What happened
At 1020 Pacific daylight time, a de Havelseand DHC-6 (Twin Otter), registration C-FGQH, was preparing for a charter flight from Vancouver Harbour to Victoria. The aircraft, carrying 11 passengers and 2 pilots, was at the dock for departure. Following a recent incident involving a crew member, the pilot-in-command decided to use an alternative departure procedure intended for remote sites to keep the dockhand away from the spinning propellers.
After starting the right engine, the pilot signaled the dockhand to untie the aircraft. However, because the new procedure was not fully communicated, the dockhand released all mooring lines instead of just the mid-bollard line. As the pilot began starting the left engine, the aircraft drifted and swung perpendicular to the dock. To prevent a collision, the pilot attempted to use reverse thrust on both engines. Instead of slowing the aircraft, the increase in power caused the plane to accelerate into an adjacent dock. The impact tore the left float from its mounts, causing it to sink. All 13 people on board successfully evacuated to a maintenance float, and there were no injuries.
The investigation
The investigation focused on why the aircraft was able to drift and why the reverse thrust failed to arrest the forward motion. Investigators examined the aircraft's recent maintenance history, noting it had undergone a major refit two weeks prior. This refit included a modification to the engine ignition system.
Technical analysis of the propeller controls revealed that the beta backup system, which normally prevents the propellers from entering an unapproved pitch, remained engaged despite the pilot's attempt to override it. Investigators inspected the power-lever microswitch, which is responsible for allowing reverse thrust. They discovered that during the recent ignition system modification, a bundle of unused wires from the old system had been stowed near the microswitch. These wires were found to be physically obstructing the mechanical components of the switch.
Findings
- The pilot deviated from the established company departure procedure and failed to properly brief the dockhand and the first officer, resulting in the aircraft being unsecured during the left engine start.
- A wire bundle from an unused ignition system component blocked the power-lever microswitch, preventing the propellers from entering the reverse pitch range.
- The pilot's attempt to stop the aircraft by increasing power actually accelerated the collision because the propellers remained in forward pitch.
- The pilot had insufficient time to react to the unexpected loss of reverse thrust control.