Incident Summary
During an aerial observation flight, the pilot reported hearing a loud noise emanating from the rear of the helicopter. Immediately after, the aircraft began rotating clockwise and descended uncontrollably, impacting the terrain in a wooded area.
Post-Accident Findings
A post-accident inspection revealed that the tail boom support strut cluster fitting on the left side had failed due to fatigue. The fitting is part of the tail boom structure that supports the rear section of the helicopter.
Relevant Airworthiness Directive
An airworthiness directive (AD) issued in 1976 mandated a daily visual inspection and a dye penetrant inspection every 200 flight hours for this fitting. A logbook entry dated August 15, 1990, stated that all applicable ADs had been complied with at that time.
Conclusion
The accident resulted from a fatigue failure of the left tail boom support strut cluster fitting during flight.