What happened
On 27 January 2007, a Tiger Moth (DH-82A), registration ZS-BXT, was conducting a private flight departing from a farm in the Oak Valley District near Grabouw. During pre-takeoff inspections, the pilot noted that the engine was running roughly on one of the magnetos, though the engine appeared to stabilize before departure.
Shortly after takeoff, the engine failed unexpectedly. The pilot immediately transitioned the aircraft into a nosedive to maintain control and confirmed that the fuel and magnetos were correctly positioned. With no suitable open area available, the pilot executed a forced landing into a nearby pear orchard. The aircraft struck several trees in a slightly nose-down attitude, causing the propeller to separate from the engine. Both the pilot and the passenger sustained minor injuries and were transported to the hospital.
The investigation
SACAA AIID investigators examined the wreckage and the engine components to determine the cause of the power loss. The investigation confirmed that the aircraft's maintenance records were up to date, with a valid Authority to Fly and a recent annual inspection. The weather at the time of the accident was clear with good visibility and light winds, and was not considered a contributing factor.
Findings
Technical examination of the engine revealed that the No. 2 and No. 3 exhaust valves had become stuck in the open position. This mechanical failure was caused by excessive carbon build-up within the exhaust valve guides, a condition brought on by excessive cylinder temperatures. This build-up led to a total loss of compression in those cylinders, resulting in the sudden engine failure during the takeoff roll.