What happened
On May 16, 2022, at approximately 09:00 UTC, an Airbus A321 (registration HA-LVR) and an Embraer ERJ170-200 (registration SP-LIA) were operating in the TC L sector of the Warsaw Flight Information Region (FIR EPWW). At the time, the Airbus was cruising at FL 360 and the Embraer was at FL 350, with flight paths converging at an angle of approximately 50 degrees.
Shortly before the incident, the air traffic controller (ACC) cleared the Airbus to descend to FL 230. However, the controller failed to update the new cleared flight level into the PEGASUS_21 surveillance system. This omission meant the system did not reflect the intended vertical separation change.
At 09:00:13 UTC, the PEGASUS_21 system generated a Short Term Collision Alert (STCA) for both aircraft. Four seconds later, the controller intervened to resolve the conflict, instructing the Embraer crew to descend immediately to FL 330 and ordering the Airbus crew to maintain FL 360. The Airbus crew, having previously received a descent clearance, queried the instruction, prompting the controller to use urgent phraseology to command the immediate cancellation of the descent. Despite these maneuvers, the aircraft reached a minimum separation of only 3.71 NM horizontally and 300 ft vertically.
The investigation
The investigation focused on the controller's failure to update the flight level in the surveillance system and the subsequent loss of separation. The investigation examined the controller's workload, which included managing eight aircraft and handling a sector split involving military traffic in the TS26 zone. The investigation also analyzed the performance of the PEGASUS_21 system and the effectiveness of the TCAS on both aircraft, noting that the TCAS did not issue resolution advisories during the separation breach.
Findings
- The primary cause was the controller's failure to notice the lack of separation between the two aircraft.
- The controller failed to input the new cleared flight level for the HA-LVR into the PEGASUS_21 system, which prevented the system from providing an earlier warning.
- The controller may have experienced "inattentional blindness," failing to perceive the conflict despite correct flight path visualization.
- Contributing factors included a high workload (controlling eight aircraft), a recent sector split, and the presence of military traffic in the vicinity.
- The controller was returning to duty after a break of more than two months.