What happened
On a Sunday evening, two long-haul flights were traveling through the Canadian Northern Control Area (NCA) on overlapping routes. The first, SAS938, a Boeing 767-300 flying from Seattle to Copenhagen, was followed approximately 13 minutes later by KLM602, a Boeing 747-300 traveling from Los Angeles to Amsterdam.
While under the radar control of the Winnipeg Area Control Centre's Trout Lake sector, the controller rerouted SAS938 to the north to avoid other traffic. Although the reroute was recorded in the local system, the change was not communicated to the Edmonton Area Control Centre's Nunavut sector. Consequently, the controller in the Nunavut sector entered the original flight plan for SAS9 8 into their display, unaware of the deviation.
As the aircraft transitioned into procedural control—where separation relies on position reports rather than radar—the spacing between the two aircraft degraded to just 9 minutes. The error went unnoticed for over an hour because position reports were relayed via HF radio through a third-party agency, causing significant delays. The discrepancy was only discovered when the Nunavut controller received a position report for SAS938 that contradicted the entered flight plan. To restore safety, the controller descended SAS938 to FL310, successfully re-establishing the required separation.
The investigation
The investigation focused on the breakdown in communication and the limitations of the air traffic control systems in the region. Investigators examined the workload of the Trout Lake controllers and the adequacy of staffing levels during the shift. The inquiry also looked into the technical capabilities of the NADS and NSiT computer systems used by Nav Canada, specifically regarding how data is transferred between different control sectors.
Findings
- The Trout Lake data controller was likely task saturated, which led to the failure to communicate the rerouting of SAS938 to the next sector.
- The NADS and NSiT computer systems lacked automatic electronic data transfer, meaning information entered in one sector had to be manually re-entered in another, increasing the risk of error.
- The lack of radar coverage and direct pilot-to-controller communication in the northern airspace prevented the timely detection of the aircraft's true position.
- The supervisor's involvement in active traffic control duties reduced the time available for high-level decision-making, contributing to the premature release of controllers from their shifts.
- Atmospheric interference and the use of a relay system (Arctic Radio) caused significant delays in receiving aircraft position reports.