What happened
On May 7, 2015, a loss of radar separation occurred within the Warsaw Flight Information Region (FIR EPWW) near the UTOLU navigational waypoint. A Boeing 737-800 was descending from SUI toward EPLL, intending to land at FL 100. Simultaneously, an AN 28 was operating at FL 110 on a route between EPPW and EPDE, positioned approximately 8NM north of UTOLU.
As the Boeing 737-800 descended, the ACC radar controller observed its descent rate and decided to transfer the aircraft to the EPLL Tower frequency. The transfer occurred near the UTOLU waypoint while the aircraft was approaching FL 110. After passing UTOLU and under Tower control, the Boeing 73 and 800 began a left turn for its approach procedure and reduced its rate of descent as it crossed FL 110. This maneuver resulted in the radar separation between the two aircraft dropping to 6.7 NM, which was below the required minimum of 7 NM.