Incident Overview
While descending from 5,000 to 3,000 feet, the pilot informed air traffic control that the left engine had failed and the engine cowling was open. After feathering the left propeller and operating the right engine at full power, the crew could not arrest a descent rate of 300 to 500 feet per minute. The crew then advised ATC that they would be landing on the water.
Evacuation and Rescue
All occupants exited the airplane through the left front pilot's emergency door. The individuals were 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 due to high-stress fatigue cracking of the cylinder hold-down studs and the #3 main bearing through-studs. The fatigue occurred as a result of cylinder fastener preload forces that were either initially inadequate or lost during service.
Maintenance History
Maintenance records indicated that the through-stud had been replaced 80 service hours prior to the accident. Examination of the cylinder hold-down studs and the #3 main bearing through-studs showed they were improperly torqued, resulting in low initial preload on the fasteners. Incorrect installation of the oversize through-studs, per existing service information, could have also been a factor in the improper torquing.
Fatigue Sequence
The locations of the fatigue origins and an edge worn into the deck indicated that the upper studs were probably the first to fail, allowing the cylinder to rock on the lower rear corner of the cylinder flange.