What happened
A de Havill and DHC-6-100 Twin Otter, operated by West Coast Air Ltd., was performing a scheduled flight from Vancouver Harbour to Victoria Harbour when a critical engine failure occurred. Shortly after departing Vancouver Harbour at approximately 1510 PST, the crew heard a loud bang and a noise resembling gravel hitting the airframe. Simultaneously, flames appeared near the No 2 engine, which immediately lost all propulsion.
At an altitude of roughly 50 to 100 feet, the aircraft experienced a sudden rightward yaw. The crew attempted to manage the emergency by applying full left rudder and deciding to land straight ahead. During the descent, the crew selected full flaps and attempted to extinguish the visible fire. Approximately 25 seconds after the initial failure, the aircraft struck the water in a nose-down, right-wing-low attitude. The impact caused the right wing and right float to detach from the fuselage. While the aircraft remained upright and partially submerged, the 15 passengers and two crew members successfully evacuated. Although the aircraft eventually sank, no serious injuries were reported.
The investigation
The investigation focused on the mechanical failure of the No 2 engine's propeller reduction gearbox (RGB). Investigators discovered that a first-stage planetary gear had fractured and disintegrated, causing the power turbine to disconnect from the propeller. Metallurgical analysis revealed that the fracture was caused by damage to the gear bore, which resulted from the bearing sleeve rotating within the bore. This instability was likely triggered by wire fragments from a faulty oil strainer screen entering the lubrication interface.
Technical analysis also examined the aircraft's configuration and the crew's response. The aircraft lacked an auto-feather system, meaning the windmilling propeller created significant aerodynamic drag. Furthermore, while the aircraft was equipped with engine oil chip detectors, it lacked a cockpit annunciator system to alert the crew to metal particles in the oil. The investigation also reviewed the maintenance history, noting that the engine had been overhauled in 1993, prior to manual revisions that clarified the correct installation of the strainer screen.
Findings
- The primary cause of the engine failure was the disintegration of a planetary gear within the propeller reduction gearbox.
- The failure originated from wire fragments released by a fractured oil strainer screen, which had been improperly installed during a previous overhaul.
- The aircraft became increasingly difficult to control as airspeed dropped below the minimum control speed (V mc), because the crew did not reduce power on the operating engine to mitigate asymmetric thrust.
- The use of full flaps during the emergency increased aerodynamic drag, further complicating the single-engine flight profile.
- The lack of an engine oil chip detector annunciator prevented the crew from receiving an early warning of the internal contamination.