What happened
On August 21, 2021, at approximately 1325 Alaska daylight time, a float-equipped de Havilland DHC-3 Otter, registration N560TR, was performing a commercial flight under 14 CFR Part 135 near Bettles, Alaska. While the pilot was on the base leg of an approach to a remote lake, a loud bang was heard, followed by a total loss of engine power.
Realizing the aircraft could not reach the intended destination, the pilot turned the aircraft into the wind and performed a forced landing on tundra-covered terrain. The impact resulted in substantial damage to the fuselage. All occupants, including the pilot and two passengers, were not injured.
The investigation
Post-accident examination of the engine revealed that the engine had seized and the propeller spline had fractured into two pieces. Investigators found a substantial amount of metal fragmentation within the oil screen. A borescope inspection showed broken internal engine components, which were so extensive that the aft accessory wall could not be removed for further inspection. A review of the aircraft's maintenance records showed no issues, and no regulatory noncompliance was identified.