What happened
On September 7, 2024, at 16:32 UTC, a serious air traffic incident involving a TCAS Resolution Advisory (RA) occurred in the Skopje Middle Sector airspace. The two aircraft involved were an Airbus A321-271NX (flight THY7NL) and a Boeing 737-8 (flight SXS2Y), both operating at flight level FL360.
Flight THY7NL, traveling from Madrid to Istanbul, requested and received permission to enter North Macedonian airspace near Debar to avoid adverse weather conditions. At approximately 16:30 UTC, the pilot of THY7NL reported having cleared the weather and requested a direct routing to waypoint DIGTI. Following a right-hand turn, the air traffic controller (ATC) observed that the aircraft's new course placed it on a collision course with SXS2Y, which was transiting from Dojran toward Skopje.
At 16:32 UTC, the Short Term Conflict Alert (STCA) activated. The controller initially instructed THY7NL to maintain its current heading, but this failed to establish separation. A subsequent instruction to descend to FL350 was issued, but seconds later, both crews reported a TCAS RA. The aircraft experienced reduced horizontal and vertical separation near LWSK before maneuvering to avoid collision. There were no fatalities or injuries and no damage to the aircraft.
The investigation
The investigation examined radar data, radio communications from the Skopje Middle Sector, and flight trajectories. The board analyzed the speed and geometry of the turn performed by THY7NL toward waypoint DIGTI. Investigators also reviewed the controller's workload, the availability of waypoint information on the controller working position (CWP), and the meteorological conditions present during the event.
Findings
- The direct routing to waypoint DIGTI was approved despite a loss of situational awareness regarding the aircraft's separation.
- The turn performed by THY7NL toward DIGTI was relatively rapid, with a horizontal speed of 451 knots. This rapid change in course contributed to the sudden conflict.
- The controller likely had an inaccurate expectation of the waypoint's location, potentially believing DIGTI was further south in Greece, as the point was not entered into the Alenia terminal database on the controller's workstation.
- The direct cause was the approval of a direct flight to waypoint DIGTI under conditions of reduced situational awareness due to an incorrect expectation of safe horizontal separation.
- Contributing factors included a lack of concentration during the final hour of the shift, high workload, and a failure to adhere to altitude regulations regarding flight direction.