What happened
On 23 November 2011, a Robinson R44 helicopter, registration ZS-RSU, was returning to Grand Central Airport (FAGC) following a commercial aerial survey task involving the tracking of a stolen vehicle near Pretoria. While the pilot was executing an into-wind approach for taxiway 35, the aircraft experienced a sudden and critical mechanical instability. The low rotor RPM warning horn activated, indicating the main rotor speed had dropped to approximately 80%, while simultaneously, the engine RPM surged beyond allowable limits.
In an attempt to stabilize the aircraft, the pilot reduced the collective and initiated an autorotative descent. The pilot also adjusted the throttle to mitigate the engine overspeed, but these actions failed to bring the engine RPM back within safe parameters. During the descent, the pilot broadcasted an emergency message to air traffic control. As the aircraft approached the ground, the pilot performed a flare to reduce forward speed and vertical descent rate. However, the helicopter's skids struck the ground with significant forward momentum, causing the aircraft to nose over and roll onto its left side. The impact caused the tailboom to separate from the main wreckage.
The investigation
SACAA AIID investigators examined the wreckage and the maintenance history of the aircraft. The investigation confirmed that the Robinson R44 was airworthy at the time of dispatch and had been maintained by an approved organization in accordance with all regulatory requirements. The crew, consisting of the pilot and one passenger, sustained no injuries during the event.
Technical analysis of the wreckage showed that the main rotor blades sustained damage consistent with the engine still producing power at the moment of impact. Investigators found no evidence of structural failure or system malfunctions prior to the onset of the RPM instability. Furthermore, the investigation found no evidence of pilot incapacity or sudden illness.