What happened
On the night of 25 September 2009, a Robinson R44 Raven II, registration ZS-RMN, was conducting a commercial vehicle tracking operation near Lawley, south of Soweto. While the pilot and a passenger were monitoring a vehicle during a left turn, the aircraft's throttle began "hunting," characterized by irregular opening and closing movements. Shortly thereafter, a high-pitched sound was heard, and the pilot observed that both the engine and rotor RPM had surged to their maximum limits.
In an attempt to stabilize the aircraft, the pilot leveled the helicopter, closed the throttle, and adjusted the collective pitch. However, these actions failed to arrest the overspeed condition. Realizing the RPM was uncontrollable, the pilot initiated an autorotation and selected a landing zone. During the subsequent emergency landing, the helicopter experienced a hard impact on its skids. The force of the landing caused the main rotor blades to strike and sever the tail boom. Both the pilot and the passenger escaped the wreckage without injury.
The investigation
An investigation by the SACAA AIID examined the mechanical state of the aircraft and the performance of its governor system. Components including the governor controller box, the governor motor, and the engine magneto were recovered and sent to the Robinson Helicopter Company for testing under the supervision of the NTS/B.
Testing of the governor controller showed it was functional, despite a minor deviation in dead band symmetry that would be imperceptible to a pilot. The magneto was found to have been overhauled in the field; while its tachometer points showed an uneven sine wave during testing, subsequent adjustments to the points and terminal connections restored the signal to within required operational duty cycles. The investigation also noted that while the aircraft had undergone maintenance for a previous overspeed condition only 20 hours prior, no structural or mechanical failures were identified in the recovered components.
Findings
- The pilot was properly licensed and qualified for the night commercial operation.
- The aircraft was properly maintained, certified, and equipped according to regulations.
- The weather conditions were clear with good visibility, though the emergency landing was conducted in darkness.
- The primary cause of the accident was an unsuccessful emergency landing following an in-flight engine and rotor overspeed condition.