Accident Overview
Approximately two minutes after takeoff, the pilot of a helicopter detected a slight 1-to-1 vibration through the cyclic control. This type of vibration is typically associated with main rotor dampers. The pilot responded by stirring the cyclic in an attempt to purge air from the blade dampers, which temporarily eliminated the issue.
Sequence of Events
The vibration recurred shortly thereafter and could not be eliminated. The intensity of the vibration increased to the point that the pilot lost control of the helicopter. An emergency landing was attempted, with touchdown occurring at approximately 60 knots. Upon impact, the helicopter came to rest inverted and was largely consumed by a post-impact fire.
Investigation Findings
Metallurgical analysis was conducted on a failed blade restraining cable. The examination revealed a catastrophic failure in a low cycle fatigue mode, consistent with a secondary cause due to short-term vibrations. Additionally, there was evidence of plastic deformation on the cable caused by vibratory loads originating from the damper systems. The extent of fire damage prevented further failure analysis of other components.