What happened
On January 25, 2006, a conflict occurred between two Airbus aircraft operating in the Prague FIR. The first aircraft, an Airbus A320 with registration F-GFKQ (call sign AFR 1983), was climbing through FL 215 toward FL 230. Simultaneously, an Airbus A321 (call sign DLH 1072) was descending from FL 240.
The incident was triggered by a series of communication errors. An air traffic controller at the Prague Approach (APP EC) issued an instruction to a different aircraft, an ATR72 (call sign DLH 3272), to descend to FL 120. The crew of DLH 1072, hearing the instruction and noting the similar call sign, mistakenly responded to the instruction and initiated a descent.
As the DLH 1072 descended through FL 233, the crew received a TCAS RA (Resolution Advisory) command to climb. This maneuver was necessary to avoid a collision with the climbing AFR 1983. The conflict was eventually resolved after the controller issued a corrected instruction to maintain FL 220, and the aircraft returned to their assigned flight levels.
The investigation
The ÚZPLN investigation examined radar data and radio communications between the crews and the Prague Area Control Center (ACC WL EC) and Prague Approach (APP EC). The investigation focused on the sequence of radio transmissions and the controllers' monitoring of the traffic.
Investigators found that the APP EC controller had a vertical filter set on the radar display from Ground to FL 150. Because DLH 1072 was flying above this altitude, the controller did not see the aircraft on the primary radar display during the initial communication. Furthermore, the investigation noted that the controller failed to notice that the crew of DLH 1072 had incorrectly acknowledged an instruction intended for DLH 3272.
Findings
- The crew of DLH 1072 repeatedly responded to instructions intended for another aircraft with a similar call sign.
- The APP EC controller failed to pay sufficient attention to the content of air-ground communications.
- The controller did not adequately evaluate the radio correspondence with DLH 1072 given the traffic situation shown on the radar.
- The controller responded to a call sign that was nearly identical to another active flight, contributing to the confusion.
- The vertical radar filter prevented the controller from seeing the DLH 1072 aircraft on the CWS display during the critical phase of the error.