATS Route Systems

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Dynamic and Flexible ATS Route Systems

55TH ANNUAL CONFERENCE, Las Vegas, USA, 14-18 March 2016

WP No. 161

Dynamic and Flexible ATS Route Systems

Presented by TOC

Summary

Dynamic and flexible ATS Route Systems provide financial benefits for airliners and contribute to a carbon neutral growth of air traffic. The implementation of any form of flexible route structures should be adjusted to the particular piece of airspace. This paper explains the different forms of flexible route structures and the challenges of implementation. IFATCA policy on the subject is reviewed.

Introduction

1.1  Dynamic and flexible ATS route systems are currently being implemented in many locations throughout the world.

1.2  The ICAO Global Air Navigation Plan (GANP) identifies “Dynamic and Flexible ATS Route Management” as a Global Plan Initiative, which will be provided by “the establishment of more flexible and dynamic route systems, on the basis of navigation performance capability, aimed at accommodating preferred flight trajectories”.

1.3  Dynamic and flexible ATS route systems are often called free route airspace. The implementation of free route airspace creates the opportunity for airspace users to benefit from direct routings more than in a fixed airway structure airspace; therefore, it contributes to meeting the global aviation industry targets for carbon neutral growth.

1.4  This Information Paper uses the term free route airspace to refer to the many variations of dynamic and flexible ATS route systems. The paper considers the free route airspace concept and details a number of ways and varying degrees to which the concept has been implemented.

Discussion

2.1.1  Unlike in traditional airspace, where airspace users plan along ATS routes, free route airspace is airspace within which users may freely plan a route between a defined entry point and a defined exit point, with the possibility to route via intermediate (published and unpublished) waypoints, without reference to the ATS route network, subject to airspace availability. Within this airspace, flights remain subject to air traffic control.

2.1.2  Free route airspace may be established within a single sector, in multiple sectors, throughout a single Flight Information Region (FIR) or may even extend across FIRs.

2.1.3  Although controllers often issue tactical direct routing, which reduces fuel consumed, airspace users cannot adjust their fuel plan and they therefore take unnecessary extra fuel, causing extra costs and fuel consumption. When users plan in free route airspace, they file a custom route that takes into account wind, weather and airspace restrictions such as military activity, which allows airspace users to plan a more fuel-efficient route.

2.1.4  It should be noted that although the terms are similar, free route airspace is different to Flexible Use of Airspace (FUA). FUA is an airspace concept in which airspace is not designated military or civil but rather is used flexibly on a day-to-day basis based on the needs of the various military and civil users in order to achieve the most efficient outcome.

2.1.5  The transition from fixed ATS routes to free route airspace may be difficult for controllers depending on the characteristics of the traffic, airspace and ATM systems. Because of this, there are a number of modified free route airspace schemes by which airspace users may be provided with more flexible routing without allowing complete free routing.

2.1.5.1  User Preferred Routes (UPRs) appeal to the imagination the most, giving the airspace user the opportunity to plan almost any route within a certain volume of airspace, given certain restrictions. When there’s a low traffic density in a volume of airspace, ATM capacity balancing could be not deemed necessary and airspace users could file UPRs that are accepted on a tactical basis – the ANSP does not have the opportunity to review the flight plan before filing. This “File & Fly without capacity balancing” is operable in many locations throughout the world including Free Route Airspace Lisbon (FRAL), the Arabian Sea/Indian Ocean UPR Zone (ASIO-Z) and the North American Route Plan (NRP).

2.1.5.2  In environments with higher traffic density, the ANSP may choose to review the UPRs on a pre-tactical basis. This way, unacceptable workload or possible system deficiencies caused by a large number of flights needing to be de-conflicted is prevented. In this “File & Fly with capacity balancing” situation, airspace users send a Preferred Route Message (PRM) to a designated ATC Flow Management Center (or other center entrusted with this task) after which the routes are reviewed, adjusted if necessary, and a TDM (Track Definition Message) is made.

