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Mobility and Transport

DANUBE FAB Cross-Border Sectors

Sofia, BulgariaBULATSA and ROMATSA

Airspace is usually arranged according to national boundaries, with one Air Navigation Service Provider (ANSP) operating exclusively within each State’s territory. These politically defined borders are geographically complex and do not take into account air traffic flows, potentially causing inefficiencies in flight routes. Cross-border Sectors (CBS) offer a solution to this problem, by redrawing airspace boundaries based on operational needs of the air traffic, rather than following the contours of a national border.

Following the establishment of DANUBE Functional Airspace Block (DANUBE FAB) the two partner States of Romania and Bulgaria implemented two cross-border sectors on 11th December 2014, the first time that cross-border sectors have been implemented under a FAB framework. In the two cross-border sectors, operational 24h a day, the States have delegated the provision of air navigation services to the national ANSP of the partner country – demonstrating the level of trust and cooperation between the two FAB partners.

The national ANSPs BULATSA and ROMATSA have therefore shared the provision of each of the air navigation services between them within the cross-border sectors. Above FL 245, air traffic services, communication services and surveillance services are provided by the cross-border ANSP, whilst navigation, meteorological and aeronautical information services continue to be provided by the territorial ANSP.

The first CBS (Sector DF 1), controlled by BULATSA over Romanian territory allows safe and efficient circumnavigation of a new military area close to the border and optimises descent procedures into two Bulgarian airports (Burgas and Varna). The second CBS (Sector DF 2), controlled by ROMATSA over Bulgarian territory, reduces transfer of control workload between ACCs, and increases controller’s ability to tactically control aircraft surrounding restricted areas. Both CBSs create more direct routes which have driven real savings in terms of distance flown.

The initiative has involved all DANUBE FAB stakeholders including the ANSPs, Militaries, NSAs and State Authorities on Transport. Through the experiences gained in establishing cross-border sectors, DANUBE FAB has developed a model approach to the aspects which need to be considered including: ensuring a sound legal basis for Air Navigation Services; reaching agreement on liability issues; determining a financial framework; and ensuring civil/military harmonisation.

All of these details, along with the benefits which the initiatives bring are expanded in detail against the award criteria in the following section.

Award criteria

  • Contribution to increasing capacity of ATM systems
  • Contribution to increasing safety of ATM systems
  • Contribution to reducing the impact of air transport on the environment
  • Early implementation
  • Potential to be replicated over the ATM network



These first cross-border sectors (CBS) implemented under a FAB framework are configured based on operational needs, optimising the use of airspace within DANUBE FAB irrespective of national boundaries, representing a further step towards the defragmentation of European airspace, demonstrating DANUBE FAB's commitment to the Single European Sky, particularly in relation to the airspace design principles, required in Commission Regulation (EU) 677/2011.

The intent to optimise the airspace boundaries of the States has been a long term goal of DANUBE FAB, which is formalised in the State Agreement. The CBS optimise conflict resolution, reduce the number of manual transfer procedures between ACCs and improve management of restricted areas, enhancing capacity and safety. The sectors improve routes based on traffic flows and optimise descent profiles, bringing savings for airspace users in terms of distance flown and decreasing environmental impact.

This pioneering project transcends the national boundaries of Romania and Bulgaria, improving performance, capacity and safety, whilst reducing environmental impact, contributing to all four high level SES goals at no additional cost to either ANSP.

Lessons can be learned from the major challenges which were overcome such as providing a legal basis for cross-border provision, resolving financial, liability and insurance issues, as well as harmonising operational, civil/military, AIS, MET and emergency procedures. Resolution of these hurdles demonstrates a solid partnership between the FAB partners and effective utilisation of the FAB’s structures for change management.

As well as providing measurable benefits for airspace users, the project serves as a case study for how cross-border Provision can be implemented, and DANUBE FAB’s methodology can be replicated elsewhere in Europe. Through the SES awards, we hope to increase visibility of our experiences and utilise the platform as an opportunity to encourage and support others in implementing similar cross-border projects.

We believe this initiative is a "best in class" example of optimising airspace and harmonising service provision, and present details of compliance with each of the criteria for the SES award in the below sections.

Contribution to reducing the impact of air transport on the environment

The introduction of cross-border Sectors allows optimised routes based on traffic flows rather than national boundaries, and enables optimised descent profiles:

  • Optimised Descent (Sector DF 1): Flights are obliged to level off at the boundary between FIRs, which before CBS implementation interfered with approach procedures into Bulgarian airports. Moving the boundary northwards allows arriving flights to optimise their descent profile to both Burgas and Varna airports.
  • Freedom to optimise routes based on traffic flows: Removing the constraints of the national borders, despite the increased complexity of circumnavigating the prohibited area north-west of Sector DF 1, there is an overall decrease in route length, and flights through Sector DF 2 can travel in a straight line rather than indirectly through waypoints on the national border.

