What happened
On a morning with clear skies and excellent visibility, an Air Canada Airbus A3/19-100, flight ACA185, was climbing through flight level 367 in the Thompson sector of the Edmonton Area Control Centre. At the same time, a Northwest Airlines Boeing 747-400, flight NWA26, was cruising at flight level 370.
Earlier that morning, the air traffic controller had been managing a potential conflict between ACA185 and a Lufthansa Boeing 747. To resolve this, the controller instructed the Air Canada crew to climb to flight level 380. However, this climb placed the aircraft on a path that would intersect with the altitude of NWA26.
While the controller was distracted by a telephone call and a misplaced flight progress strip from another sector, the separation between the two aircraft decreased significantly. At 1004:06, the aircraft's onboard collision-avoidance systems issued resolution advisories, commanding the Northwest Airlines aircraft to climb and the Air Canada aircraft to descend. The aircraft successfully maneuvered to pass each other with only 0.7 nautical miles of horizontal separation, far below the required five nautical miles.
The investigation
The investigation examined the controller's actions, the use of the Integrated Situational Display System (ISiT), and the communication between the flight crews and air traffic control. Investigators looked into why the conflict between the climbing Air Canada flight and the cruising Northwest Airlines flight was not identified before the aircraft reached critical proximity.
Findings
- The controller failed to identify that the planned climb for ACA185 would create a conflict with NWA26.
- Confirmation bias likely prevented the controller from noticing the potential danger, as his attention was focused on the highlighted conflict between the Air Canada and Lufthansa aircraft.
- The controller's ability to monitor the separation plan was compromised because he was working alone and chose to handle administrative tasks, such as processing incoming estimates and retrieving a misplaced flight strip, during the critical window.
- The controller did not utilize available decision support tools, such as the ISiT's predicted track lines, which could have helped identify the conflict.
- The use of non-urgent phraseology by the controller delayed the Air Canada crew's response to the descent instruction, increasing the period of risk.
- While both crews notified the controller of the TCAS maneuvers, they did not use the recommended specific phraseology, meaning the controller was unaware that the aircraft were deviating from their assigned altitudes.