What happened
On an unspecified date, an Alaskan Airlines MD-80 (ASA131) was cruising at flight level 3CO30 while traveling from Seattle to Anchorage. Simultaneously, a Canadian Airlines Boeing 747-400 (CDN4) was descending from flight level 350 toward Vancouver, British Columbia, after arriving from Tokyo.
During the descent, the air traffic controller at the Vancouver Area Control Centre cleared the Boeing 747-400 to descend to 17,000 feet without referencing the nearby MD-80. As the aircraft approached the INHAM intersection, the crew of CDN4 spotted the other aircraft and received a TCAS traffic advisory followed by a resolution advisory to climb. Because the crew could visually confirm the other aircraft and determined there was no immediate danger, the captain followed company manual procedures and continued the descent. The aircraft eventually passed within 700 feet vertically and 1 nautical mile horizontally of the MD-80, violating the required separation minimums of 2,000 feet vertically or 5 nautical miles horizontally.
The investigation
Investigators examined the actions of the west sector radar controller, who was managing four aircraft at the time of the incident. The investigation focused on the controller's use of conflict-identification tools and the accuracy of flight progress strip annotations. It was noted that the controller had failed to follow standard practices, such as scanning the flight progress board and radar display before issuing clearances.
Furthermore, the investigation looked into the administrative handling of flight strips. The controller did not annotate the descent for CDN4 as required by operational manuals, and the strip for ASA1131 lacked critical timing information. These omissions, combined with a local practice of using single strips, made identifying potential conflicts more difficult. The investigation also examined the controller's environment, noting that he was engaged in a non-operational conversation with a nearby colleague during the period in question.
Findings
- The west sector controller failed to utilize available tools and procedures designed to maintain separation minima.
- Inadequate annotation of flight strips for both the Boeing 747-400 and the MD-80 reduced the ability to detect a potential conflict via the flight progress board.
- A non-essential conversation involving the controller likely acted as a distraction, compromising his ability to monitor the aircraft under his control.