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 through FL 233.
The incident was triggered by a series of communication errors on the Prague Approach (APP EC) frequency. An air traffic controller (ATCO) issued a frequency change instruction intended for an ATR72 (call sign DLH 3272). Due to the similarity in call signs, the crew of DLH 1072 mistakenly responded to this instruction. Subsequently, the APP EC controller issued a descent instruction to the ATR72, which the crew of DLH 1072 also mistakenly acknowledged and followed, initiating a descent from FL 240.
As the aircraft trajectories converged, the TCAS system on DLH 1072 issued a Resolution Advisory (RA) for a climb. The crew of AFR 11983 also experienced a TCAS RA, necessitating a descent. The conflict was resolved through the combined actions of the flight crews and the ACC Prague radar controller, who issued instructions to stop the climb of AFR 1983 and the descent of DLH 1072.
The investigation
An investigation by the ÚZPLN examined radar data, radio communications, and the actions of the involved flight crews and air traffic controllers. The investigation focused on the sequence of instructions issued by both the Prague Radar (WL EC) and Prague Approach (APP EC) sectors.
Investigators noted that the APP EC controller was utilizing a vertical filter on the radar display that set the ground limit to FL 150. Because DLH 1072 was flying above this threshold, the aircraft was not visible on the controller's primary radar display. Furthermore, the investigation found that the controller failed to notice that the crew of DLH 1072 was acknowledging instructions meant for a different aircraft, and did not react to the incorrect read-back of the descent instruction.
Findings
- The crew of DLH 1072 repeatedly responded to instructions intended for another aircraft (DLH 3272) due to the high similarity between the two call signs.
- The APP EC controller demonstrated insufficient attention to the content of air-ground communications.
- The APP EC controller failed to identify that the crew of DLH 1072 was incorrectly acknowledging instructions meant for another aircraft.
- The controller's use of a vertical radar filter prevented the visibility of DLH 1072 on the controller workstation during the critical phase of the incident.
- The minimum separation between the aircraft was maintained at 1,800ft vertically and 4.95 NM horizontally during the resolution maneuvers.