What happened
On an unnamed date, a Boeing 747-400 operating as Northwest Airlines Flight 11 was traveling from Detroit to Tokyo. While transiting the Winnipeg Area Control Centre airspace, the aircraft was cleared to climb from flight level 310 to FL350. Simultaneously, a Syncrude Citation 560, flying as SYN21 from Calgary to Fort Mcoded, was also assigned FL350.
During the flight, a controller in the La Biche sector incorrectly updated the flight progress strip for the Boeing 747-400, reverting the altitude from FL350 back to FL310. This error occurred while the controller was managing a high workload and coordinating a reroute for another aircraft. Because of this incorrect notation, the controller believed there was no conflict between the two aircraft.
As the aircraft moved through the sector, a series of rapid controller handovers occurred. The third controller in the sequence received both aircraft at FL350. Although the pilots of both aircraft identified themselves and correctly reported their altitude of FL350, the controller failed to verify this against the flight progress strip or the radar data tags. At 1409 mountain standard time, the pilots received a TCAS resolution advisory. The Syncrude Citation 560 descended to FL330 while the Boeing 747-400 climbed, preventing a collision as the vertical separation had dropped below 2000 feet.
The investigation
The investigation focused on the sequence of documentation errors and the lack of secondary defenses. Investigators examined the actions of three successive controllers in the La Biche sector and the impact of rapid shift changes on situational awareness. The investigation also looked into the technical capabilities of the radar processing system and the adequacy of monitoring practices within the Edmonton ACC.
Findings
- The primary cause of the loss of separation was the first controller's incorrectly reverting the altitude on the flight progress strip from FL350 to FL310, which prevented subsequent controllers from identifying the potential conflict.
- The third controller failed to cross-reference the altitude reported by the Boeing 747-400 with the information on the flight progress strip or the radar data tags.
- Rapid successive controller changes contributed to a lack of situational awareness, as the incoming controllers had insufficient time to fully assess the traffic situation.
- The radar processing system lacked automated conflict-alerting software at the time of the event.
- There was no effective method in place to monitor whether controllers were adhering to standard strip-marking procedures.