What happened
On September 5, 2007, at 04:11 UTC, a loss of separation occurred between an ATR 72, registration HB-AFJ, and an Airbus A320, registration HA-LPH, at the boundary of the Prague and Warsaw Flight Information Regions (FIRs).
The incident began when the crew of the ATR 72 reported their position to Prague ACC, but controllers could not establish radar contact. The aircraft's SSR transponder was malfunctioning; specifically, it was only responding to certain radar sensors, making it invisible to others. This technical failure meant that Warsaw ACC (EPWW) lacked radar information regarding the ATR 72's position as it entered their airspace.
Simultaneously, the Airbus A320 was climbing through FL 200. Due to a combination of uncoordinated flight plan changes from Warsaw ACC and a complex sector handover within Prague ACC, the two aircraft converged. At 04:13:18, the separation between the aircraft dropped below 5 NM with a vertical separation of only 100 ft. The situation was further complicated by radio frequency interference (coupling) and a lack of clear communication during the transition of air traffic services between Prague sectors.
The investigation
The ÚZPLN investigation focused on the technical performance of the ATR 72's transponder and the coordination procedures within Prague ACC. The investigation established that the ATR 72's transponder was intermittently failing due to a faulty weight-on-wheels (WOW) sensor.
Investigators also examined the handover process between Prague ACC sectors LM and L. They found that a significant sectorization change was underway, which lasted 11 minutes—longer than usual. During this period, the controllers lacked sufficient information to effectively manage the conflict. The investigation also reviewed the coordination between Prague and Warsaw ACCs, noting that Warsaw had issued conflicting instructions for the Airbus A320 without proper coordination with Prague.
Findings
- The primary cause was the malfunctioning SSR transponder on the ATR 72, which prevented accurate radar identification and contributed to an inadequate assessment of the traffic situation by Warsaw ACC.
- Warsaw ACC issued a climbing clearance for the Airbus A320 that conflicted with the existing flight plan for the ATR 72 without coordinating the change with Prague ACC.
- A controller in Prague sector LM made an incorrect assumption that separation was being maintained by Warsaw ACC and failed to verify this with other controllers.
- The handover of air traffic services between Prague sectors LM and L was insufficient, leaving the incoming controller with inadequate information to manage the conflict.
- The simultaneous change in sectorization and the rotation of personnel increased the risk of error during the critical period.