The design of DANUBE FAB airspace is based on the objective of maximising the efficient use of airspace while ensuring the required level of safety for air traffic operations. According to the DANUBE FAB State Agreement, the airspace is organised without regard to national borders and is based on common criteria for lateral and vertical delineation. The airspace under DANUBE FAB consists of the Bucharest and Sofia Flight Information Regions, including portions of the high seas where Romania and Bulgaria provide air navigation services as notified in their national Aeronautical Information Publications (AIPs) and in accordance with ICAO regional agreements.
Free Route Airspace was first implemented individually by BULATSA and ROMATSA during night-time operations in November 2013. On 30 March 2017, DANUBE FAB ANSPs, together with HungaroControl, introduced significant changes to the structure of their respective airspaces through the South Eastern European Night Free Route Airspace (SEEN FRA) initiative.
In preparation for the transition to 24-hour Free Route Airspace operations, ROMATSA and BULATSA commissioned EUROCONTROL to conduct a full-scale real-time simulation (RTS) between 27 October and 12 November 2015 at the EUROCONTROL Experimental Centre in Brétigny-sur-Orge, France. The RTS simulated FRA operations across the entire DANUBE FAB airspace, with a total of 18 sectors each operated by an executive and a planning controller. The exercise demonstrated that 24-hour FRA could be introduced beyond the current night-time implementation if controllers are supported with appropriate advanced tools.
The progressive implementation of FRA continued beyond the initial phase. In August 2015, cross-border night-time FRA operations were introduced between Romania and Hungary. In March 2017, night-time FRA operations were extended across Romania, Bulgaria and Hungary. Slovakia joined in December 2018. In November 2019, Romania, Bulgaria, and Hungary transitioned to 24-hour FRA, with Slovakia continuing night-time operations. Slovakia implemented 24-hour FRA in January 2021. In February 2022, the Republic of Moldova joined 24-hour FRA operations and an interface with Baltic FRA was established via Slovakia. The Czech Republic joined in February 2023. FRA is now operational 24 hours a day across Romania, Bulgaria, Hungary, Slovakia, the Republic of Moldova, and the Czech Republic, forming the South-East Europe Free Route Airspace (SEE FRA).
The airspace configuration within DANUBE FAB has evolved substantially to support the implementation of Free Route Airspace (FRA) and improve the efficiency and flexibility of airspace management. A key milestone was reached in July 2021, when Romania eliminated the ATS route network above Flight Level 105 (FL105), allowing the extension of 24-hour FRA operations to lower levels across the entire București FIR.
This change enabled aircraft operators to plan and fly optimised trajectories using direct segments, without reference to fixed route structures. It marked the transition to a fully performance-based environment, increasing route efficiency, reducing fuel consumption, and contributing to the overall environmental objectives of the FAB.
FRA is now applied H24 above FL105 across the entire DANUBE FAB airspace, including both the București and Sofia FIRs. The airspace structure is configured through multiple sectorisation scenarios, tailored to traffic flows, seasonal variations, and operational demand. These scenarios enable flexible management of capacity while ensuring a high level of safety.
The concept of flexible use of airspace plays an essential role in maintaining a balance between civil and military needs. Sector configurations are developed in close coordination with military stakeholders, allowing for dynamic allocation of airspace volumes while safeguarding mission requirements.
Environmental considerations are also integrated into the airspace design process. The application of FRA across a wider vertical range supports more efficient climb, cruise, and descent profiles, resulting in measurable reductions in emissions and improved flight efficiency.
Through these configurations, DANUBE FAB ensures that its airspace structure remains responsive, interoperable, and aligned with European strategic goals for the modernisation of air traffic management.
DANUBE FAB is the first Functional Airspace Block in Europe to implement cross-border airspace sectors under a fully operational framework. Since December 2014, two cross-border sectors (CBS) have been established based on operational needs, optimising the use of airspace regardless of national borders. These sectors allow air traffic services to be provided by the ANSP of one state in the sovereign airspace of the other, above Flight Level 245.
Sector DF 1 is managed by BULATSA and extends into Romanian airspace, while Sector DF 2 is managed by ROMATSA and extends into Bulgarian airspace. The establishment of these sectors is based on the Joint Designation Act signed by the Ministers of Transport of both states, in accordance with Article 20 of the DANUBE FAB State Agreement. Air traffic, communication, and surveillance services are provided by the designated ANSP, while navigation, aeronautical information, and meteorological services remain the responsibility of the host state.
This arrangement required the resolution of complex legal, financial, and operational challenges. A dedicated agreement between the ANSPs and a specific supervision framework coordinated by the National Supervisory Authorities were put in place to address liability, cost allocation, insurance, and civil-military coordination. Each ANSP remains liable for the services it provides, under the laws of the state where the service is delivered.
Civil-military coordination continues to be performed within national boundaries, and joint procedures have been developed between the national airspace management cells. For search and rescue operations, the host state retains full responsibility.
The implementation of cross-border sectors in DANUBE FAB has brought significant operational benefits, including improved conflict resolution, reduced controller workload, optimised descent profiles, and more efficient routing. These innovations contribute to enhanced safety, increased capacity, and lower environmental impact.
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