2.1.5.3  When a more orderly traffic flow is desired (for example in high traffic density airspace), so-called Flex Tracks or an “Organized Track System” can be used. Flex Tracks are published by an ANSP on a daily basis for flights between major aerodromes and are activated during peak traffic periods. Airspace users send in a Preferred Route Message after which the ANSP assimilates and validates a common preference of the day, taking possible restrictions into account. The Flex Tracks are then published in a Track Message via AFTN or NOTAM. This way the airspace users can benefit from more direct routes and forecast winds, while ATC is facilitated in achieving separation and airspace capacity by limiting the number of confliction points. Organized Track systems are used in Australia (AUSOTS), England (North Atlantic Organized Track Structure. NAT OTS) and the Pacific (PACOTS).

2.1.5.4  If more predictability is required, Direct Route Segments may be implemented. Direct Route Segments are additional routes that are published by the ANSP in order to provide more flexibility for flights operating on the fixed ATS route structure. Direct Route Segments such as those in the Melbourne FIR provide connections between ATS routes and often replicate direct routing that is issued by controllers on a tactical basis.

2.2.1  A traditional fixed route network means expected traffic streams and creates routine, causing a less error prone environment. When controllers issue direct routing, the situation might not be the same every time but can be seen as somewhat similar. Conflict detection by the controller is based on experience and known ‘hotspots’. Before giving a direct routing the situation is checked for potential crossing or future conflicts. Also the direct routings can be used as a way to quickly solve certain conflicts.

2.2.2  In a free route airspace environment, these certainties do not exist anymore. Instead of fixed streams, traffic flies along optimal routes that may conflict in unusual locations close to sector boundaries or reserved airspace; therefore, conflict detection should be one of the main issues to take into account when implementing free route airspace. The solutions to this issue depend on the traffic situation of the affected airspace.

2.3 After the implementation of free route airspace, several area control centres reported significantly increased complexity due to the shifting of conflict areas. Instead of the existence of a single waypoint where traffic centralized, a big area of conflict was now formed, making conflict detection more difficult. Furthermore direct routings could no longer be used as a way to solve potential conflicts. However, other centres have reported no issues with the free route airspace implementation, with controllers adapting easily to the new situation. This deviation could be caused by the differences in system support or airspace volume, sector complexity and/or capacity. The estimated effect of the implementation of free route airspace should be determined per centre.

2.3.1  In a study conducted in 1997 the effect of the then so-called Free Flight on Mental Workload, Monitoring and System Workload was tested; although an older study, the results are still relevant. The participating controllers reported that the workload in the Free Flight environment was lower than they anticipated, however conflict detection was considered and proven to be significantly more difficult. Also the number of Flight Information requests (such as predicted route and intent information) increased significantly compared to a so-called Controlled Flight (airway structure) situation. Participating controllers expressed their concern about relying on STCA as a safety net, depriving themselves of sufficient time and options to solve a conflict situation.

2.3.2  In several ATC environments paper strips are still used. By using the estimated times over waypoints the paper strips give an overview of conflicts in the medium term. However, in a free route airspace environment only entry and exit points are used so the estimates don’t give a clear view on possible conflicts. This is especially problematic in case of a surveillance system failure in high traffic volume airspace where paper strips are used as a back-up.

2.3.3  Conflict detection tools can help the controller to recognize conflicts away from known hotspots; however, an automated surveillance system including electronic strips or a stripless system is a precondition in this case.

2.3.3.1  An example for such a tool can be found within the MUAC (Maastricht Upper Area Control) centre. This centre has implemented a Direct Route Airspace (Traffic can file direct tracks between certain points but existing routes can also be filed and flown. A full FRA environment will be implemented in several steps) within high traffic loads environment. Their so-called NCW (Near Term Conflict Alert Window) tool automatically extrapolates the aircraft tracks, taking controller input into account. The tool shows the traffic, which will come within 10 NM separation within the next 10 minutes, displaying the conflict information in a separate window on the screen. When pointing at such a conflict, the conflicting aircraft are also paired on the screen to get a quick overview of the traffic situation.

