Narrative
The pilot was conducting an agricultural spray operation. While initiating a turn at the south end of a grove, he reduced power and heard a noise. The helicopter yawed slightly and lost all lift, descending straight down. The pilot did not have time to review instruments. Ground crew members reported that they had refueled the helicopter and added chemical shortly before departure. A short time later, they heard the pilot call on the radio that he had crashed. They located the helicopter and pilot at the south end of the grove. Upon arrival, the main rotor was still turning and the engine was running. The pilot was in the cockpit. Attempts to shut down the engine using cockpit controls were unsuccessful due to jamming. The ground crew then accessed the engine compartment and shut off the engine using the fuel control lever. The pilot stated he was coming out of a turn when the engine lost power and the helicopter went down. One ground crewmember reported that the pilot had mentioned during a previous servicing stop that the engine was running hot and losing power.
Examination
Post-accident examination of the engine fuel control revealed that the pump drive clutch assembly was fractured into two pieces. According to the technician who performed the disassembly, this fracture caused disengagement from the N1 flyweight assembly. During normal operation, the N1 flyweight assembly drives the N1 servo system, which regulates fuel supply based on engine speed. With the N1 servo system disengaged, the fuel control remained in the start position, supplying an amount of fuel insufficient to sustain normal engine operation. The technician further observed excessive wear on the pump drive clutch, the mating surface on the flyweight assembly, and the bearing located below the drive clutch. The failure of the pump drive clutch was attributed to this wear, which occurred over an extended period of operation and was not caused by a single incident or impact.
Probable Cause
The fracture of the pump drive clutch assembly due to excessive wear, resulting in disengagement from the N1 flyweight assembly and subsequent insufficient fuel supply to sustain engine operation.