What happened
On the afternoon of 16 September 2024, a Robinson R44 Raven II helicopter, registration ZT-RBY, departed from Doornhaag Farm in the North West province. The private flight, operated under Part 91 regulations, was destined for the Tswalu Nature Reserve in the Northern Cape.
While the aircraft was transitioning at approximately 20 feet above the ground at a speed of 20 knots, the pilot experienced a low rotor RPM warning and audible alarm. This loss of rotor speed caused the helicopter to descend rapidly. The pilot attempted an emergency landing on nearby farm terrain; however, during the maneuver, the tail rotor blades struck a farm fence. The subsequent hard impact caused the main rotor blades to strike the tail boom, resulting in substantial damage to the aircraft. There were no fatalities and no injuries to the pilot or the passenger.
The investigation
SACAA AIID examined the mechanical, environmental, and operational factors surrounding the incident. The investigation confirmed that the pilot held a valid Commercial Pilot Licence and the aircraft had a current Certificate of Airworthiness. Maintenance records showed the helicopter had undergone its most recent 100-hour inspection in August 2024, with no defects reported at the time of departure.
Investigators analyzed the atmospheric conditions at the time of the accident. Although the physical elevation of the site was 4,165 feet, the high temperature of 28°C resulted in a density altitude of 6,700 feet. This significantly higher density altitude reduced the engine's effective performance and the aircraft's lift capabilities.
Findings
- The primary cause of the loss of engine power was excessive power demand, where the power required for flight exceeded the power available due to the high-density altitude.
- The high-density altitude environment significantly impaired the helicopter's ability to maintain lift.
- The aircraft was operating at the upper limits of its weight capacity, which likely exacerbated the performance degradation in the thin air.
- The pilot was unable to safely recover the aircraft's altitude due to the low height at which the low RPM warning occurred.