What happened
On a flight path between Ottawa and Vancouver, an Airbus A3/20 operated by Air Canada, identified as ACA139, experienced a loss of separation with another aircraft near Broadview, Saskatchewan. The aircraft was flying at flight level 350 when the flight crew received a Traffic Alert and Collision Avoidance System (TCAS) resolution advisory instructing them to descend. Simultaneously, the crew of a Boeing 737 operated by Canadian Airlines International, CDN636, received a conflicting advisory to climb.
The two aircraft passed within approximately 1.3 miles horizontally and 900 feet vertically. Both crews responded immediately to the TCAS alerts, and no injuries or damage were reported. The incident occurred while ACA139 was deviating north of its assigned route to avoid thunderstorm activity, eventually turning back toward its original track in a manner that intersected the path of CDN636, which was flying at the same altitude due to turbulence.
The investigation
The investigation focused on the air traffic control environment within the Winnipeg Area Control Centre's Broadview sector. At the time of the event, the controller was managing moderate to heavy traffic density complicated by widespread thunderstorms, which forced several aircraft to fly at non-standard altitudes or deviate from assigned tracks.
Investigators examined the controller's mental model and situation awareness. While the controller had been alerted to a potential conflict via flight progress strips, the investigation found that the controller had previously analyzed the situation and concluded no conflict existed. The investigation also looked into the functionality of the radar system's traffic conflict warning capabilities and the regulatory requirements regarding pilot deviations from assigned tracks.
Findings
- The controller failed to recognize the developing conflict between ACA193 and CDN636, failing to maintain the required separation.
- The controller's previous assessment that the aircraft were de-conflicted led to a cognitive bias where subsequent changes in the aircraft's direction were not sufficiently noticed.
- High traffic density, complex coordination requirements, and limited time contributed to the failure to detect the change in aircraft status.
- The pilot of ACA139 was not required by regulation to request clearance to return to the original track after deviating for weather.
- The traffic conflict warning system within the radar system was not operational at the time of the occurrence.
- CDN636 was operating at a non-standard altitude due to excessive turbulence.