Accident Description
On a regularly scheduled flight from Vancouver Harbour water aerodrome to Victoria Harbour water aerodrome, a de Havilland DHC-6-100 Twin Otter with floats (serial number 086) operated by West Coast Air Ltd. departed at approximately 1510 Pacific standard time. The aircraft carried two crew members and 15 passengers. Shortly after lift-off, a loud bang and a noise similar to gravel hitting the aircraft were heard. Simultaneously, a flame appeared from the forward section of the No 2 (right-hand) engine, and the engine lost all propulsion. The aircraft was estimated to be between 50 and 100 feet above the water. About 25 seconds later, the aircraft struck the water in a nose-down, right wing-low attitude. The right float and right wing separated from the fuselage upon impact. The aircraft remained upright and partially submerged. Occupants evacuated through the main entrance door and pilot doors, then gathered on top of the left wing and fuselage. Several vessels, including a public transit SeaBus, arrived within minutes. The SeaBus deployed an inflatable raft, and occupants were taken ashore and transported to hospital for observation. No serious injuries were reported. The aircraft later sank, but all wreckage was recovered within five days.
Flight Sequence and Crew Actions
The accident flight was the fifth take-off of the day for the crew. Normal pre-flight checks and safety briefings were completed per standard operating procedures. Take-off proceeded normally until the aircraft approached 70 knots indicated airspeed, when the propulsion loss caused a sudden yaw to the right. The pilot flying applied full left rudder. Both crew members acknowledged the engine power loss and agreed to land straight ahead. They selected full flaps, identified the No 2 engine as affected, pulled the fire extinguisher T-handle, and activated the fuel emergency shut-off valve. Take-off power was maintained on the No 1 engine until immediately before impact. The crew did not attempt to manually feather the right propeller, and the aircraft was not equipped with an optional auto-feather system. Weather conditions were visual with an easterly wind of 5 to 10 knots; weather did not contribute to the occurrence.
Engine Failure
Post-accident inspection of the No 2 engine, a Pratt & Whitney PT6A-20 (serial number 21867), revealed that the power turbine had disconnected from the propeller due to a fractured first-stage planetary gear in the propeller reduction gear box (RGB). The gear fracture caused extensive internal damage, and the power turbine disk oversped and shed all blades as designed. Metallurgical analysis determined that the planetary gear fracture resulted from surface damage on the gear bore caused by rotation of the bearing sleeve within the bore. The most probable scenario is that small wire fragments from the planetary gear oil strainer screen contaminated the lubrication oil and entered the bearing sleeve interface, binding the sleeve to the planetary gear shaft (axle). This caused the bearing sleeve to rotate within the gear bore. The manufacturer, Pratt & Whitney Canada Corp., conducted a tear-down and analysis (report TL-1626) and found that the oil strainer had a deformed Teflon insert, pinched O-ring, bent neck, and fretting, suggesting improper installation. The screen wires exhibited fatigue fractures, some with smearing and corrosion indicating they were not recent. Release of wire fragments may have initiated or contributed to the bearing sleeve distress.
Investigation Findings
The aircraft was not equipped with flight-instrument-panel annunciators for the magnetic chip detectors, nor was it required by regulations. The auto-feather system was also not installed. The calculated take-off weight and centre of gravity were within limits. The Universal 30A cockpit voice recorder was recovered and provided information. No flight data recorder was required or installed. The engine fire warning system did not activate because the internal fire was not detected by the system; the crew discharged the fire extinguisher after seeing flame reflection, but it would not have been effective. The fire was extinguished by water immersion upon impact. The aircraft had undergone a 100-hour inspection about 46 flight hours prior, which included chip detector checks and oil sample analysis; no anomalies were detected. The unsafe condition developed over a longer period, but no evidence was apparent at the last inspection.
Probable Cause
The most probable scenario is that the bearing sleeve bound to the associated planetary gear shaft (axle) because small pieces of wire from the individual planetary gear oil strainer screen contaminated the lubrication oil and entered the bearing sleeve / planetary gear shaft (axle) interface. The binding caused the bearing sleeve to rotate within the gear bore, leading to fracturing of the planetary gear and subsequent engine power loss.