On June 7, 1991, a Hiller UH-12E (registration N507HA) operated by San Joaquin Helicopters was involved in an aviation accident near Huron, CA. Investigators recorded the probable cause as: Fatigue of the No. 3 cylinder connecting rod bolt due to consistent engine overspeed power settings by the pilots. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive; 5 related events involving the same aircraft type or operator are linked below.
A helicopter experienced a total loss of power while returning from an aerial application flight. The pilot attempted an autorotation and sideslip between almond trees but failed. Investigation revealed a fatigued connecting rod bolt due to engine overspeed.
Introduction
A helicopter conducting aerial application operations experienced a complete loss of engine power while returning to the loading area. The subsequent forced landing resulted from a mechanical failure.
Sequence of Events
After completing an aerial application flight, the pilot was returning to the loading area when the helicopter sustained a total loss of power. During the subsequent autorotation, the pilot attempted to sideslip the helicopter between almond trees but was unable to do so.
Examination Findings
An examination of the engine and subsequent metallurgical analysis disclosed that the No. 3 cylinder connecting rod bolt had failed due to fatigue. Company maintenance personnel reported that the fatigue was caused by consistent engine overspeed power settings by the pilots flying the helicopter.
Probable cause
Fatigue of the No. 3 cylinder connecting rod bolt due to consistent engine overspeed power settings by the pilots.