History of Flight
On 6 December 2013, at approximately 1230Z, a Robinson R44 Raven II helicopter, registration ZS-RZI, departed from Wonderboom Aerodrome (FAWB) for a post-maintenance acceptance flight. The flight was intended to return to the same aerodrome. The purpose of the flight was to test the helicopter after a mechanical fuel pump had been replaced.
During take-off, as the helicopter passed through the transition phase at an altitude of about 40 feet above ground level, the pilot reduced the throttle setting to decrease engine power. However, closing the throttle did not reduce power, causing engine revolutions to increase and an overspeed condition to develop. The pilot immediately shut down the engine and performed a safe landing. After landing, a maintenance engineer was called to tow the helicopter to the maintenance facility.
Injuries and Damage
No injuries were reported among the sole occupant, the pilot. The helicopter sustained no damage during the landing, although the engine was damaged by the overspeed condition.
Investigation Findings
Inspection of the engine and its components revealed that a piece of a Stanley knife was wedged between the butterfly valve and the housing of the fuel control valve, keeping the valve in the open position. The aircraft maintenance engineer (AME) who had replaced the mechanical fuel pump admitted to using the blade of a Stanley knife to clean the gasket from the old fuel pump and to having disassembled the knife to remove the blade. The AME also acknowledged that he did not cover the opening of the pipe leading to the fuel control valve intake while performing the replacement.
Probable Cause
The investigation determined that the engine overspeed was caused by a foreign object restricting the operation of the butterfly valve within the fuel control valve. A contributory factor was improper inspection after maintenance.