What happened
On 6 December 2013, a Robinson R44 II helicopter, registration ZS-RZI, departed Wonderboom Aerodrome for a post-maintenance acceptance flight. The flight was intended to verify recent work performed on the aircraft's mechanical fuel pump.
As the helicopter transitioned through approximately 40 feet of altitude, the pilot attempted to reduce engine power by adjusting the throttle. However, the power level failed to decrease, leading to an immediate engine overspeed condition. The pilot reacted quickly by shutting down the engine and performing a safe landing at the aerodrome. There were no injuries and the aircraft sustained no structural damage during the landing.
The investigation
Following the incident, the aircraft was moved to a maintenance facility for a detailed inspection of the engine components. Investigators discovered that a fragment of a Stanley knife blade was wedged between the butterfly valve and the housing of the fuel control valve. This debris had physically prevented the butterfly valve from closing, thereby keeping the valve in an open position.
Subsequent inquiries established that during the recent replacement of the mechanical fuel pump, an aircraft maintenance engineer (AME) had used a dismantled utility knife to scrape old gasket material from the mounting surface. The investigation found that the AME had failed to cover the intake leading to the fuel control valve during this process, allowing the metal fragment to enter the system.