What happened
On September 28, 2024, a loss of lateral separation occurred within the Warsaw FIR, specifically in the TC Low area control sector (ACC). The incident involved two aircraft: a Boeing 737-800, registration OM-HEX, and an Airbus 321, registration OY-TCE.
At the time of the event, the Boeing 737-800 was cruising at FL350, navigating toward the REVMA waypoint. The Airbus 321 was operating at FL360, navigating toward point OLBUV with intentions to land at EPPO. Before the two aircraft passed one another, the air traffic controller issued a clearance to the Airbus 321 to descend from FL360 to FL310. This instruction resulted in the horizontal distance between the two aircraft decreasing to 4.5 NM, with a vertical separation of only 700 feet. This loss of separation lasted for approximately 5 seconds.
The investigation
The investigation examined the sequence of events and the instructions issued by the area control center. The investigation concluded that the findings provided by the investigating entity were sufficient to close the case.
Findings
- The direct cause of the incident was the issuance of a conflicting descent clearance to the Airbus 321 by the air traffic controller.
- An error in the controller's personnel preventive measures contributed to the occurrence.