On July 6, 1985, a Hughes 369HS (registration N5410W) operated by Rushmore Helicopters, Inc. was involved in an aviation accident near Keystone, SD. This summary draws on records from the U.S. National Transportation Safety Board (NTSB) historical archive.
A helicopter's main rotor hub and blades separated in flight 279 hours after an overhaul by a repair station not authorized by the manufacturer. The torsion-tension strap pack failed due to fatigue cracking from corrosion pits, with nine failed laminants exceeding the allowable limit of three.
Accident Overview
A helicopter experienced an in-flight failure of the main rotor system, resulting in separation of the main rotor hub and blades. The incident occurred 279 flight hours after the hub had been overhauled. The overhaul was performed by a repair station that was not authorized by Hughes Helicopters for hub overhauls. The helicopter subsequently crashed into a densely wooded area.
Post-Accident Examination
Investigators examined the main rotor system and found that the torsion-tension strap pack had failed. The failure was attributed to fatigue cracking within the stainless steel laminants of the strap pack. Further metallurgical examination showed evidence of corrosion pitting on the laminant surfaces. These corrosion pits served as the initiation points for the fatigue cracks.
Condition of the Strap Pack
The manufacturer specifies a maximum allowable number of laminant failures in any one strap of three. Examination of the failed strap pack revealed that nine laminants had failed due to fatigue.