2.3.3.2  Another function used in MUAC is the PROBE tool, which is divided into a short and a long term PROBE. The short term PROBE automatically activates when a cleared flightlevel is selected in the controller label and checks for possible conflicts within the next 2 minutes. The long term PROBE can be activated manually and checks for potential conflicts in an 8 minute window.

2.3.3.3  Other centres that have implemented free route airspace use other forms of conflict detection. Hungary for example uses 5 different layers of conflict detection, starting with conflict detection based on Flight Plan information. Furthermore a medium term conflict detection system extrapolates the current track using an adjustable time line. STCA will give a mild pre-warning 3 minutes before a conflict occurs while the standard STCA warning is given 2 minutes in advance.

2.3.3.4  IFATCA Policy is:

ATS 3.20 Conflict Detection Tools

Conflict Detection Tools (CDTs) are computer based Controller Tools that identify conflicts and then provide system generated conflict advice to controllers.

Responsibility and legal implications should be fully addressed before implementation of CDTs.

During degraded modes, clearly defined operational procedures must exist. Nuisance and false alerts must be kept to an absolute minimum.

 

TOC considers this policy still valid.

2.3.3.5  IFATCA Policy is:

AAS 1.4 Required Navigation Performance (RNP) and Area Navigation (RNAV)

Track prediction vectors should be available on situation displays to be used to control airspace where random routes are permitted.

 

Although TOC still considers that a well-implemented and sufficient conflict detection tool is necessary in free route airspace, this does not necessarily have to be a Track Prediction Vector. Trajectory prediction tools are still deemed necessary in order to make a proper conflict consideration. As discussed above, conflict detection tools are also eligible in some cases to compensate for the lack of predictability in traffic flows.

Furthermore the existing policy uses the term “random routes”, while ICAO speaks of “dynamic and flexible ATS routes” in the GANP. To better align with ICAO terminology and reduce the specificity of the policy, it is proposed to reword the current IFATCA policy, as follows:

Trajectory prediction and conflict detection tools should be available on situation displays used to control airspace where dynamic and flexible ATS routes are permitted.

 

2.3.4 Sectorisation needs to be reviewed before implementing free route airspace. Sometimes the sectorisation used in the fixed route is also applicable in a free route airspace environment; however, other cases have been reported in which the sectorisation had to be adjusted and was tuned according the most preferred routes. According to the “Free Route Developments in Europe” from Eurocontrol, sector design criteria should, at least, take into account:

  • the principle traffic flows and orientation
  • minimising short transits trough sectors
  • minimising sector and ACC re-entry
  • positions of airspace reservations
  • coherency with adjoining fixed route sectors and link routes to SIDs and STARs
  • civil/military coordination aspects (European Free Route Developments, Edition No 1.0, page 11)

2.3.4.1 A system in which it is easy to include or exclude certain sectors from the control sequence could reduce the need for re-sectorisation. In some cases only a limited number of flights will only ‘scratch’ a sector, or unexpectedly enter it in short-term notice. In such cases an adjustable system, sending details to the other sector or giving notice to disregard the flight, would help to keep the overall view of applicable traffic streams.

2.3.5 Military cooperation is of great importance to optimize the airspace use. Flexible use of military airspace makes more direct tracks available and plannable for the aircraft operators. The use of airspace reservations can be communicated through AUPs (Airspace Utilization Plan) if known well in advance. However the same Flexible Use of Airspace might call for short-term notice reservations.

2.3.5.1  In some case an active TRA (Temporary Reserved Area) does not impact major traffic streams and a tactical solution in which the controller vectors the traffic clear of the military airspace could be sufficient. In this case local procedures have to be defined with the military to cover cases such as a communication failure in which case an aircraft will revert back to its filed flight plan. When a larger amount of traffic is affected or in non-surveillance airspace, standard or specially created points can be used in order to reroute traffic to stay clear of the restricted area.

