What happened
On June 17, 2017, a Cessna U206E seaplane, registration LN-BEM, was conducting a private flight in Norway. After an initial leg from Sola seaplane base to Helgøysund and a subsequent landing at Frøylandsvatnet, the pilot prepared for takeoff from the lake. During the departure climb, at an altitude of approximately 200–250 feet, the engine suffered a sudden and total loss of power accompanied by a loud bang and visible smoke.
To avoid nearby residential buildings, the pilot executed a sharp turn to the east. Despite the difficulty of controlling the aircraft at low speed, the pilot managed a hard landing on the water. The aircraft bounced before settling and eventually slid into the shoreline, where it struck land at roughly 18–20 knots and flipped onto its back. All three occupants escaped the aircraft without injury, aided by the use of four-point safety harnesses.
The investigation
The Norwegian Safety Investigation Authority (NSIA) examined the engine, the wreckage, and flight data. Investigators found that the engine was nearly devoid of oil upon inspection. Metallurgical analysis of the connecting rod from cylinder number four revealed a fracture caused by extreme tension and overheating, with temperatures reaching between 900 and 1,000 °C.
Further examination of the engine components showed that the bearing shells for rod number five had been deformed and worn down, creating excessive clearance. Additionally, evidence of detonation was found on the piston crown of cylinder number five. An oil film was also observed on the water near Helgøysund, suggesting a leak may have occurred earlier in the flight.
Findings
- The primary cause of the accident was the loss of engine power following the failure of connecting rod number four.
- The rod failure was triggered by the bearing seizing due to friction and overheating.
- This overheating was caused by significant amounts of engine oil being vented out of the engine through the crankcase ventilation system.
- The oil loss likely began after the departure from Sola, as evidenced by an oil slick observed by a witness near Helgøysund.
- Excessive clearance in the bearing for rod number five, likely resulting from previous detonation, allowed oil to escape the lubrication circuit.
- The pilot's decision to steer away from residential areas and the use of specialized safety belts prevented injuries to the crew and passengers.