History of the Flight
On the day of the event, Air Inuit Ltd. flight AIE304, a DHC-8-102 (registration C-FCJD), departed La Grande-Rivière, Quebec, for Puvirnituq at 1328 Eastern Standard Time, with 3 crew members and 28 passengers aboard. The aircraft was cleared to climb to flight level (FL) 250 under instrument flight rules (IFR), but the crew requested and received authorization to maintain FL230, which was reached at 1349. At 1357, the crew switched to the en route frequency. Radar coverage was lost at 1416, about 175 nautical miles (nm) north of Chisasibi.
At 1406, flight AIE880, a DHC-8-314 (registration C-GUAI), departed Puvirnituq for La Grande-Rivière with a 3-member flight crew. The flight planned for FL220, but the crew requested FL250. Due to AIE304 at FL230 on an opposite heading, AIE880 was instructed to maintain FL220 until the aircraft passed.
At 1421:35, while cruising at FL230, AIE304 began a slow descent without the captain noticing. Over 14 minutes, the aircraft lost just over 700 feet at an average rate of about 50 feet per minute. The descent went undetected because the captain's attention was focused on the landscape outside, and the autopilot was in vertical speed (VS) mode, selected due to recurring pitch oscillations in altitude-hold mode. The first officer had left the cockpit for physiological reasons; the captain was the sole pilot at the controls.
At 1435:26, the first officer of AIE880, who was the pilot flying (PF), received a traffic advisory (TA) and disconnected the autopilot, initiating a shallow turn. Two seconds later, the captain of AIE304 heard a TA and then a resolution advisory (RA) requesting climb. He disconnected the autopilot and turned right, banking up to 38°. The aircraft lost about 50 feet before climbing. The AIE880 PF made a left-climbing turn but then descended as per the RA. The aircraft passed each other at 1436:12 with approximately 1500 feet vertical separation and 0.8 nm horizontal separation. Both flights returned to their assigned altitudes and landed without incident.
Personnel Information
Both flight crews were certified and qualified for the flight. Duty and rest times were within prescribed limits; there was no evidence of fatigue. The flying experience of the pilots varied, with the AIE304 captain having 4500 hours on the DHC-8 as captain, and the AIE880 captain having 300 hours as captain and 8500 total flying hours.
Aircraft Information
Autopilot and Altitude Alerting
The AIE304 autopilot was in VS mode to maintain altitude, a work-around for pitch oscillations in altitude-hold mode. VS mode maintains the vertical speed at activation; it is possible a slight descent was already present when selected. The aircraft was equipped with an altitude warning light, but it did not flash, and daylight reduced its visibility. The captain did not notice the light once the aircraft deviated more than 250 feet from the selected altitude; the light was illuminated for about 9 minutes. The other aircraft in the fleet had an audible altitude alert, but the DHC-8-102 did not.
TCAS Reaction
Following the TA, the AIE304 first officer observed the TCAS display indicating a target ahead. The captain disconnected the autopilot and turned right. AIE880's PF made a shallow right turn, then a left-climbing turn before descending as per the RA. The two aircraft were converging with clear visibility; the PF of AIE880 believed the visual indication suggested a turn. The TCAS advisory led to the avoidance maneuvers.