2.3.5.2  Responsibility for this separation in the tactical phase lies fully with the controller. In such a case a controller tool indicating a potential TRA penetration could be useful. This tool could be activated on different levels; based on the Flight Plan (pre-tactical level), a warning in the label and a last minute warning.

2.3.5.3 An example for such a penetration warning tool comes again from the MUAC centre. As soon as the controller makes a route related input (for example, adds a fix where the aircraft is cleared direct to in the label) the trajectory-based system automatically calculates if this route will take the aircraft trough an active TRA. This calculation is also made for aircraft on a filed route as soon as a military area becomes active. Should the predicted aircraft route penetrate an active TRA in any way, a visual warning is displayed in the label.

2.3.6 Whether or not a sector or FIR is suitable to implement free route airspace depends on the surveillance level and controller tools available. A surveillance environment is not necessarily an enabler for FRA; however, the volume and complexity of traffic crossing a sector at any given time should be taken into account. The Melbourne FIR, for example, implemented free route airspace in non-surveillance airspace, in a very large sector with low traffic volume and complexity.

2.3.6.1  Although many ANSPs report an increase of capacity after the implementation of free route airspace, other contrary reports state that capacity has been reduced slightly because of the increased complexity. In order to manage an increase in complexity, the ANSP can define some complex areas as only for fixed ATS routes or provide more predictable options such as an Organised Track Structure rather than allowing UPRs.

2.3.6.2  According to the IFTACA Technical and Professional Manual, Sector Capacity is determined as follows:

ATS 3.7 Sector Capacity Values

Sector Capacity: The maximum number of flights that may enter a sector per hour averaged over a sustained period of time, to ensure a safe, orderly and efficient traffic flow.

 

Currently Sector Capacity is often calculated using occupancy counts, which are defined as follows:

ATS 3.7 Sector Capacity Values

Occupancy Counts: the number of flights occupying a sector simultaneously during a specified period of time.

 

2.3.6.3  Since flight plans are still filed on the basis of published significant points, calculations concerning traffic volume and sector demand take place similar to the existing fixed airspace structure. Therefore Flow Management often does not change with the implementation of FRA. However, as mentioned before, FRA could cause an increase in task complexity. A tool that measures workload – taking complexity into account – instead of just occupancy might provide a better image on the actual Sector Capacity.

2.3.7 In the process of transitioning from a fixed ATS route structure to free route airspace, it might be possible that both fixed routes and UPRs are used at the same time so it is imperative that the correct route information is available to the controller at all times. The ATS system should be able to process not only the standard route flight plans but also those containing flexible routes. In order to keep track of the different routes filed, controllers may be provided with a tool that displays the flight planned route of the aircraft. This could be displayed upon request and/or automatically when assuming the aircraft on first contact.

2.3.7.1 IFATCA Policy is:

AAS 1.4 Required Navigation Performance (RNP) and Area Navigation (RNAV)

In airspace where random routes are permitted, the ATS system should be capable of processing random routing flight plans and controllers should be able to amend/update such information.

 

Although the intent of this policy is still valid, again the term random routes should be replaced by “dynamic and flexible ATS routes” in order to reflect ICAO terminology. As mentioned in the previous paragraph, situations in which both flexible and standard routes are filed occur; in such cases, the ATS system should be able to process these flight plan in order to provide the controller with the appropriate information. TOC considers this policy still valid. A slight rewording is proposed, since the processing of flight plans by the ATS system also includes the possible changes and adjustments made by the controller.

It is therefore proposed that the policy is amended to read:

In airspace where dynamic and flexible ATS routes are permitted, the ATS system should be capable of processing associated flight plans.

 

2.4 Although free route airspace implemented in single FIRs is beneficial for the airspace users, even more benefit may be obtained by implementing free route airspace across FIR boundaries. In Europe, such a cross border concept is already in the roadmap of most Functional Airspace Blocks, and more and more states are working on implementation. The Arabian Sea/Indian Ocean UPR Geographic Zone provides another example of ANSPs within a region implementing free route airspace across national and FIR boundaries.

