Incident Overview
During cruise flight at 8,500 feet mean sea level, the pilot heard a loud noise and the engine began running rough. The pilot started to divert to a nearby airport when a second loud bang emanated from the engine compartment, engine rpm dropped dramatically, and white smoke filled the cockpit. While being vectored to an alternate airport, the cowling popped open and remained open, substantially increasing the airplane's rate of descent.
Emergency Landing
The pilot set up to land in a vineyard by lowering the landing gear and approaching the field at 45 degrees to the rows. The landing was initiated at 75 knots when the left main landing gear snagged a grape vine row pole and wires. The airplane veered left and stopped almost immediately. The pilot reported that his head hit the dashboard during the landing and that his passenger in the right seat was unconscious after the landing. The airplane was equipped only with lap belts; shoulder harnesses or straps were not installed. The calculated landing decelerating force was approximately 19.5 g's, resulting in both occupants receiving serious head injuries.
Post-Accident Examination
Examination of the engine revealed that the number 4 cylinder had departed the engine along with its piston. The engine case halves were fractured directly above the number 4 cylinder location. The two upper right cylinder hold-down studs were present and appeared undamaged, while the bottom studs were sheared across their diameter above the engine case, and the left-hand side studs were sheared across their diameters at the engine case surface. The number 4 cylinder base pad surface of the crankcase exhibited fretting at the 3 and 9 o'clock positions.
Investigation Findings
The observed condition is consistent with the right-hand and bottom cylinder hold-down nuts backing off during engine operation, allowing the number 4 cylinder to move in rhythm with the piston until the remaining cylinder studs on the left side failed under cyclic load. According to the investigation, the backing off of the cylinder hold-down nuts would not normally occur if the nuts had been properly torqued when installed. The number 2, 4, 5, and 6 cylinders had been removed and replaced or reinstalled 186.2 hours prior to the accident. The under-torqued cylinder hold-down nuts were not detected during the 100-hour inspection performed 88.1 hours before the accident.