What happened
On June 4, 2016, at 15:19 UTC, a serious loss of separation occurred within the controlled airspace of the Republic of Bulgaria, specifically in the Sofia-West sector. The incident involved two commercial aircraft: an Airbus A320-232, registration TC-ATK, and a Boeing 737-8F2, registration TC-JGF.
At the time of the event, the Boeing 737-8F2 was climbing through flight level 340 toward flight level 380. Simultaneously, the Airbus A320-232 was transiting the same airspace at flight level 370. As the aircraft trajectories converged, the Airborne Collision Avoidance Systems (ACAS/TCAS) on both aircraft activated. The crew of the Boeing 73 and 8F2 received a "Climb" Resolution Advisory (RA), while the crew of the Airbus A320-232 received a "Level Off" RA. Despite the automated warnings, the aircraft passed each other with a minimum horizontal distance of 2.6 NM and a vertical separation of only 500 feet.
The investigation
The investigation examined radar data, Mode S RA downlink messages, and airborne recordings to reconstruct the flight paths. Investigators analyzed the coordination between air traffic control (ATC) sectors and the specific responses of the flight crews to the automated advisories. The study included a comparison of flight data recorder (FDR) altitudes and vertical rates against the timing of the RA downlink messages to determine the effectiveness of the collision avoidance maneuvers.
Findings
- The investigation established that the incorrect assessment of the air situation by the air traffic controller was a primary factor, as permission to change the trajectory of TC-JGF was issued without the necessary coordination between the SCL and SCU sectors.
- The crew of the Airbus A320-232 (TC-ATK) failed to respond appropriately to the "Level Off" RA, continuing to climb at a rate of approximately 1000 ft/min despite the instruction to stabilize altitude.
- The crew of the Boeing 737-8F2 (TC-JGF) responded to the "Climb" RA by increasing their rate of climb to meet the required vertical speed.
- Coordination procedures between the controllers of the vertically separated sectors were performed inefficiently, contributing to the loss of separation.