What happened
Approximately ten minutes into the flight, the pilot reported that the engine began running roughly. During troubleshooting, the pilot noted that while most instruments appeared normal, the fuel flow was low. In an attempt to rectify the issue, the pilot advanced the throttle, switched fuel tanks, and activated the fuel boost pump.
Immediately following these actions, a loud bang was heard coming from the engine, resulting in a total loss of all engine power. The pilot subsequently performed a forced landing on an access road located adjacent to a highway. The occupants were not injured.
The investigation
A post-accident examination of the engine revealed that the number four cylinder had separated from the crankcase. Investigators examined the fractures of the cylinder hold-down studs and found no clear evidence of fatigue cracking; instead, the fractures were determined to be representative of overstress separations. Additionally, investigators observed fretting damage on the mating cylinder flange and the crankcase boss, as well as metal folds on the cylinder flange/hole wall corners.