Modelling the total annual traffic over the cross-border sectors, the above operational improvements provide the following benefits over an annual period (see footnote for methodology):

  • For Sector DF 1, the benefits equate to a reduction in distance flown of 16,900 NM , with subsequent reductions, every day, of approximately 104,000 kg of fuel, 330,000 kg of CO2 emission, 1,380 kg of NOx emissions and 2,280 minutes of flight time.
  • For Sector DF 2, these benefits equate to a reduction in distance flown of 29,200 NM, with subsequent reductions, every day, of approximately 417,000 kg of fuel, 1.32 million kg of CO2 emissions, 5,270 kg of NOx emissions and 3,730 minutes of flight time.

Contribution to increasing capacity and safety of the ATM system

The cross-border sectors optimise conflict resolution, reduce the number of manual transfer procedures and allow additional airspace to manage restricted areas which lowers controller workload, therefore enhancing capacity and improving safety.

  • Conflict resolution: The introduction of a new military prohibited area (DEVESELU), just north of the Romanian/Bulgarian border intersected major traffic flows. The subsequently revised route structure, designed to circumnavigate the prohibited area introduced a conflict point just as traffic enters north into BUCURESTI FIR. The new cross-border Sector DF 1 moves the SOFIA ACC boundary northwards, allowing this conflict point in Romania to be foreseen and handled within a single sector by BULATSA. This increases controller ability to manage hazardous conflict situations, increasing safety, and reduces coordination between the two ACCs to communicate potential conflicts, reducing workload and therefore increasing individual controller capacity.
  • Reduction in manual procedures: Westbound traffic crossing the border had to re-enter Bulgarian airspace multiple times due to the complex contours of the national border. The new cross-border Sector DF 2 moves the sector boundary southwards, removing the need for multiple transfers between ROMATSA and BULATSA, and eliminating the manual input to the ATM System which was required to enable the system to cope with multiple crossings. This reduces potential confusion from erroneous messages and enhances safety, whilst reducing controller workload.
  • Additional space to manage restricted areas in: Military areas within BUCURESTI FIR are very close to the national border, and tactical intervention is often needed to coordinate traffic with the Romanian military. Sector DF 2 moves the previous sector boundary southwards allowing additional space and time for ROMATSA to tactically coordinate traffic around military areas, reducing workload and increasing safety.

Early Implementation "First Mover" and Innovative aspects

DANUBE FAB is the first to implement cross-border sectors under a FAB framework. Many airspace reorganisation projects can be achieved through cooperation between ANSPs, whereas the CBS project requires Air Traffic Services to be provided in non-sovereign airspace, requiring extensive cooperation and trust between all FAB layers including State, military, ANSPs and NSAs.

There were many hurdles to implementation, for which there was no "best practice" mechanism, guidance or European standards to refer to. This meant innovative approaches were needed to reach the necessary resolutions, noted below in relation to each challenge.

State level

Legal basis for cross-border provision:

  • The DANUBE FAB Governing Council issued a decision agreeing that ATS (including alerting services), Communication and Surveillance will be provided in the CBS airspace not in the state’s sovereignty, whilst the provision of the remaining ANS would remain the responsibility of the sovereign state.
  • Based on the Governing Council’s decision, the two ANSPs signed a written agreement on the establishment of the cross-border sectors, enabling the signature of the "Joint Designation Act" by the Ministers of Transport from each state.

Financial Issues: The financial framework for the cross-border sectors was a key consideration since the parties identified considerable difficulties in implementing a system for the reimbursement of costs incurred by an ANSP in the cross-border sector from another charging zone. A pragmatic and simple solution was agreed by all parties.

  • Both states agreed to change the charging zones along the CBS boundaries.
  • Both States accepted that the en-route charging zones cannot be changed during a reference period and fixed them for a period of 5 years over RP2, to be re-visited for RP3.
  • Both States accepted that charging zones cannot be segregated using vertical limits. Despite the cross-border sectors being above FL 245, the ANSPs accepted this shortcoming and changed the whole charging zone from the ground upwards.

Liability: No responsibility or liability guidelines exist at either European or global level.

  • DANUBE FAB decided that each service provider is responsible and liable for the services it provides in the cross-border sectors, but under the national laws of the territorial state.


Civil-Military Coordination: Whereas ATS in the cross-border sectors is the responsibility of the neighbouring state, civil-military coordination is a sovereign responsibility.

  • It was agreed to retain civil-military coordination within the national boundary of each state, and the neighbouring ACC in control of the CBS is duly informed of actions to be taken in response to military requirements.
  • An accompanying letter of agreement on coordination procedures between national Airspace Management Cells was jointly established from both civil and military perspectives, which specifies the relevant processes required for pre-tactical airspace management.


Insurance: There are no clear guidelines for ANSP insurance arrangements in non-sovereign airspace.

