Synopsis
At approximately 0930Z on 11 August 2019, a Robinson R44 Raven II helicopter (ZS-TTC) departed from a private farm in Vaalwater, Limpopo Province, for a scenic flight with the pilot and two passengers on board. The helicopter weighed 2390 lbs at takeoff, with full fuel tanks. After takeoff, the pilot entered a hover and transitioned forward at 10 to 20 knots along the left side of the Vaalwater Dam. Trees lined the right side of the dam. The pilot reported that wind and trees disrupted airflow into the tail rotor system, causing main rotor RPM to decrease and a drop in engine power. To recover, the pilot applied a flare to increase main rotor RPM, but the tail rotor stinger struck the water, causing the main rotors to flap downward and sever the tail boom. The helicopter subsequently impacted the water and was recovered to a hangar in Wonderboom Aerodrome. The pilot and one passenger were uninjured; the other passenger sustained minor injuries and was hospitalized. The accident occurred in daylight conditions (wind 280°/7kt, temperature 21°C, visibility CAVOK).
Factual Information
Investigation revealed that the helicopter took off toward high trees ahead. The calculated safe operating weight for the ambient conditions (pressure altitude 4739 ft, outside air temperature 20°C) was 2120 lbs, but the helicopter's actual weight was 2390 lbs—270 lbs above the limit. The helicopter was operating between 10 and 20 knots indicated airspeed; for a safe transition and climb, 50 knots or above would be required. The out-of-ground effect ceiling vs. gross weight chart indicated the helicopter was above its performance capability. The height-velocity diagram advises against operating in the low-speed regime where the accident occurred. The pilot held a valid commercial pilot license (3192.6 hours on type) and a Class 1 medical. The helicopter had a current certificate of airworthiness and had flown 70.2 hours since its last mandatory periodic inspection. Weather was not considered a contributing factor.
Findings
The helicopter was above the required weight for the altitude and conditions, reducing its performance. The pilot maneuvered the helicopter to the left before reaching flying speed (50 knots) to avoid trees, but the helicopter lost lift, impacted the water, and severed its tail boom.
Probable Cause
The helicopter took off in a direction where there were high trees ahead and, as its performance was reduced by being above the required weight, the helicopter could not clear the obstacle; the pilot manoeuvred the helicopter to the left before reaching its flying speed (of 50kts). However, the helicopter lost lift, resulting in an impact with the water surface and severing its tail boom.