What happened
During a period of intense thunderstorm activity near Fort Mcally, Alberta, two aircraft experienced a significant loss of separation. An Air Mikisew Jetstream 31, operating flight AM 304, and a WestJet Boeing 737, flight WJA 255, were both en route to Fort McMurray. Due to severe weather, air traffic controllers had authorized several aircraft to deviate from their flight paths as necessary to avoid thunderstorms.
The controller was managing multiple arrivals and departures, all of which were navigating around a line of convective activity. While the aircraft were following parallel courses, the Boeing 737 was cleared to 9000 feet, while the Jetstream 31 was cleared to descend to 7000 feet. As the faster-moving Boeing 737 overtook the Jetstream 31, the lateral and vertical separation dropped below the required minimums. At the closest point, the aircraft were only 1.4 nautical miles apart and separated by just 100 feet vertically. The situation prompted a TCAS resolution advisory (RA) for the WestJet crew, which commanded a climb.
The investigation
The investigation examined the controller's management of the airspace, the impact of weather on flight paths, and the communication between the cockpit and air traffic control. Investigators looked into the controller's recent training, the workload at the Edmonton Area Control Centre, and the specific maneuvers performed by both aircraft during the conflict. The investigation also reviewed the use of safety alert phraseology and the adherence to collision avoidance protocols.
Findings
Several factors contributed to the loss of separation. The controller had issued deviations without restrictions to three different aircraft in close proximity, which complicated the maintenance of separation. The controller's strategy for separation relied on the Jetstream 31 maintaining a specific descent rate and track, but this plan failed when the aircraft altered its heading for weather without notifying the controller.
The controller failed to use required safety alert phraseology when instructing the Jetstream 31 to climb, which prevented the pilots from recognizing the urgency of the situation. Additionally, the WestJet crew did not specify whether they were climbing or descending during their TCAS maneuver, partly due to an unrelated medical emergency on board. This lack of communication, combined with the controller issuing a turning instruction to the Boeing 737 while it was responding to a TCAS RA, delayed the restoration of safe separation.