Serious incident Airbus A321-211 (WL) G-TCDG,
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ASN Wikibase Occurrence # 268175
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Date:Tuesday 23 July 2019
Time:11:36 LT
Type:Silhouette image of generic A321 model; specific model in this crash may look slightly different    
Airbus A321-211 (WL)
Owner/operator:Thomas Cook Airlines
Registration: G-TCDG
MSN: 6122
Year of manufacture:2014
Engine model:CFMI CFM56-5B3/P
Fatalities:Fatalities: 0 / Occupants:
Aircraft damage: None
Category:Serious incident
Location:53 nm E of Varna -   Bulgaria
Phase: En route
Nature:Passenger - Scheduled
Departure airport:London-Gatwick Airport (LGW/EGKK)
Destination airport:Bourgas Airport (BOJ/LBBG)
Investigating agency: AAIU Bulgaria
Confidence Rating: Accident investigation report completed and information captured
Thomas Cook Airlines flight MT1052 (call sign TCX1NB), an Airbus A321-200 registered G-TCDG, and Travel Service flight TVS3729, a Boeing 737-800 registered OK-TVM, were involved in a serious airprox incident in Bulgarian airspace about 53 nm east of Varna.
Both flights were en route in the Bulgarian airspace under the control of sector VBL of the Sofia Area Control Centre (ACC).
At 08:30:55 h UTC (11:30 local time) the flight crew of TCX1NB started a descent to FL150 to point ESENA after an instruction issued by the Radar Air Traffic Controller (EXE ATCO) from the VBL sector. At 08:31:43 h UTC the flight crew of TVS3729 started a climb to FL 220 to point AMTOV after an instruction issued by an EXE ATCO from the same sector VBL.
At 08:31:02, the MTCD (Medium Term Conflict Detection) of the Automated System SATCAS detected a potential conflict of the trajectories of TCX1NB and TVS3729, which was automatically displayed as red dot on the track label of the two aircraft on the Controller Working Position of sector VBL.
At 08:34:30, the EXE ATCO issued instructions to TCX1NB to stop their climb at FL200 at a time while the flight was in fact descending: “TCX1NB stop climb at level 200 due to traffic”. TCX1NB replied correctly: “Stop descend FL200, TCX1NB". To avoid a potential conflict, the controller then instructed TVS3729 to “stop climb at FL190”. With no response, the controller called again: "TVS3729 stop climb and return to FL190".
At 08:35:20, the flight crew of TVS3729 replied: “Are you cleared us before to FL220 so we stop the climb now to 190?”.
The controller then instructed the flight crew of TCX1NB to "stop descend immediately, maintain FL210".
Flight TVS3729 then again asked form clarification of their clearance: "Sofia, TVS3729, do you confirm we descend now to FL190".
The controller confirmed the instruction "TVS3729 Stop climb immediately and descend back to FL200". TVS3729 replied: "We descend FL200 and you’ve cleared us to FL220"
At 08:35:47 the flight crew of TCX1NB received a Climb Resolution Advisory from their traffic alert and collision avoidance. The flight crew stopped the descent and started to increase its vertical speed to the required 1500ft/min.
At the same time SATCAS generated a visual indication tot he controller in bright red colour with sound alarm for STCA between TCX1NB at FL209 and TVS3729 crossing FL206 in a climb. The distance between the two aircraft was 7,9 nm.
At 08:35:57 TCX1NB was issued a subsequent resolution advisory (Level Off). This was immediately followed by an instruction from the controller to turn 20 degrees to the left.
At 08:36:37 the flight crew of TCX1NB received a 'Clear of Conflict' message.
Both aircraft continued to their destination without further incident.

At 08:36:02 UTC, the standard for minimum radar separation was infringed and the closest point of approach between the two aircraft was reached 2,87 NM horizontally and 800 ft vertically.

Immediate causes of the serious incident:
- Ineffective interaction in the EXE/PLN team [PLN=Planning air traffic controller];
- EXE and PLN air traffic controller did not detect the conflict in VBL sector;
- Lack of a preliminary plan for resolving potential conflicts;
- EXE and PLN air traffic controller did not react in accordance with the procedures for detecting and resolving the conflict;
- Lack of expediency of EXE and PLN air traffic controller in providing Air Traffic Services.
The main cause of the serious incident was that the EXE ATCO issued a conflict clearance for descend of the aircraft, which led to a hazardous convergence with another aircraft performing a climb after issued clearance.
The following was identified as a contributing factor to the serious incident:
The high load of the EXE and PLN air traffic controller due to switching from backup to main AATMS and change in the sector configuration

Accident investigation:
Investigating agency: AAIU Bulgaria
Report number: 
Status: Investigation completed
Duration: 2 years and 2 months
Download report: Final report


AAIU Bulgaria

Revision history:

29-Sep-2021 09:06 harro Added
29-Sep-2021 09:20 harro Updated [Narrative]
29-Sep-2021 09:24 harro Updated [Narrative]

Corrections or additions? ... Edit this accident description

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