  • The DANUBE FAB ANSPs analysed the impact of CBS operations on their insurance arrangements, and have taken the necessary steps to ensure that their insurance policies provide adequate coverage for service provision within CBS and were considered when issuing of the "Joint Designation Act".

AIS: Consistency of Aeronautical Information provided to airspace users in cross-border Sectors.

  • Mechanisms were developed to ensure consistency across data from each ANSP’s Aeronautical Information. Whilst responsibility for NOTAM provision remains with the host state, procedures are in place to re-distribute NOTAMs whose subject is of interest to CBS operations to the ATS unit providing the service in the area concerned.

Technical: Utilisation of CNS equipment outside of national boundaries.

  • Communication and Surveillance services are provided cross-border as per the Joint Designation Act, in order not to create any additional investments, whereas navigation remains the responsibility of the host State.
  • Service level agreements for the use of CNS means were designed and implemented to ensure that data consistency and integrity is shared effectively.

Search and Rescue: There are no clear guidelines which define lines of responsibility for emergencies in the CBS.

  • A Decision was taken that Search and Rescue services be provided by the host state, according to its national legislation, and ACCs developed procedures in place to notify each other in case of emergency situations.

Promoting partnership and Contribution to Change Management

It is clear that resolving all of the above issues required a solid partnership between Romania and Bulgaria within DANUBE FAB, and there was a willingness and pragmatism in making the project a success. The change was managed through a robust governance and consultation process to ensure comprehensive communication between FAB stakeholders and effective decision making.

DANUBE FAB has the right structures in place to support, oversee and ensure delivery of complex projects such as cross-border sectors. The many organisations, committees and task forces involved in the project are tied together by a State, National Supervisory Authority (NSA) and ANSP Agreements, and Rules of Procedure which set out clearly documented structures and responsibilities, and are managed effectively to ensure the rules on paper translate into effective working bodies.

The DANUBE FAB has the mechanisms to allow decisions to be made at the highest political level based on the proposals of working level committees utilising clear communication channels, as well as similarly effective horizontal communication channels between the Ministries of Transport, Ministries of Defence, NSAs and ANSPs. This enables consensuses to be reached between FAB stakeholders, decisions to be proposed and escalated upwards to the Airspace Policy Body and Governing Council, and work delegated downwards effectively in short timescales.

This close working relationship allowed all of the innovative changes in the previous section to be proposed, discussed, approved and implemented quickly and efficiently, from the decision to go ahead with implementation from the Governing Council in February 2013, through to full operations in December 2014.

Consultation is a vital component in change management, to ensure staff involvement in the FAB, and manage any reluctance which may arise, particularly for aspects such as cross-border services. Through the regular Social Consultation Forums, which take place on a 6 monthly basis, social partners and professional organisations were briefed on the plans for cross-border Sectors, with all suggestions and concerns taken on board to help optimise the operational design of the sectors and improve the transition arrangements, such as through enhanced training and certification.

Consultation with Airspace Users was also important to ensure the end users were consulted about the proposed benefits. Dialogues were held alongside the consultation process for the DANUBE FAB RP2 Performance plan, and the initiative was well received by Airspace Users.

Potential to be replicated over the ATM Network

DANUBE FAB's cross-border sectors provide an excellent demonstration of the sort of benefits that FABs could and should deliver: namely optimised use of airspace regardless of national boundaries.

Crucially this case study provides a model for how this can be achieved: resolving key issues which surround cross-border initiatives: legal, financial, liability, responsibility and insurance challenges, sharing and harmonisation of services such as AIS, MET, Search and Rescue and technical systems. Aligning operational methods and showing the change is safe to multiple NSAs, and ultimately enabling the change through political will and cooperation, effective governance and open consultation.

DANUBE FAB has learned many lessons from implementing cross-border Sectors, which are equally relevant to other airspace regions throughout in Europe. DANUBE FAB would be pleased to use the SES Awards as a platform to share experiences, and to promote, encourage and advise on other cross-border initiatives.

Footnote: Derivation of statistical data for benefits

  1. Traffic data source: Demand Data Repository DDR2 / Filtered traffic - developed by EUROCONTROL/ Network Manager.
  2. Type of traffic: initial (m1) traffic (according to flight plan) during the year 2015 via LUGEB (Sector DF 2) and ORTIP, RASUB and APROB (Sector DF 1).
  3. Assessment tool: SAAM (System for Air Traffic Assignment and Analysis at Macroscopic Level, ed.4.9.3).
  4. Methodology: the assignment of 2015 traffic via LUGEB (Sector DF 2), respectively via ORTIP, RASUB and APROB (Sector DF 1) points has been successively performed for the networks VST1412 (VST - very short term) and VST1413, in force before and immediately after CBS sectors implementation. The resulted traffic samples have been intersected with the sectors volumes corresponding to Sector DF 1 and, respectively, Sector DF 2. The traffic assignment on both VST1412 and VST1413 has been made according to the shortest route principle. The scenario economy estimations have been obtained using the dedicated SAAM functionality.

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