Introduction
On 4 June 2016 at 15:19 UTC, a serious incident occurred in the controlled airspace of the Republic of Bulgaria, sector Sofia-West. The incident involved an Airbus A320-232 aircraft, registration marks TC-ATK, operated by Atlasjet as flight KKK5LP, and a Boeing 737-8F2 aircraft, registration marks TC-JGF, operated by Turkish Airlines as flight THY7FX. The Aircraft Accident Investigation Unit (AMRAIU) classified the occurrence as a serious incident.
Factual Information
The two aircraft passed each other with a minimum horizontal distance of 2.6 nautical miles and a vertical separation of 500 feet. The automated collision avoidance systems (ACAS) on board both aircraft reacted, and both pilots reported a TCAS Resolution Advisory (RA). The incident took place in the upper airspace of Bulgaria, within sector Sofia-West, during daytime hours.
Findings and Causes
The investigation commission concluded that the serious incident resulted from an incorrect assessment of the air situation and issuing of permission for changing the trajectory of aircraft THY7FX without carrying out relevant coordination by the air traffic controller of sector SCL with the controller of sector SCU. Additionally, an uncommanded climb of aircraft KKK5LP and no response to the TCAS RA by the flight crew were noted as contributing factors.
Conclusion
The incident involved a violation of the standard safe minimum separation between aircraft transiting in the upper airspace of Bulgaria. No further details on injuries or damage were reported in the investigation source.