Incident Summary
A turbine-powered helicopter was involved in an accident during takeoff. According to the pilot, he began a hover approximately three feet above the ground, turned the helicopter into the wind, and initiated a takeoff. Witnesses reported that the helicopter had started moving forward and reached an altitude of about 25 feet when they heard a change in the engine sound.
Pilot Statement
The pilot stated, "As power was brought in, a loud metallic type pop was heard." He reported that the helicopter yawed left after the pop and did not respond to control inputs. The helicopter flew over an embankment, and its rotor blades struck the side of the embankment. Following the blade strike, the helicopter rolled onto its left side, then onto its right side, and came to rest.
Examination Findings
Examination of the helicopter and its components revealed that the attachment bolts connecting the driveshaft to the geared portion of the driveshaft coupling had failed. Metallurgical examination showed that two of the bolts had failed due to fatigue, while the other two bolts exhibited evidence of overload failure. Additionally, after disassembly, a tension (centering) spring was discovered to be missing from the transmission end of the driveshaft; this missing spring would have allowed the bolt heads to contact the grease retaining plate.
The last annual inspection of the helicopter and engine was performed on November 3, 1993.