Incident Overview
On 7 May 2015, a loss of radar separation occurred between two aircraft in the Warsaw Flight Information Region (FIR EPWW) near the UTOLU navigation point. The aircraft involved were a Boeing 737-800 (B738) and an AN-28. The B738 was descending to flight level (FL) 100 for landing at EPLL, having approached from SUI towards UTOLU. The AN-28 was en route from EPPW to EPDE at FL 110, located approximately 8 nautical miles north of UTOLU with radar separation initially maintained.
Sequence of Events
The radar controller (ACC), observing the descent rate of the B738, decided to transfer the aircraft to TWR EPLL frequency. The B738 was handed off to TWR EPLL near UTOLU while it was descending through FL 110 en route to FL 100. After passing UTOLU, the B738, now on TWR frequency, initiated a left turn in accordance with the approach procedure and reduced its descent rate as it crossed FL 110—the same level occupied by the AN-28 (still on ACC frequency and airspace). This resulted in a reduction of radar separation to 6.7 NM, below the required minimum of 7 NM, at the moment the B738 crossed FL 100.
Contributing Factors
The investigation identified that the controller was on his third duty shift after one day off. The number of open sectors was appropriate, but traffic density was at the upper limit of capacity. The controller held a valid license and medical certificate, with a refresher course completed in October 2013. Clearances had been issued for the B738 to descend to FL 110 and for the AN-28 to enter the same FL 110. The aircraft were on converging courses, and attention was not paid to the continuously decreasing distance resulting from the diminishing groundspeed difference between the two aircraft. The B738 was transferred to TWR EPLL before the aircraft had crossed levels, without checking its arrival route to the airport. No descent rate was assigned to the B738 crew.
Probable Cause
The probable cause, as stated in the source, is errors in ATM operations ("Błędy w pracy ATM").
Safety Actions
The investigating body took the following preventive actions: (1) The final report was forwarded to the LSM ACC GAT to brief the involved controller on the findings, aiming to reduce the likelihood of similar occurrences. (2) The report and collected materials were sent to the ATS Personnel Training Centre to be included in the database of investigated events used by instructors during initial and refresher training for air traffic controllers.
No safety recommendations were formulated by the commission.