What happened
On 3 December 2006, a Robinson R44 Astro, registration ZS-RKW, was conducting a private flight from a smallholding in Randfontein toward Wonderboom Aerodrome. The flight, which included two passengers, began after the pilot had completed an earlier solo trip to the same location.
During the return leg, the pilot established a hover just above the ground to stabilize manifold pressure. As the aircraft attempted to transition into forward flight to clear a three-foot fence located approximately 30 feet ahead, the pilot increased the collective pitch. This maneuver triggered the low rotor RPM warning horn. In an attempt to stabilize the aircraft, the pilot leveled the cyclic and lowered the collective to descend into ground effect; however, the aircraft struck the ground, causing the skid gear to collapse and the fuselage to slide approximately two meters.
The investigation
SACAA AIID examined the aircraft's maintenance and performance capabilities. The investigation found that the aircraft had exceeded its 100-hour mandatory periodic inspection (MPI) by 20 hours, meaning the Certificate of Airworthiness was invalid at the time of the accident.
Investigators also analyzed the environmental conditions and aircraft loading. The flight took place in high-density altitude conditions, with a temperature of 29°C and a pressure altitude of 5,500 ft. While the aircraft remained within its maximum takeoff weight, the investigation noted that the helicopter was operating near its weight limit for a hover in ground effect and was significantly overweight for an out-of-ground-effect maneuver under these specific atmospheric conditions.
Findings
- The primary cause of the accident was a loss of rotor RPM when the aircraft transitioned from a hover in ground effect to out of ground effect.
- The pilot's limited experience on the Robinson R44 Astro type contributed to the difficulty in managing the power requirements.
- The pilot failed to perform an accurate weight and balance calculation, relying instead on a mental calculation that did not account for the performance limitations imposed by the high density altitude.
- The aircraft was operating with an invalid Certificate of Airworthiness due to a lapsed mandatory periodic inspection.