What happened
On a recent arrival into Vancouver, two large aircraft experienced a loss of separation while being vectored toward Runway 26R. The first aircraft, an Air Canada Boeing 767-200, was arriving from the northeast. Simultaneously, a Singapore Airlines Airbus A340-300 was inbound from the southwest.
As the Air Canada flight was positioned on right base, descending from approximately 4,800 feet toward 3,000 feet, the Singapore Airlines aircraft was established on left base, maintaining an altitude of 4,000 feet. During the turn toward the final approach, the two aircraft closed the distance to within 600 feet and 1.5 nautical miles of each other before separation was restored.
At approximately 11:02 PDT, the crew of the Boeing 767-200 contacted the arrival low controller, reporting a descent from 6,000 feet to 5,000 feet. The aircraft was turning left to a heading of 080° at a speed of 250 knots with a descent rate of 1,000 feet per minute. The controller cleared the aircraft to 3,000 feet and issued a warning regarding potential turbulence from preceding heavy traffic. Following this, the controller cleared the aircraft for the ILS approach and instructed a speed reduction to 170 knots.
The investigation
Investigators analyzed radar data and radio communications to reconstruct the sequence of events. The investigation established that the controller likely expected the Air Canada flight to reach its cleared altitude of 3,000 feet before lateral separation was compromised. However, the instruction to reduce speed to 170 knots caused the Boeing 767-200 to decrease its rate of descent.
By the time the controller identified the reduced descent rate, the separation had already been lost. To mitigate the conflict, the controller directed the Singapore Airlines aircraft to turn left, which helped reduce the closure rate, and instructed the Air Canada crew to expedite their descent. The Air Canada crew responded by increasing their descent rate and tightening their turn to the localizer.
Findings
- The controller failed to ensure vertical separation was maintained before lateral separation was lost.
- The use of a vectoring procedure that placed two aircraft on reciprocal tracks at similar altitudes increased the controller's monitoring burden and the risk of error.
- A commanded change in airspeed can naturally lead to a modification in an aircraft's rate of descent.