What happened
On a daylight flight in Quebec, two Air Inuit Ltd. aircraft were traveling on opposing tracks under instrument flight rules. The first, a DHC-8-102 (registration C-FCJD), was flying from La Grande-Rivière toward Puvirnituq at flight level 230. The second, a DHC-8-314 (registration C-GUAI), was traveling from Puvirnituq toward La Grande-Rivière, initially assigned to flight level 220.
To avoid pitch oscillations caused by the autopilot's altitude-hold mode, the crew of C-FCJD utilized vertical speed (VS) mode to maintain its altitude. During the flight, the captain of C-flyJD was left alone at the controls while the first officer briefly left the cockpit. During this period, the aircraft began a gradual descent of approximately 50 feet per minute, a change that went unnoticed for 14 minutes as the captain focused on the coastal landscape.
At 1435 EST, the Traffic Alert and Collision Avoidance System (TCAS) issued a traffic advisory, followed quickly by a resolution advisory. The crew of C-GUAI performed a maneuver involving a left turn and descent, while the captain of C-FCJD initiated a sharp right turn. The two aircraft passed each other with a vertical separation of approximately 1500 feet and a horizontal separation of 0.8 nautical miles. Both aircraft subsequently returned to their assigned altitudes and landed safely.
The investigation
Investigators examined the use of the autopilot's vertical speed mode and the crew's monitoring of flight instruments. The investigation looked into the lack of an audible altitude alert on C-FCJD, noting that the aircraft's altitude warning light was difficult to see in daylight. The probe also assessed the crew's reaction to the TCAS advisories, the adequacy of simulator training, and the lack of coordination due to the first officer's absence from the cockpit.
Findings
- The crew of C-FCJD used vertical speed mode as a workaround for autopilot oscillations, which allowed for an imperceptible descent that was not detected by the pilot.
- The captain's attention was diverted from the flight instruments to the external landscape, preventing the detection of the altitude loss.
- The absence of simulator training regarding TCAS maneuvers contributed to the crews' inappropriate lateral maneuvers (turns) in response to vertical instructions.
- The lack of an audible altitude warning on this specific aircraft model reduced the likelihood of the crew noticing the deviation.
- The first officer's absence from the cockpit meant there was no secondary monitor to detect the altitude deviation or assist with the TCAS alerts.