On October 21, 1998, a Mooney M20C (registration N6405U) was involved in an aviation accident near Englewood, CO. Investigators recorded the probable cause as: Improper torquing of the camshaft gear retaining bolts during the engine overhaul, leading to loss of torque and subsequent failure of the valve train. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
During climb after departure, the engine lost power. The pilot attempted to return but made an emergency landing in a field. Examination revealed camshaft gear retaining bolts lost torque due to improper torquing during overhaul.
Accident Narrative
Following departure, while climbing, the engine lost power. The pilot reported the emergency to the tower and attempted to return to the airport. Determining that he would be unable to make it back, he executed an emergency landing in a field located one and a half miles southeast of the airport.
Postaccident Examination
A postaccident examination of the engine revealed that the camshaft gear retaining bolts, which connect the camshaft gear to the camshaft, had lost torque. As a result, the lobes failed to open their respective valves, creating a discontinuity in the gear train. The tread on each of the four camshaft gear bolts exhibited shiny, worn patterns. According to a Textron Lycoming engineer, loss of camshaft retaining bolt torque is indicative of the bolts being improperly torqued during the engine overhaul. Aircraft maintenance records indicated that the engine was overhauled on March 15, 1985, and had accumulated 833 hours since that overhaul.
Probable cause
Improper torquing of the camshaft gear retaining bolts during the engine overhaul, leading to loss of torque and subsequent failure of the valve train.