2.4.1 When implementing free route airspace across FIR boundaries, it is important to consider the additional complications that may arise. AIS publications must provide clear procedures to airspace users for flight planning; these procedures should be regionally harmonised as much as possible (to simplify the flight planning process) while still taking into account the different ATS system performance of each of the participating FIRs.

2.4.1.1 Information sharing is essential in cross-border free route airspace. The latest information should be available at all time and all parties should be able to process this info. An electronic form of information sharing is preferable since this is less error-prone.

Conclusions

3.1  Dynamic and flexible ATS route systems (often referred to as free route airspace) contributes to meeting the global aviation industry targets for carbon neutral growth and allow airspace users to benefit from plannable direct routings.

3.2  Airspace users may freely plan a route between a defined entry and exit point, without reference to the ATS route network and subject to airspace availability. Flights remain subject to air traffic control.

3.3  Free route airspace may be established within a single sector, in multiple sectors or may even extend across Flight Information Regions. Because a fully flexible route structure might not be possible for all airspace, there are a number of schemes by which airspace users may be provided with more flexible routing such as User Preferred Routes, an Organized Track System and Direct Route Segments.

3.4  Since implementing any form of free route airspace may increase controller workload, shifting from standards and recognisable patterns to a more ad hoc way of working, several enablers need to be taken into account before implementation.

3.5  After the implementation of free route airspace, increased complexity due to shifting conflict areas has been reported in several centres. Conflict detection and trajectory prediction tools that could help the controller to recognize new conflict areas are deemed necessary in high traffic volume sectors.

3.6  Sectorisation, military cooperation and sector capacity should be taken into account when implementing any form of free route airspace. In order to obtain even more benefits from the free route airspace concept, cross border implementation may be considered.

Recommendations

4.1  It is recommended that:

AAS 1.4 Required Navigation Performance (RNP) and Area Navigation (RNAV)

Track prediction vectors should be available on situation displays to be used to control airspace where random routes are permitted.

is amended to read:

Trajectory prediction and conflict detection tools should be available on situation displays used to control airspace where dynamic and flexible ATS routes are permitted.

4.2  It is recommended that:

AAS 1.4 Required Navigation Performance (RNP) and Area Navigation (RNAV)

In airspace where random routes are permitted, the ATS system should be capable of processing random routing flight plans and controllers should be able to amend/update such information

is amended to read:

In airspace where dynamic and flexible ATS routes are permitted, the ATS system should be capable of processing associated flight plans.

References

Technical and Professional Manual, 2015 edition- IFATCA

DOC 4444 PANS-ATM – ICAO

Global Air Navigation Plan – ICAO

Guidance Material and Best Practices from Fixed to Flex Routes – IATA

Specification for Short Term Conflict Alert Edition 1.0 – Eurocontrol, 2007

European Free Route Airspace Developments – Eurocontrol, February 2012

European Route Network Improvement Plan, Part 1 “European Airspace Design Methodology – Guidelines”- Eurocontrol, June 2015

Off Air Routes Planning Manual – Airservices Australia

“UPR trials and establishment of ASIO UPR Zone” – WP/4 Combined ASIOACG and INSPIRE Working Group Meeting, 2013, IATA

“Free Route; una implantación premature” – Ignacio Baca, Spanish ATC Magazine, n ̊81 2014

Presentation FRAL Project – Vanda Cruz, March 2011

The effect of Free Flight on Air Traffic Controller Mental Workload, Monitoring and System Performance – National Aerospace Labatory (NLR), Amsterdam & Catholic University of America, Washington DC

With special thanks to:

Kenrick Taylor, Airservices Australia

Adrian Stefan, EGATS, Eurocontrol MUAC

Ignacio Baca, USCA, Spain

Christian Radu, Romania

Mavrák Gábor and Vulkan Farkas, HATCA, Hungary

Razvan Bucuroiu, Eurocontrol

Last Update: October 1, 2020  

January 23, 2020   134   Jean-Francois Lepage    2016    

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