What happened
On September 21, 2009, a significant loss of separation occurred in the TRA 65 airspace within the Prague FIR. A military L-159 A was conducting a training flight between FL 200 and FL 280. Simultaneously, a Travel Service Boeing 737-80 0, registration OK-TVK, was descending through the area.
While the air traffic controller (NSEL EC) had previously managed a conflict between other aircraft, the controller failed to terminate radar vectoring for the Boeing 737-800 after it had been cleared to follow a specific heading. This resulted in the commercial aircraft inadvertently entering the restricted TRA 65 airspace.
As the aircraft converged, the pilot of the L-159 A received an instruction from the MACC controller to descend immediately to FL 200. Following this instruction, the military pilot performed a steep left-hand spiral descent from approximately 21,700 feet to 14,900 feet at a vertical rate of 6,000 feet per minute. During this maneuver, the pilot did not visually identify the conflicting traffic. The Boeing 737-800 crew, utilizing TCAS II, identified the potential conflict and visually monitored the military aircraft. The closest point of approach occurred at 08:23:11 UTC, with a horizontal separation of 4.83 NM and a vertical separation of 800 feet.
The investigation
The ÚZPLN investigation examined radar data, flight recorder information from both aircraft (AMOS and DFDAU), and air traffic control recordings. The investigation focused on the actions of the NSEL EC and MACC controllers, as well as the flight paths of both aircraft. The analysis utilized high-precision multilateration radar data to accurately reconstruct the flight trajectories, as standard radar processing can sometimes smooth out the extreme vertical maneuvers characteristic of military aircraft.
Findings
- The primary cause of the separation infringement was the failure of the NSEL EC controller to terminate radar vectoring for the Boe 737-800, which allowed the aircraft to enter the TRA 65 airspace.
- The NSEL EC controller failed to respond to both the DAIW (warning of potential entry into restricted airspace) and the STCA (Short Term Conflict Alert) warnings.
- The NSEL EC controller did not coordinate the situation with the MACC controller.
- The MACC controller, while recognizing the danger and issuing an immediate descent instruction to the L-159 A, failed to coordinate the situation with ACC Praha.
- The pilot of the L-159 A was heavily focused on executing the flight training program and did not visually observe the conflicting traffic during the high-rate descent.
- The air traffic controller was managing a high workload (traffic density coefficient KH 3) in a complex sector involving both civil and military coordination.