What happened
On an unnamed date, a WestJet Boeing 737-700, registered C-FWSX, was performing a scheduled flight from Vancouver to Edmonton. While navigating through the Alberta High airspace near Rocky Mountain House, the aircraft was cleared by air traffic control to descend from flight level 370 to flight level 320. Simultaneously, a Cessna 525 (registration C-GPOS) was established at flight level 320, traveling from Springbank to Grande Prairie.
During the descent, the crew of the WestJet aircraft responded to a traffic alert and a collision avoidance system resolution advisory. At 0950 mountain standard time, the vertical separation between the two aircraft had dropped to 600 feet, with a lateral distance of 4.8 nautical miles, failing to meet the required 1000-foot vertical and 5-nautical-mile lateral separation standards.
The investigation
The investigation focused on the actions of the Alberta High specialty controller, who was managing both radar and data position responsibilities at the time. The controller was actively processing numerous flight data strips and communicating with multiple sectors and aircraft.
Investigators examined the flight data strips for both aircraft, noting that the strips for C-FWSX and C-GPOS were positioned adjacent to one another on the data board. While the controller had recorded flight level 330 for the WestJet aircraft, the strip for the Cessna indicated flight level 320. The investigation also reviewed the functionality of the conflict alerting software and the impact of potential nuisance alerts on controller responsiveness.
Findings
- The controller likely inadvertently transposed the altitude assigned to the Cessna 525 when issuing the clearance to the Boeing 737-700.
- A "Hear Back Type II Error" occurred, in which the WestJet crew correctly read back the incorrect altitude of flight level 320, but the controller failed to recognize the error and instead confirmed the incorrect clearance.
- The conflict alerting software failed to provide a timely traffic warning, which limited the window of opportunity for the controller to intervene.
- The frequency of nuisance alerts in the system may lead to controller desensitization, increasing the risk that genuine traffic conflicts are dismissed as false alarms.