What happened
On 26 November 2009, a Robinson R22 Beta helicopter, registration ZS-SRV, was conducting a training flight near Rand Aerodrome when it experienced a sudden loss of altitude. The flight, involving a commercial instructor and a student pilot, was performing a left-hand circuit. While flying the downwind leg, the aircraft encountered a 14-knot tailwind.
As the helicopter began to descend, the student pilot attempted to regain altitude by raising the collective. This action triggered the low rotor RPM warning light. The instructor immediately took control, attempting to restore engine power by lowering the collective and increasing the throttle; however, the rotor speed continued to decay rapidly. To avoid an imminent collision with a wire fence located ahead of the aircraft, the instructor executed a right-hand turn. During this maneuver, the left-hand skid made contact with the ground, causing the helicopter to roll onto its left side.
The investigation
SACAA AIID investigators examined the flight sequence and the mechanical state of the aircraft. The investigation confirmed that the Robinson R2-2 Beta was airworthy, with a valid certificate of airworthiness and recent maintenance inspections. The crew members held valid licenses and medical certificates.
Technical analysis focused on the aerodynamic impact of the tailwind and the student's control inputs. The investigation noted that the tailwind increased the demand on the tail rotor, which in turn diverted engine power away from the main rotor. Furthermore, the investigation found that the aircraft's registration markings were inconsistent, as it displayed two different markings (ZS-SRV and ZS-RVS).
Findings
- The primary cause of the accident was the loss of main rotor RPM during the training exercise.
- The student pilot failed to compensate for a 14-knot tailwind, which contributed to the initial loss of altitude.
- The instructor's attempt to avoid a wire fence resulted in the left skid digging into the turf, leading to the rollover.
- The rapid decay of rotor RPM made recovery nearly impossible at such a low altitude.
- The sudden increase in the angle of attack during the instructor's takeover likely increased drag and further accelerated the rotor speed decay.