Accident Sequence
The helicopter was contracted by Georgia Power Company to perform inspections and maintenance on transmission power lines. After completing a shield wire splice procedure, both the pilot and passenger heard a popping sound from the rear of the helicopter. The helicopter then began to spin violently to the right. To avoid striking the shield wire, the pilot increased the collective pitch, which accelerated the turn. After clearing the wire and making a second turn, the pilot reduced the throttle to slow the spin, leveled the aircraft, and subsequently collided with trees in a level flight attitude.
Examination Findings
The pilot did not report any mechanical malfunctions prior to the accident. Post-accident examination of the helicopter revealed extensive damage: the main rotor blades had separated, the left skid was detached, the windshield was broken, and there was upward crush damage along the bottom of the fuselage aft of the skid. The tail rotor assembly had separated at the driveshaft, crush damage was noted along the sides of the tail section, and the horizontal stabilizer had detached from the tail.
Laboratory Analysis
The tail rotor assembly and part of the tail rotor drive shaft were sent to the NTSB Materials Laboratory in Washington, D.C., for further examination. The Materials Lab determined that the tail rotor assembly failed due to a fatigue fracture of the input gearshaft in the tail rotor gearbox assembly. This fracture led to jamming of the gearbox, which caused an overstress fracture of the tail rotor drive shaft. The failure pattern was consistent with the engine driving the shaft at the forward end while the aft end was stopped.