Incident Overview
While descending from 5,000 feet to 3,000 feet, the pilot informed air traffic control that the left engine had failed and the engine cowling was open. The crew reported that after feathering the left propeller and applying full power on the right engine, they could not arrest a rate of descent between 300 and 500 feet per minute. The crew then notified ATC of their intention to land in the water.
Emergency Ditching
All occupants exited the aircraft through the left front pilot's emergency door. The individuals remained in the water for approximately 30 minutes before being rescued. One passenger was in cardiac arrest when retrieved from the water.
Examination Findings
Examination of the left engine revealed that the #2 cylinder had separated from the engine in flight. The separation resulted from high stress fatigue cracking of the cylinder hold down studs and the #3 main bearing thru-studs. The fatigue in the studs occurred because the cylinder fastener preload forces were either initially inadequate or lost during service. Maintenance records indicated that the thru-stud had been replaced 80 service hours prior to the accident. Further examination showed that the cylinder hold down studs and the #3 main bearing thru-studs were improperly torqued, leading to low initial preload on the fasteners. Additionally, incorrect installation of the oversize thru-studs, per existing service information, could have been a factor in the improper torquing. The locations of the fatigue origins and the edge worn into the deck suggest that the upper studs were probably the first to fail, allowing the cylinder to rock on the lower rear corner of the cylinder flange.
Probable Cause
The total loss of left engine power was caused by an in-flight separation of the #2 cylinder. The cylinder separated due to high stress fatigue cracking of the cylinder hold down studs and the #3 main bearing thru-studs. Factors contributing to the accident were improper torquing of the studs and failure of maintenance personnel to properly comply with service information.