The number 4 cylinder and piston separated from the engine during cruise flight and the airplane collided with vine trellises and terrain during an emergency landing in a vineyard. While in flight at 8,500 feet mean sea level, the pilot heard a loud noise and the engine started running rough. He started to divert to a nearby airport when there was another loud bang from the engine compartment; engine rpm dropped dramatically, and the cockpit started to fill with white smoke. He was being vectored to a nearby airport when the cowling popped open and remained open, substantially increasing the airplane's rate of descent. The pilot set up to land in a vineyard by lowering the airplane's landing gear and approaching the field at 45 degrees to the vineyard rows. He started the landing at 75 knots when the left main landing gear snagged a grape vine row pole and wires. The plane veered to the left, and stopped almost immediately. He recalled that his head hit the dashboard during the landing and his passenger in the right seat was unconscious after the landing. Shoulder harnesses or straps were not installed on this airplane, only lap belts. The calculated landing decelerating force was about 19.5 g's, which resulted in both occupants receiving serious head injuries. Post accident examination of the engine revealed that the number 4 cylinder had departed the engine along with the 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 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. This 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 the cyclic load. 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 that was performed 88.1 hours prior to the accident.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:

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