What happened
On 15 October 2015, a Robinson R44 Raven II, registration ZS-RRZ, was conducting a private game capture operation at Balmoral Farm in Ammerskloof, Mpumalanga. While the pilot was hovering at an altitude of approximately 10 to 15 meters above the ground to assist in netting livestock, an unusual loud noise was heard within the cockpit.
Following the noise, the pilot observed a high engine RPM accompanied by a low rotor RPM indication. The low rotor RPM warning horn activated, and the clutch actuator caution light illuminated. Recognizing the severity of the situation, the pilot immediately lowered the collective to attempt an autorotation. However, because the aircraft was hovering at a height exceeding the safety parameters defined in the Pilot's Operating Handbook (POH), the pilot was unable to maintain sufficient rotor RPM. The helicopter descended rapidly and struck the ground with significant force, coming to rest on its belly. The pilot and passenger were uninjured, though the aircraft sustained substantial damage to its skids and underbelly.
The investigation
SACAA AIID investigators examined the wreckage and the aircraft's maintenance history. The investigation focused on the drive system and the circumstances of the descent. The inquiry confirmed that the aircraft's documentation and maintenance records were up to date, and the lower belt tension actuator bearing had been serviced in accordance with the required maintenance schedule only months prior to the event.
Investigators also reviewed the flight profile against the manufacturer's Height/Velocity diagram. They determined that the pilot's position in the hover was within the "Avoid" shaded area of the POH, which specifies flight profiles that are unsafe in the event of a power or transmission failure. The investigation also noted that while the aircraft was being operated for private use, the registration transfer from the previous owner to the new private owner had not yet been finalized.
Findings
- The primary cause of the transmission failure was the seizing of the lower belt tension actuator bearing.
- This bearing failure caused the vee-belts to become displaced and slack, leading to the loss of power transmission between the engine and the rotor system.
- The pilot was operating within the unsafe altitude/velocity parameters recommended by the POH for an emergency of this nature.
- The lack of sufficient forward airspeed and altitude prevented the pilot from successfully executing a flare to cushion the impact.
- The pilot's emergency procedures, specifically the immediate lowering of the collective, were performed correctly according to the manufacturer's instructions.