Investigation Summary
The investigation into Flight 831 faced immense challenges. The DC-8 impacted a muddy field at a 55-degree angle, creating a crater that quickly filled with rainwater and fuel, while the aircraft itself was pulverized into small fragments. At the time, Canadian regulations did not require Cockpit Voice Recorders (CVR) or Flight Data Recorders (FDR), leaving investigators with only radar data and wreckage analysis. The inquiry focused on the Pitch Trim Compensator (PTC), a system designed to automatically adjust the nose-up trim as the aircraft accelerated to counteract the DC-8's natural tendency to tuck its nose down at high speeds. If the PTC failed or moved to an uncommanded full-down position, it could induce a dive that, at high speeds, would cause a 'hydraulic stall.' This occurs when the aerodynamic hinge moments on the elevators exceed the maximum output of the elevator hydraulic boost cylinders, effectively locking the control surfaces and making recovery impossible.
Final Conclusions
The official commission of inquiry released in 1965 stated the cause was 'undetermined.' However, it cited a probable failure of the Pitch Trim Compensator (PTC) system or the vertical stabilizer's drive motor. The report suggested the crew may have been misled by a faulty PTC or were unable to overcome a runaway trim condition, leading to a high-speed dive and subsequent loss of control.
Photographic Evidence (1)
Video Analysis
Killer Safety Feature | Trans-Canada Air Lines Flight 831 — An analysis of the 1963 crash of a Trans-Canada Air Lines DC-8 near Montreal, exploring the potential role of the Pitch Trim Compensator system in the disaster.
If playback is blocked, the owner has disabled embedding — use the link above.
Systems & Failure Modes
Pitch Trim Compensator (PTC)
An electromechanical system that adjusts the elevators to compensate for 'Mach tuck' at high speeds.
Horizontal Stabilizer Trim
The system used to maintain the aircraft's pitch attitude; a runaway or mismanaged trim can lead to uncontrollable dives.
Hydraulic Actuators
Components that move the flight control surfaces; at extreme speeds, aerodynamic forces can exceed the power of these actuators.
Interesting Facts
- 01At the time, it was the deadliest aviation accident in Canadian history.
- 02The aircraft was only nine months old, having been delivered in February 1963.
- 03The impact was so violent that the aircraft dug a crater approximately 8 to 10 feet deep and 150 feet wide in the soft, waterlogged soil.
- 04Investigators ruled out sabotage by the FLQ (Quebec Liberation Front) after extensive explosive residue testing.
- 05The lack of flight recorders led to significant changes in Canadian aviation safety regulations.
Safety Actions & Advisories
Flight Recorder Mandates
Following the investigation's inability to determine a definitive cause, Canada moved to mandate the installation of Flight Data Recorders and Cockpit Voice Recorders on all large commercial aircraft.
DC-8 PTC System Review
Douglas Aircraft Company and operators reviewed the PTC system design and pilot procedures for handling trim malfunctions at high speeds.