What happened
On an evening in Ontario, an Airbus A3/20 operated by Air Canada, flight ACA3578, and a Boeing 757 operated by Northwest Airlines, flight NWA853, experienced a loss of separation while traveling on reciprocal tracks. The Air Canada flight was traveling eastbound toward the Red Lake VOR, while the Northwest Airlines flight was traveling northwest.
Seeking relief from turbulence, the crew of NWA853 requested a climb to FL370. The air traffic controller approved this climb, even though the altitude was inappropriate for the aircraft's direction of flight. While the controller was managing this altitude change and coordinating a direct routing for ACA3578, the two aircraft converged. The controller eventually noticed the conflict and issued urgent instructions to change altitudes and headings. Despite the intervention, the aircraft passed within 1.5 nm horizontally and 600 feet vertically, failing to meet the required 5 nm horizontal or 2000 feet vertical separation.
The investigation
The investigation focused on the operational environment within the Winnipeg Area Control Centre (ACC). It was established that the Winnipeg specialty was undergoing airspace restructuring, which had led to significant understaffing. On the night of the event, the center was missing a team supervisor and a low-level controller. The acting supervisor was instead performing active controlling duties, leaving the sector without standback supervision.
Investigators examined the controller's actions and workload. The controller was managing both radar and data positions simultaneously under heavy traffic complexity. The investigation found that the controller's mental picture of the aircraft tracks was inaccurate because he relied on projected track lines rather than verifying the flight progress strip for ACA3578. Furthermore, while the controller had marked the altitude change on the flight progress strip, he failed to implement the required 5 nm track offset for the aircraft flying at an inappropriate altitude.
Findings
- The controller cleared NWA853 to an altitude inappropriate for its direction of flight and failed to issue necessary offset instructions to maintain separation.
- The controller failed to monitor the flight paths of the two aircraft closely enough to prevent the loss of separation.
- High workload, driven by the simultaneous management of radar and data positions, reduced the controller's ability to monitor traffic.
- Significant staffing shortages in the Winnipeg ACC meant there was no supervisor available to provide oversight or assist with the high workload.
- The lack of an automated conflict alerting system at the time meant the controller had to rely solely on manual scanning, which was interrupted by other coordination tasks.
- The use of non-standard phraseology instead of specific safety alert language delayed the crew's response to the conflict.