On September 18, 1986, a Hiller UH-12E (registration N688SB) operated by West Farm Air was involved in an aviation accident near Lemoore, CA. Investigators recorded the probable cause as: The number 3 connecting rod bolts failed, and the remaining connecting rod bolts had torque values approximately 50% less than required. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter rolled onto its side during an autorotation after the pilot heard a loud pop and saw an engine fire, losing cyclic control. Investigation found connecting rod bolt failure and insufficient torque values.
Incident Overview
The pilot of a helicopter reported that while returning to the loading area after spraying a field, he heard a loud pop emanating from the engine compartment. Shortly thereafter, he observed a fire in the engine compartment and lost forward and aft control of the helicopter. The pilot immediately executed an autorotation. The helicopter rolled over onto its side on touchdown.
Post-Accident Investigation
Post-accident examination of the engine disclosed that the number 3 connecting rod bolts failed. Further examination revealed that the remaining connecting rod bolts had torque values approximately 50% less than the required values.
Probable cause
The number 3 connecting rod bolts failed, and the remaining connecting rod bolts had torque values approximately 50% less than required.