Introduction
On 23 July 2019 at 08:36 UTC, a serious incident (Airprox) occurred in the controlled airspace of the Republic of Bulgaria involving two commercial aircraft: a Boeing 737-800 aircraft with registration marks OK-TVM, operated by Travel Service a. s. (call sign TVS3729), and an Airbus A321-200 aircraft with registration marks G-TCDG, operated by Thomas Cook Airlines Ltd. (call sign TCX1NB). The incident was investigated by the Aircraft, Maritime and Railway Accident Investigation Unit Directorate of the Ministry of Transport, Information Technology and Communications of the Republic of Bulgaria.
Factual Information
Flight History
TCX1NB was performing a charter flight from EGKK (London Gatwick) to LBBG (Burgas). TVS3729 was operating a charter flight from LBWN (Varna) to LFRS (Nantes). Both aircraft were cruising in Bulgarian airspace under the control of family sector VBL of the Sofia Area Control Centre (ACC).
At 08:30:55 UTC, the flight crew of TCX1NB started a descent to FL150 en route to point ESENA, after receiving an instruction from the executive air traffic controller (EXE ATCO) of sector VBL. At 08:31:43 UTC, the flight crew of TVS3729 began a climb to FL220 toward point AMTOV, also following an instruction from the same EXE ATCO.
Conflict Detection and Warnings
At 08:31:02 UTC, the Medium Term Conflict Detection (MTCD) function of the automated air traffic control system SATCAS detected a potential conflict between the trajectories of TCX1NB and TVS3729. This was automatically displayed as a red dot in the line zero of the track labels of both aircraft on the controller working position of sector VBL.
At 08:35:50 UTC, SATCAS generated a visual indication in bright red color accompanied by an audible alarm indicating a Short-Term Conflict Alert (STCA). At that time, TCX1NB was crossing flight level 209 descending, while TVS3729 was crossing flight level 206 climbing.
The EXE ATCO issued a course of delayed instructions to move the aircraft away from each other.
Aircraft Systems
On board TCX1NB, the traffic alert and collision avoidance system (TCAS) activated. The specific resolution advisory (RA) issued by the TCAS was not detailed in the report.
Injuries and Damage
The report states that no injuries to persons were recorded, and there was no damage to either aircraft as a result of the incident.
Conclusion
The investigation determined that the serious incident was classified as an airprox. The probable cause was not explicitly stated in the source material; therefore, none is provided here.