Accident Summary
The pilot reported that the engine was operating normally while flying at 6,500 feet mean sea level when the engine "blew up." He elected to execute a gear-up forced landing to a corn field about two miles from the airport. However, the airplane impacted a ditch during the emergency landing.
Engine Examination
A teardown inspection of the engine revealed a hole in the crankcase approximately 3 inches by 3 inches above the number 4 cylinder, which included the 11 and 1 o'clock deck studs. The number 4 cylinder had separated from the crankcase, and the number 4 connecting rod was not attached to the number 4 crankshaft pin. The number 4 connecting rod and rod cap exhibited signatures consistent with an overload event that deformed and/or sheared the threads on the nuts and bolts.
Cylinder number 4 deck stud nuts 6, 7, and 8 were reported missing from the engine at the accident scene and were subsequently found inside the engine cowling of the aircraft. Cylinder number 4 deck studs 6, 7, and 8 were securely attached to the engine crankcase and their threads were clean and undamaged. The 2 o'clock cylinder deck stud was fractured and missing. The remaining deck studs and thru-bolts all exhibited ductile fractures indicative of tensile overload.
Break-away torque measurements were recorded for the thru-bolts and deck studs of the other five cylinders. The inspection revealed that numerous thru-bolts and studs had break-away torque values that did not comply with the manufacturer's limitations.
Maintenance History
The aircraft engine's logbook indicated that the engine received an annual maintenance inspection on February 16, 2005. The engine logbook entry stated: "Removed and overhauled all 6 cylinders. Installed new rings, ground valves, cleaned and gapped spark plugs." The tach time indicated 5,380.5 hours at the time of the annual inspection. At the time of the accident, the tach time was 5,662.24 hours, which was 281.74 hours since the cylinders had been replaced.