What happened
The aircraft was operating at a cruising altitude of 5500 feet when the standby power annunciator light illuminated. This warning was immediately followed by a complete loss of oil pressure. The pilot initiated a return toward the airport; however, the engine experienced a progressive reduction in power output. Concurrently, the propeller moved to a feathered position. During the subsequent emergency landing attempt at night, the aircraft struck overhead power lines and trees before crashing.
The investigation
A detailed teardown of the engine was conducted to determine the root cause of the mechanical failure. Disassembly of the reduction gearbox scavenge pump revealed that a retaining nut and tang washer located inside the pump had backed off from their intended position. These components lodged between the spur gears, which caused the scavenge pump drive shaft to shear. This mechanical separation resulted in the total loss of oil pressure, which allowed the propeller system to feather automatically.