FTW03LA188

The 6,533-hour pilot reported that the engine developed a vibration during cruise flight. The pilot suspected a runaway propeller, and elected to shut down the engine. During the forced landing, the airplane went through a fence and impacted terrain. Examination of the engine revealed that the number three cylinder had separated from the 200 horsepower Lycoming engine. Metallurgical evaluation by the NTSB Materials Laboratory revealed that the crankcase bolts were found to be fractured from stress fatigue cracking, and concluded that fatigue failure on cylinder mounting hardware is usually the result of insufficient torque loading.

On July 18, 2003, about 1535 central daylight time, a Mooney M20F single-engine airplane, N353JT, was substantially damaged when it impacted the ground during a forced landing following a loss of engine power near Kerrville Municipal Airport (ERV), Kerrville, Texas. The commercial pilot and passenger sustained minor injuries. The airplane, registered to and operated by the pilot, was operating under 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed, and a visual flight rules (VFR) flight plan was filed for the personal flight. The flight originated from the San Angelo Regional Airport (SJT), near San Angelo, Texas, at 1450.

The 6,533-hour pilot reported in the Pilot/Operator Aircraft Accident Report (NTSB Form 6120.1/2) that 40 minutes into a planned one hour and 26 minute flight to San Antonio, Texas, the airplane experienced an engine vibration. The pilot stated that he reduced manifold pressure and began a slow descent, and at 1,400 agl, "the alternator belt failed and I heard the propeller started to runaway, with significantly increased vibration." The pilot added, "to prevent a real runaway," he called ERV Unicom and told them that he was shutting down the engine and planning to land in a field about 20 miles west of ERV airport. During landing roll, the airplane went through a fence and impacted the bank of a creek bed.

The FAA inspector, who responded to the site, reported substantial structural damage throughout the fuselage, wings, and flight controls. The number three cylinder of the engine had separated from the crankcase and was hanging by hoses and cables.

The wreckage of the airplane was released for recovery to the Air Salvage of Dallas (ASOD) facility in Lancaster, Texas, and the Lycoming IO-360-A1A engine, serial number L-3459-51A, was removed from the airframe for examination. Maintenance records revealed that the engine was manufactured on March 22, 1967, and had a total flight time of 4,180.8 hours, with 64.2 hours since the last annual inspection and 191.6 hours since overhaul.

On August 12, 2003, the owner/representative of the airplane and NTSB personnel removed broken bolts from the crankcase and the number three cylinder. The hardware was examined by the NTSB's material laboratory and found to be fractured from stress fatigue cracking, and concluded that " a fatigue failure on cylinder mounting hardware is usually the result of insufficient torque loading." The laboratory also found that wear damage to treads of two of the bolts was consistent with movement between the cylinder and the engine case, after fracture of these pieces, but prior to the final separation of the cylinder from the case.

Examination of the maintenance records by an FAA airworthiness inspector did not reveal any anomalies.