What happened
On 25 October 2019, a Robinson R44 helicopter, registration ZT-RAL, was conducting a training flight for an instrument rating revalidation near Wonderboom Aerodrome. The flight crew included a pilot, a designated flight examiner (instructor), and an authorized officer from the regulator's personnel licensing department.
After completing several training exercises, the aircraft approached the operator's hangar. Dissatisfied with the pilot's approach, the instructor took control of the aircraft to initiate a go-around. During the subsequent climb, the helicopter's low rotor RPM warning horn activated at approximately 200 feet above ground level. Although the instructor initially managed to stabilize the aircraft, the warning horn and low rotor RPM light illuminated again at 300 feet during a right turn.
Unable to recover the rotor speed at such a low altitude, the pilot attempted an emergency landing to avoid nearby buildings. The helicopter struck treetops and branches before hitting a streetlight pole. The impact caused the aircraft to strike the ground hard, resulting in the destruction of the helicopter. While the pilot and instructor sustained minor injuries, the authorized officer suffered serious injuries.
The investigation
The SACAA AIID investigation focused on the sequence of events during the go-around maneuver and the mechanical state of the aircraft. Investigators examined the aircraft's maintenance records, finding that the annual periodic inspection had been completed recently and the aircraft was within its weight and performance limits for the prevailing 36°C temperature. The investigation also reviewed the flight controls and the RPM governor, which was found to be in the "ON" position.
Findings
- The instructor's sudden takeover of the controls during the go-around was the catalyst for the instability.
- The instructor likely applied more collective pitch than the engine could provide or incorrectly adjusted the throttle, leading to a rapid drop in rotor RPM.
- As the blades attempted to maintain lift by increasing pitch, the resulting increase in drag exceeded the available engine power.
- The loss of rotor RPM led to a loss of lift, making a controlled descent inevitable.
- The low altitude prevented the crew from establishing a successful autorotation.