What happened
On the morning of 04 November 2013, an Air Tractor 502B, registration ZS-ZZX, was performing a low-level geophysical survey for the mining industry near Kimberley. While flying at an altitude of approximately 25 meters above ground level, the pilot experienced a sudden loss of engine torque. Although the engine remained running, the aircraft was unable to maintain sufficient altitude to clear the terrain.
In an attempt to find a safe landing site, the pilot executed a right turn to follow a valley in an easterly direction. The aircraft eventually descended toward the banks of the Vaal River, west of Barkly West. During the forced landing on rocky terrain, the aircraft's undercarriage collapsed, causing the propeller to strike the ground. The impact resulted in substantial damage to the airframe, engine, and propeller, though the pilot escaped the wreckage without injury.
The investigation
SACAA AIID investigators examined the wreckage and the aircraft's maintenance history. The investigation focused on the engine's performance and the mechanical state of the fuel control system. A post-accident inspection of the engine revealed that the fuel control unit throttle linkage had become disconnected.
Specifically, investigators found that the bolt securing the linkage was missing its self-locking castellated nut and split pin. This failure caused the engine to revert to an idle state, stripping the pilot of throttle response and preventing the necessary power for climbing. The investigation also reviewed the maintenance records, noting that a Pratt & Whitney PT6A-34AG engine had been recently installed on the aircraft following a maintenance event in October 2013.