Accident Sequence
At 1020 Pacific daylight time, a de Havilland DHC-6 (Twin Otter) float plane, registration C-FGQH, serial number 106, was at a dock preparing for a charter flight from Vancouver Harbour, British Columbia to Victoria. The aircraft had 2 pilots and 11 passengers on board. The number 2 (right-hand) engine was started normally, and the pilot-in-command (PIC) signalled the dockhand to untie the aircraft. The dockhand disconnected the auxiliary power unit, confirmed the untie signal, and untied both mooring lines from the dock.
The PIC then began starting the number 1 (left-hand) engine. During start, the unsecured aircraft drifted free and swung right to a position approximately perpendicular to the dock. As the number 1 engine spooled up, with reverse selected on the number 2 engine, the aircraft accelerated forward and veered in a left-hand arcing turn toward an adjacent dock. The PIC applied full reverse on both engines to stop forward motion, but a mechanical fault prevented the propellers from going into reverse. The increased power accelerated the aircraft forward. The PIC shut down the engines using the fuel control levers. The aircraft struck the dock, and the left float was ripped from its mounts, causing the aircraft to tip left as the float sank. All 13 people aboard escaped onto a maintenance float; there were no injuries.
Background
The company's normal start procedure at the Vancouver Harbour base required the aircraft to be securely tied to the dock during start, with propellers selected to FEATHER. The pilots were to ensure both engines and propellers were functioning in forward and reverse before signalling release of mooring lines. Two days prior, a company pilot had been struck by a spinning propeller while securing an aircraft to a mooring line, heightening sensitivity to propeller dangers.
On the accident day, the PIC decided to use an alternate procedure from the company's standard operating procedures (SOP), designed for remote sites to avoid working near spinning propellers. This involved starting the right engine, untying the mid bollard, pivoting the aircraft right so the left propeller was clear of the dock, starting the left engine, then untying the heel of the float. This plan was not fully briefed to the first officer or the dockhand.
Propeller Control System and Modification
The aircraft was equipped with reversible-pitch, fully-feathering, constant-speed propellers with a beta backup system to prevent blades from moving into unacceptable low pitch or reverse in case of mechanical failure. The pilot could override beta backup by twisting the power lever grip before moving into reverse, activating a microswitch beneath the left power-lever gate. The microswitch's mechanical components were exposed within a panel-covered compartment.
The accident aircraft had returned to service about two weeks prior after a major refit that included modifying the ignition system from glow plugs to spark ignition per DHC-6 Service Bulletin 6/527. During this modification, unused wires from the old system were isolated, tie-wrapped, and stowed near the microswitch. The service bulletin did not specify clearance requirements, require removal of unused wires, or highlight special care near the microswitch.
Findings
An inspection after the accident revealed that the wrapped wire bundle had come into contact with the exposed mechanical components of the microswitch, impeding its operation. No flight or maintenance personnel had reported reverse thrust problems in the two-week period.
The PIC signalled the dockhand to untie, expecting only the mid-bollard line to be released per the alternate procedure. Due to the lack of full briefing, the dockhand released all lines. As the PIC started the left engine, the aircraft pivoted clockwise to perpendicular with the dock. At some point, the wire bundle blocked the microswitch, causing the beta backup system to remain engaged and preventing the propellers from moving into reverse pitch. As engine power increased with propellers in forward pitch, the aircraft moved forward. The PIC's attempt to apply increased reverse power accelerated the aircraft forward until it struck the dock. The PIC had insufficient time to respond.
Under normal start conditions (aircraft tied to dock), the fault might have been discovered during post-start checks before lines were released.
Safety Action
Following the occurrence, the company inspected all aircraft in its fleet for similar risks; none were found. Transport Canada reviewed Service Bulletin 6/527 with Bombardier Aerospace and worked with the company to incorporate additional instructions for isolating and stowing unused wires near the microswitch.