Incident Summary
Shortly after takeoff, while climbing through approximately 1,000 ft above ground level (AGL), the pilot of a helicopter experienced airframe vibration accompanied by a grinding sound. In response, the pilot entered autorotation. At approximately 200 ft AGL, the tail rotor and a portion of the tail rotor gear box separated from the helicopter. Following the separation, the helicopter rotated an estimated 720 degrees before impacting the ground.
Maintenance Directives and Inspection Background
An airworthiness directive (AD) and a related service bulletin (SB) were in effect at the time of the accident. These directives required the installation of rivets within 300 hours of flight time to prevent possible loss of tail rotor control. Due to the helicopter's seasonal use, it had accumulated less than 160 hours since the issuance of the AD. Additionally, the SB mandated a daily visual inspection of the tail rotor blades for debonding, among other checks.
Post-Accident Examination
One of the tail rotor blades was found to be missing its abrasion strip. Examination of the intact tail rotor blade revealed approximately 40% debonding; however, this blade would have passed the tap test as specified in the manufacturer's service bulletin. The failed blade was estimated to be 90% debonded. The daily visual inspection required by the service bulletin reportedly did not detect the ongoing debonding process.