A321 / B738, en-route, south eastern Bulgaria, 2016

A321 / B738, en-route, south eastern Bulgaria, 2016

Summary

On 8 September 2016, an Airbus A321 en route in Bulgarian airspace at FL 350 was given and acknowledged a descent but then climbed and came within 1.2nm of a descending Boeing 737. The Investigation found that the inexperienced A321 First Officer had been temporarily alone when the instruction was given and had insufficient understanding of how to control the aircraft. It was also found that despite an STCA activation of the collision risk, the controller, influenced by a Mode ‘S’ downlink of the correctly-set A321 cleared altitude, had then added to the risk by instructing the 737 to descend.

Description

On 8 September 2016, an Airbus A321 (TC-ATF) being operated by Atlasjet on a scheduled international passenger flight from London Gatwick to Istanbul under callsign KKK8YJ lost prescribed separation in day VMC against a Boeing 737-800 (TC-JVS) being operated by Turkish Airlines on an international passenger flight from Zurich to Istanbul under callsign THY4AV whilst both aircraft were tracking south-eastwards towards a common waypoint at FL 350 and FL 370 respectively in Class ‘C’ airspace. The two aircraft passed at FL 363 with 1.2 nm horizontal separation.

Investigation

After the classification of the event as a Serious Incident by the Aircraft Accident Investigation Unit (AAIU) of the Aircraft, Maritime and Railway Accident Investigation Unit Directorate at the Bulgarian Ministry of Transport, Information Technology and Communications, an Investigation Commission was appointed by the Minister. Recorded ATC data from Sofia ACC including Mode ‘S’ downlink data was available to inform the Investigation and the FDR data of both aircraft involved were also available.

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