What happened
During a descent from 5,000 feet to 3,000 feet, the pilot reported to air traffic control that the left engine had failed and that the engine cowling was open. Despite feathering the left propeller and applying full power to the right engine, the crew was unable to stop a descent rate of 300 to 500 feet per minute. The crew subsequently notified authorities of their intention to perform an emergency landing in the water.
Following the impact, all occupants evacuated the aircraft through the emergency door on the left front side of the cockpit. The occupants remained in the water for roughly 30 minutes before rescue operations were completed. During the recovery, one passenger was found to be in cardiac arrest.
Findings
An investigation of the left engine determined that the #2 cylinder had detached from the engine while in flight. This separation was caused by high stress fatigue cracking of the cylinder hold down studs and the #3 main bearing thru-studs. The fatigue was attributed to the fastener preload forces being either insufficient from the start or lost during the aircraft's service life.
Maintenance documentation showed that the thru-stud had been replaced 80 service hours before the accident. Analysis of the hardware revealed that the studs and thru-studs had been improperly torqued, leading to inadequate initial preload. Additionally, the installation of oversize thru-studs may have contributed to the torque issues. The investigation concluded that the upper studs likely failed first, causing the cylinder to rock on the lower rear corner of its flange.