Incident Overview
During cruise at 5,500 feet, the standby power annunciator light illuminated, followed by a loss of oil pressure. The pilot initiated a turn back toward the airport, but the engine progressively lost power and the propeller moved to a feathered position. An emergency landing was attempted at night. During the landing, the aircraft struck a power line and trees, then crashed.
Investigation Findings
Post-accident examination included an engine teardown. Disassembly of the reduction gearbox scavenge pump revealed that a retaining nut and tang washer within the scavenge pump had backed off. These components lodged between the spur gears, causing the scavenge pump drive shaft to shear. As a result, oil pressure was lost and the propeller feathered.
The official probable cause findings identified multiple occurrence phases:
- Airframe/component/system failure or malfunction during cruise: the oil scavenge pump was loose, jammed, and failed totally, initiated by the loose retaining hardware.
- Loss of engine power (total) due to mechanical failure during cruise: oil starvation.
- Forced landing during emergency descent.
Additional Contributing Factors
The investigation noted that the forced landing occurred at night, and the aircraft collided with transmission wires and trees during the landing attempt.
The sequence of events was: initial component failure in the scavenge pump led to oil starvation, total engine power loss, and a forced landing that resulted in collision with obstacles.