What happened
At 13:10 Newfoundland daylight time, a loss of separation occurred between three aircraft within the CYMON sector of the Gander Area Control Centre. The aircraft involved included British Airways flight BAW93, flying from London Heathrow to Toronto; Air Canada flight ACA861, traveling from Heathrow to Halifax; and United Airlines flight UAL971, en route from Milan to Washington.
While operating at flight level 340, the crew of BAW93 requested a climb to flight level 360. The sector controller granted this clearance immediately. As the aircraft ascended to the new altitude, the required separation between BAW93 and the other two aircraft was lost. The controller eventually identified the conflict and issued a new clearance to BAW93 to restore the necessary distance.
The investigation
The investigation focused on the actions of the CYMON sector controller and the performance of the radar data processing system (RDPS) situational display (RSiT). It was determined that the controller issued the climb clearance without performing the standard check for conflicting traffic. Furthermore, the controller failed to mark the flight progress strip for BAW93 to indicate the change in altitude, a deviation from his standard procedure.
Technical analysis of the RSiT revealed that overlapping aircraft targets caused garbled radar data. This interference prevented the controller from visually identifying that BAW93 was climbing until the aircraft had already leveled off at flight level 360. The investigation also noted that the controller was relying heavily on the radar display rather than cross-checking the physical flight progress strips.
Findings
- The primary cause of the incident was the inadvertent issuance of a climb clearance to BAW9t3 by the sector controller.
- The controller failed to verify the surrounding traffic for potential conflicts before approving the altitude change.
- The controller did not update the flight progress strip, which prevented him from detecting the error through manual cross-checks.
- Overlapping radar targets on the situational display obscured the altitude change from the controller's view.
- A decrease in traffic density and the controller being near the end of his shift may have contributed to a lapse in attention.