What happened
The incident occurred while the aircraft was conducting a cable patrol flight at an altitude between 300 and 500 feet above ground level. During this phase of operation, the pilot heard a loud noise, followed immediately by a sharp increase in engine RPM and a simultaneous decay in rotor RPM. These symptoms indicated a sudden separation between the powerplant and the main rotor system.
In response to the mechanical failure, the pilot initiated an autorotation to regain lift without engine power. The intended landing area was a nearby parking lot; however, the pilot had to maneuver the aircraft to avoid obstacles including trees and a vehicle. The helicopter ultimately touched down hard on the right skid, which collapsed upon impact and resulted in damage to the main rotor blades.
The investigation
A subsequent examination of the wreckage revealed that the forward coupling splines on the lower pulley drive shaft had failed. This component failure effectively disconnected the engine from the main rotor assembly. Investigators found that the boot designed to retain lubricant in the spline area was missing. Consequently, both the spline and its receptacle were dry of lubricant.
Records indicated that the accident occurred approximately 71 hours after the couplings had been repacked with lubricant. The absence of the retaining boot allowed the lubricant to escape, leading to dry operation and eventual mechanical failure of the splines under load.
Findings
The primary factor contributing to this event was the failure of the forward coupling splines on the lower pulley drive shaft. This failure was directly caused by the lack of lubrication due to a missing retaining boot. The pilot's decision to attempt an autorotation into a populated area with obstacles resulted in a hard landing and subsequent structural damage to the helicopter's skid and rotor system.