On October 6, 1996, a Bell 47G-5 (registration N7925J) was involved in an aviation accident near Nunda, SD. Investigators recorded the probable cause as: Failure of six bolts attaching the accessory drive gear to the crankshaft due to fatigue cracking resulting from inadequate pre-load torque. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter experienced total engine power loss during an agricultural operation. The pilot performed an autorotative landing, resulting in damage. Investigation revealed failure of six attachment bolts due to fatigue from inadequate pre-load torque.
Event Description
During the second agricultural load of an operation, the helicopter experienced a total loss of engine power. The pilot executed an autorotative landing, but the helicopter sustained damage during the occurrence.
Engine Examination
A subsequent examination of the engine revealed that six bolts attaching the accessory drive gear to the crankshaft had failed. Further analysis of the attachment bolts showed evidence of fatigue cracking. Investigators also found indications that inadequate pre-load torque had been applied to the assembly. The engine had accumulated approximately 400 hours of operation since its last overhaul.
Probable cause
Failure of six bolts attaching the accessory drive gear to the crankshaft due to fatigue cracking resulting from inadequate pre-load torque.