What happened
During approach operations, a loss of separation occurred between two inbound aircraft, QF652 and QF/QF601. Radar data indicated that as the aircraft passed one another, QF652 was at 9,000 ft and QF601 was at 8,000 ft, with a lateral separation of 2.3 NM. Subsequent tracking showed QF652 at 8,700 ft with the lateral distance to QF601 increasing to 2.6 NM. This breach occurred because the required separation of 3 NM or 1,000 ft was not maintained, with the minimum recorded separation dropping to 2.5 NM and 700 ft.
The investigation
The investigation focused on a descent clearance issued by the approach controller. The controller inadvertently issued a clearance to QF652 that was actually intended for flight QF62. Although the flight crew read back the incorrect call sign, the controller did not identify the mismatch between the aircraft label and the verbal response, likely because the response seemed consistent with the instruction provided.
At the time of the incident, the controller reported feeling alert and well-rested, noting a moderate workload. While the controller noted that techniques exist to differentiate similar call signs—such as appending the aircraft's position in the sequence or using the 'heavy' designation—these methods were not utilized in this instance. Both aircraft involved were 'heavy' aircraft, which limited the effectiveness of such identifiers.
Findings
- The primary cause of the separation loss was the incorrect descent clearance issued to QF652 due to call sign confusion.
- The controller failed to employ mitigation strategies, such as emphasizing specific digits or repeating the call sign, to prevent confusion between similar flight numbers.
- Both aircraft were operating on the same frequency with highly similar call signs.
- While Airservices Australia has established processes to monitor and report call sign confusion, the investigation noted that the distribution of monthly conflict reports to operators had previously been paused during a system upgrade.