What happened
On a morning in June 2001, a Cessna 172, registration N8522U, was flying from Washington state to Nanaimo, British Columbia, at 4500 feet. Simultaneously, a West Coast Air de Havill de Havill DHC-6 Twin Otter, call sign C606, was operating a scheduled flight from Vancouver Harbour to Victoria. Both aircraft were flying under visual flight rules (VFR) within the Victoria terminal airspace.
At approximately 1018 Pacific daylight time, the two aircraft came into close proximity. The pilot of the Cessna 172 noticed the Twin Otter and performed an immediate, rapid descent of 200 feet to avoid a collision. The crew of the Twin Otter did not observe the Cessna 172 in time to take evasive action, as the co-pilot was occupied with cockpit duties and the captain did not visually acquire the other aircraft until the Cessna 172 was already passing below and in front of them.
The investigation
The investigation focused on why the conflict was not detected by air traffic services and why the pilots failed to see each other in time. Investigators found that the Victoria terminal complex was operating with significant staffing shortages. Due to absences, the unit was running with only three controllers when five were scheduled, and one of those three was on a scheduled break.
Furthermore, the shift supervisor had mistakenly believed a fourth controller would arrive at 1000, when they were not actually scheduled until 1015. While traffic restrictions had been implemented to manage the workload, these restrictions expired 15 minutes before the additional controller arrived. This created a window where workload increased significantly without the necessary personnel to manage the sector.
Findings
- The pilots of both aircraft failed to visually identify each other in time to prevent the proximity event.
- No traffic advisories were issued by air traffic control to either aircraft.
- The controller failed to detect the conflict because his attention was divided by supervisory duties and an increasing workload.
- The controller did not integrate the flight path of the Cessna 172 into his plan when radar-identifying the Twin Otter.
- The shift supervisor did not open the VFR traffic advisory sector in a timely manner and did not utilize available methods to mitigate the increasing workload.
- Neither aircraft was equipped with a traffic alert and collision-avoidance system (TCAS), which likely would have alerted the pilots to the danger.