On May 9, 1997, a Mooney M20C (registration N7853V) was involved in an aviation accident near Torreon, NM. Investigators recorded the probable cause as: The piston pin plug working in the bore for an extended period of time, resulting in an aluminum ball in the number four cylinder forward piston pin bore, metal particles throughout the oil system, and damage to connecting rod bearings. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
At 8,500 ft, a pilot saw oil pressure at zero, then oil temperature rose and engine ran rough. After a highway landing, a gust pushed the aircraft off the road into sandy knolls. Investigation found metal particles from a worn piston pin plug.
Accident Sequence
While in cruise flight at an altitude of 8,500 feet mean sea level, the pilot reported that the oil pressure gauge indicated a reading at or near zero. The pilot contacted air traffic control and began searching for a suitable location to land. Subsequently, the oil temperature began to rise and the engine started to operate with a slight roughness. As the aircraft approached a highway on which the pilot intended to land, the propeller governor ceased to regulate engine RPM, and a strong hot odor emanated from the engine.
During the landing roll on the highway, a gust of wind caused the aircraft to depart the left side of the road. The airplane then impacted several sandy knolls.
Post-Impact Examination
A post-impact examination of the engine revealed an aluminum ball located in the forward piston pin bore of the number four cylinder. Investigators stated that this was the result of the piston pin plug working in the bore for an extended period of time. This condition accounted for metal particles throughout the oil system and a rub mark on the interior of the cylinder wall. The oil journal leading to the number two cylinder was found with metal particles emanating from it. The number two and number three connecting rod bearings showed significant damage.
Probable cause
The piston pin plug working in the bore for an extended period of time, resulting in an aluminum ball in the number four cylinder forward piston pin bore, metal particles throughout the oil system, and damage to connecting rod bearings.