What happened
On a morning flight in Alberta, a WestJet Boeing 737-700, operating as WJA28, was traveling from Fort McMurray to Edmonton at flight level 340. Simultaneously, an Air Canada Jazz Bombardier CL-600-2B19, operating as JZA8369, was flying from Calgary to Fort McMurray at flight level 330. Both aircraft were under the radar control of the La Biche Sector within the Edmonton Area Control Centre.
At approximately 0722 mountain standard time, the sector's conflict alert system triggered an alarm. The crew of JZA8369 reported performing a descending turn to the right following a TCAS resolution advisory, which had identified an intruder above them. The two aircraft were found to be only 1.4 nautical miles apart laterally and 300 feet apart vertically, significantly violating the required separation standards of 5 nautical miles laterally or 1000 feet vertically.
The investigation
The investigation focused on the activities of the La Biche Sector controller, who was simultaneously managing both radar and data position responsibilities. At the time of the incident, the controller was heavily engaged in preparing for an anticipated period of high traffic volume, which included processing numerous flight data strips and communicating estimates to other sectors. The investigation also examined the functionality of the radar data processing system's conflict alert (CA) software.
Findings
- The La Biche Sector controller was preoccupied with data position duties, which prevented active monitoring of the radar environment.
- The controller incorrectly believed the WestJet aircraft was at flight level 320 when he issued a clearance for it to descend to 10,000 feet.
- The controller issued a descent clearance to WJA22 that caused it to descend through flight level 330, directly into the path of JZA8369.
- Perceptual confusion likely occurred when the controller, while reviewing similar flight data strips, associated the altitude of a different aircraft (SYN22) with the WestJet flight.
- High workload led to task shedding, where the controller failed to verify the actual altitude of the aircraft during initial contact and during the descent clearance.
- Under certain flight geometries, the conflict alerting software may fail to provide a timely traffic warning before separation is lost.
Safety action
- The Edmonton Area Control Centre has implemented a specific corridor for traffic between Edmonton and Fort McMurray, utilizing assigned eastbound and westbound altitudes and lateral segregation.
- NAV CANADA is developing RNAV routes to provide permanent lateral separation along this corridor.