What happened
On a scheduled evening flight from Montreal-Dorval Airport (YUL) to Toronto (YYZ), a Douglas DC-8 departed runway 06R at approximately 18:28 following minor boarding delays. Weather conditions at the time included fog, light rain, and overcast skies with visibility limited to 4 miles. After a normal takeoff, the crew reported passing 3,0-00 feet and received clearance for a left turn toward St. Eustache. This communication served as the final contact from the flight.
Shortly after this report, the aircraft deviated significantly from its intended path, turning roughly 55 degrees to the right while entering a rapid descent. At 18:33, approximately 16.9 miles from the airport, the aircraft struck the ground at high speed, traveling between 470 and 485 knots at a descent angle of about 55 degrees. The impact caused the total disintegration of the airframe. There were 118 fatalities among the occupants, with no survivors.
Findings
Investigators could not determine the exact cause with absolute certainty but identified a chain of events involving the aircraft's trim settings. It was determined that the pilot had applied near-maximum nose-down trim to the horizontal stabilizer. This action initiated a dive so severe that recovery was impossible because the hydraulic motor for the stabilizer had stalled, preventing the crew from trimming the aircraft back out of the extreme position.
Several factors may have prompted the application of this trim:
- Potential icing of the Pitot system.
- A possible failure of a vertical gyro, which could have provided misleading flight attitude information to the pilot.
- An unprogrammed and unnoticed extension of the Pitch Trim Compensator (PTC).
Ultimately, the unprogrammed extension of the Pitch Trim Compensator was identified as the most probable cause. This extension would have moved the control column and elevators into a nose-up position, prompting the pilot to counteract the force with significant nose-down trim. The resulting combination of PTC extension and extreme nose-down trim created severe stability and control issues, leading to divergent oscillations and an uncontrollable dive.