What happened
On 19 February 2024, a Robinson R44 Raven II, registration ZT-RBZ, was engaged in a game culling and capturing operation at Elandsheuwel Farm near Potchefstroom. The flight, conducted under visual meteorological conditions, had departed from Doornhaag Farm earlier that day. During the operation, the pilot had removed three of the four helicopter doors to facilitate the task.
While repositioning the aircraft at an altitude of approximately 50 feet above ground level and a forward speed of 30 knots, the pilot experienced a sudden drop in main rotor RPM, which fell from 102% to 87%. This triggered the low rotor RPM warning horn and instrument panel light. Although the pilot attempted to increase throttle and lower the collective to recover, the aircraft could not maintain sufficient rotor speed. The helicopter performed a hard landing, during which the main rotor blades struck the tail boom, severing it. Despite the substantial damage to the aircraft, there were no injuries to the pilot or the two passengers on board.
The investigation
SACAA AIID examined the flight conditions and performance parameters of the aircraft. The investigation confirmed that the pilot was medically fit and the aircraft was well-maintained, with a valid Certificate of Airworthiness and a recent periodic inspection. The investigation focused on the aerodynamic state of the helicopter during the maneuver.
Investigators analyzed the impact of high-density altitude on the aircraft's performance. At the time of the accident, the ambient temperature was 32°C at an elevation of 4,641 feet, resulting in a calculated density altitude of approximately 9,600 feet. The investigation also reviewed the pilot's flight path relative to the manufacturer's recommended velocity-height diagrams.
Findings
- The pilot was operating the helicopter within the prohibited shaded area of the velocity-height diagram, specifically at 50 feet AGL and 30 knots.
- The required power for the maneuver exceeded the power available due to the high-density altitude conditions.
- The engine manifold pressure was at 23 In Hg, which was insufficient to maintain the necessary rotor RPM under the prevailing atmospheric conditions.
- The loss of rotor RPM led to a main rotor stall, causing the blades to strike and sever the tail boom during the descent.