What happened
On 15 August 2008, a Beechcraft F33A, registration ZS-RMA, was competing in the 'Race of Champions' air race. The flight originated from Tzaneen Aerodrome with a planned return to the same location. Approximately 70 minutes into the competition, while flying between Marble Hall and Vivo, the aircraft experienced a sudden loss of power followed by engine spluttering and total failure.
In response to the loss of power, the pilot identified an open field at Appelfontein farm near Dendron and initiated an emergency forced landing. During the descent, the aircraft struck a one-meter-high ridge at high speed, causing the plane to bounce and lose control. The aircraft subsequently impacted several thorn bushes, resulting in substantial damage to the right wing, flaps, propeller, and landing gear. Despite the significant impact, there were no injuries to the two occupants on board.
The investigation
SACAA AIID investigators conducted a teardown inspection of the Continental IO-520 engine to determine the cause of the power loss. The examination revealed that the camshaft drive gear and its mating idler gear had suffered significant failure.
Metallurgical analysis determined that the camshaft drive gear failed due to fatigue, which caused four teeth to detach and subsequently damage the idler gear. The investigation found that the nitriding heat treatment on the drive gear failed to achieve the required surface hardness, leading to insufficient resistance to fatigue. Additionally, the root radii on the cam drive gear were smaller than those on the idler gear, creating a stress-concentration effect that contributed to the failure.