Incident Overview
While flying at an altitude between 300 and 500 feet above ground level on a cable patrol flight, the pilot heard a loud noise. Immediately thereafter, the rotor RPM began to decay while the engine RPM increased sharply. The pilot initiated an autorotation to a nearby parking lot, but had to maneuver to avoid trees and a vehicle. The helicopter touched down hard on the right skid, collapsing the skid and causing damage to the main rotor blades.
Investigation Findings
An investigation determined that the forward coupling splines on the lower pulley drive shaft, part number 269A5559-3, had failed. This failure effectively disconnected the engine from the main rotor, leading to the loss of rotor RPM. The investigation also revealed that the rubber boot designed to retain lubricant in the spline area was missing, and the spline and receptacle were dry of lubricant. The accident occurred approximately 71 hours after the couplings had been repacked with lubricant.
Conclusion
The failure of the forward coupling splines resulted from a missing lubricant retention boot and a consequent lack of lubricant, which led to the disconnection of the engine from the main rotor. No further causal factors were identified in the source material.