Piper PA-31-310 N1009S accident near Cabazon, California

Probable cause

A total loss of left engine power due to the failure of the aft crankshaft counterweight retaining washer for reasons that could not be determined during postaccident examinations.

NTSB analysis

The pilot reported that, while in cruise flight about 20 miles from the departure airport, he heard a loud bang come from the left engine and then saw oil gushing out of it, followed by a loss of engine power. The pilot feathered the propeller and attempted to return to the airport. When he realized that the airplane could not maintain altitude, he made a forced landing on uneven, vegetated desert terrain, which resulted in substantial damage to the airplane.

A visual examination of the left engine revealed that the engine crankcase had fractured above the Nos. 5 and 6 cylinders. The postaccident teardown examination of the engine revealed that the aft crankshaft counterweight was partially separated from the crankshaft. The trailing edge roller pin, retaining washer, and part of the retaining snap ring were found displaced from the counterweight and were subsequently located in the oil sump. The retaining washer was fragmented into multiple pieces, which allowed the roller pin to move within its respective slot with the retaining snap ring still installed. As the roller pin became partially extracted from the counterweight, it contacted a connecting rod, which resulted in a catastrophic failure of the engine. Removal of the remaining portion of the snap ring revealed that it was installed correctly. The reason for the failure of the retaining washer could not be determined.

Text from the NTSB. Open the NTSB docket for WPR11LA401 for the full record.