What happened
On August 10, 2003, a Continental Airlines Boeing 757-200, flight COA422, departed Seattle-Tacoma International Airport for Anchorage, Alaska. During the initial departure phase, the Seattle Sector 3 radar controller mistakenly cleared the aircraft to FL370, despite the flight plan specifying FL310. The pilot read back the incorrect altitude, and the controller failed to notice the error. Simultaneously, a Seattle data controller, distracted by a telephone call, recorded the flight plan altitude of FL310 on the flight progress strip without verifying the actual clearance.
As the Boeing 757-200 entered Canadian airspace, it was handed over to the Vancouver Area Control Centre. The West sector controller acknowledged the aircraft but did not notice that the plane was climbing through its planned altitude. During the transition to the Holberg controller, the discrepancy remained undetected. At the same time, an Asiana Airlines Boeing 747-400, flight AAR284, was cruising at FL370 on a route from Anchorage to Los Angeles.
By the time the Holberg controller realized that COA422 was at the same altitude as AAR284, the aircraft were on reciprocal tracks and had already lost the required lateral separation. The situation escalated when the radar system briefly lost track of AAR284 due to coverage limitations. Ultimately, the risk of a collision was only averted when both crews responded to TCAS Resolution Advisories, which prompted the Boeing 757-200 to climb and the Boeing 747-400 to descend.
The investigation
The investigation focused on the breakdown in communication and monitoring between the Seattle and Vancouver air traffic control centers. Investigators examined the accuracy of flight progress strips, the effectiveness of the radar data processing system, and the mental models of the controllers involved. The probe also looked into the impact of controller fatigue, noting that the Holberg controller had returned to work for an afternoon shift after only a short rest following a midnight shift. Furthermore, the investigation assessed the lack of automated conformance monitoring and conflict detection technology in the Vancouver ACC at the time of the event.
Findings
- The primary cause of the risk of collision was that the Seattle ARTCC controller inadvertently issued an incorrect altitude clearance to COA422, which was not detected by any subsequent controllers.
- Controllers in the Vancouver West and Holberg sectors maintained a mental model based on erroneous information, failing to update their awareness despite the aircraft reporting its actual altitude during radio transmissions.
- The lack of automated conflict detection or conformance monitoring in the Vancouver ACC meant there was no technological alert to warn of the altitude deviation or the impending loss of separation.
- The Holberg controller's alertness may have been compromised by a sleep debt resulting from a short changeover between shifts.
- The pilot of COA422 did not report reaching the cleared altitude, an omission that prevented the West controller from identifying the discrepancy.