What happened
While cruising at 5,500 feet, the crew observed the standby power annunciator light illuminate, which was immediately followed by a drop in oil pressure. In an attempt to return to the airport, the pilot experienced a continuous decline in engine performance, eventually resulting in the propeller moving into a feathered position. During the subsequent nighttime emergency landing, the aircraft struck transmission lines and trees before crashing.
Findings
An investigation involving the disassembly of the engine's reduction gearbox scavenge pump determined that a retaining nut and tang washer had become loose within the assembly. These components moved into a position where they became lodged between the spur gears, causing the scavenge pump drive shaft to shear. This mechanical failure led to total oil starvation within the lubricating system, which triggered the loss of oil pressure and the subsequent propeller feathering.