On September 23, 1993, a Bell UH-1B (registration N3979C) operated by Clearwater Management Co. Inc. was involved in an aviation accident near Pierce, ID. Investigators recorded the probable cause as: SEPARATION OF THE 90 DEGREE GEARBOX AFTER FATIGUE FAILURE OF THE GEARBOX ATTACH BOLTS. FACTORS INCLUDE HILLY/MOUNTAINOUS TERRAIN. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
While maneuvering near a mountainous refueling site, a helicopter's 90-degree gearbox detached. The pilot attempted a run-on landing on a logging road; a hard flare caused skid collapse and tail boom separation. Metallurgical analysis revealed fatigue failure of all six attach bolts.
Incident Overview
During maneuvering near a refueling site in hilly and mountainous terrain, the 90-degree gearbox separated from the aircraft. The pilot subsequently attempted a run-on forced landing on a nearby logging road.
Landing and Post-Impact Damage
During the flare, the aircraft landed hard. The impact resulted in a collapse of the skids and separation of the remaining portion of the tail boom.
Metallurgical Findings
Metallurgical analysis of the gearbox attach bolts revealed that all six bolts had experienced reverse-bending fatigue failure.
Component History
Operator records indicated that the gearbox was a new surplus part that had accumulated 136 hours of operation since new. Historical records that would confirm the unit's total time since being shipped to the Philippines as a new surplus part in 1981 could not be located.