What happened
On 17 December 2008, a Robinson R22 helicopter, registration ZS-RNU, was conducting a private flight in the Free State. The aircraft departed from Junctionspruit farm with a passenger on board, intending to return to the same location. After approximately 50 minutes of flight, the aircraft began to lose altitude while cruising at 5,000 ft AMSL.
As the helicopter descended to roughly 200 ft AGL, the pilot received both visual and aural warnings indicating low rotor RPM. In an attempt to stabilize the aircraft, the pilot pushed the cyclic forward and lowered the collective at an airspeed of 40-50 kts. However, this maneuver resulted in a further loss of altitude and a second low RPM warning. Seeking a safe landing spot, the pilot attempted a right turn to find level ground and avoid rocks. During the landing attempt at Killarney farm, the right skid struck a rock, causing the aircraft to bounce and roll onto its left side. The pilot immediately shut down the engine, and both occupants exited the aircraft without injury.
The investigation
An investigation by the SACAA AIID established that the aircraft was well within weight limits, with only about 7 US gallons of fuel remaining at the time of the incident. The weather was reported as calm with clear skies and a temperature of 30°C. The investigation confirmed that the aircraft had undergone its last maintenance inspection approximately 86 hours prior to the accident. The investigation focused on the pilot's flight maneuvers following the activation of the low RPM warnings.
Findings
- The pilot was flying a validly endorsed Robinson R22.
- The aircraft experienced a significant loss of lift due to a rotor stall condition.
- The pilot used an incorrect technique to recover from the low rotor RPM condition.
- The impact with a rock during the landing maneuver caused substantial damage to the main rotor blades, hub, tail rotor, and fuselage.