What happened
On a night involving significant weather-related delays at Calgary International Airport, two WestJet aircraft—a Boeag 737-800 (registration C-GWSA) and a Boeing 737-600 (registration C-GWSI) — came dangerously close to each other. The first aircraft was traveling from Vancouver to Calgary, while the second was en route from Comox to Calgary.
While navigating near the OPALE intersection, air traffic control issued radar vectors and descent clearances to both aircraft. At 22:45:45 mountain daylight time, the lateral separation between the two planes dropped to 4.7 nautical miles, and the vertical separation decreased to just 70 feet, failing to meet the required 5 nm lateral or 1000 feet vertical standards. Both flight crews received Traffic Alert and Collision Avoidance System (TCAS) resolution advisories and performed maneuvers to avoid a collision. The air traffic control conflict alert system was also triggered.
The investigation
Investigators analyzed digital flight data recorder information from both aircraft and correlated it with radar data, air traffic control communications, and crew observations.
At the time of the incident, Calgary International Airport was operating at a reduced capacity due to poor weather, necessitating flow control measures such as ground delays and metering. Although flow control had officially ended at 22:30, holding and vectoring delays were still in effect for several inbound flights.
Investigators found that the Calgary En route sectors had been combined into a "midnight configuration" prematurely. This consolidation occurred without the required consultation with the on-duty shift manager, as the controllers involved were following an established routine to manage their individual workloads. The resulting single controller was managing a complex traffic situation over a vast area, which significantly impacted his ability to monitor inbound traffic from the west.
Furthermore, the handover briefing between the outgoing and incoming controllers was insufficient. The relieving controller was not clearly informed of the specific flow control instructions previously issued to aircraft inbound from the west, leading him to believe all flights were already cleared to hold at OPALE. When he realized holding clearances had not been issued, he attempted to meter the aircraft using radar vectors, which increased the complexity of the traffic.
Findings
- The loss of separation occurred because the two aircraft were vectored on converging courses and cleared to the same altitude.
- The premature combination of airspace sectors created an excessive workload for the radar controller, hindering effective monitoring.
- An inadequate handover briefing prevented the relieving controller from having an accurate understanding of the traffic metering plan.
- The controller's initial climb instruction to one aircraft lacked the necessary urgency, which may have impacted the vertical separation distance.
- The controller could have reduced workload and risk by clearing eastbound flights to hold at the published OPALE intersection rather than using vectors.