What happened
On 30 May 2003, a Jabiru SP aircraft, registration ZU-CBL, was conducting a PPL training flight departing from Swartkops Air Force Base (FASK). The flight, which included a student pilot and an instructor, proceeded toward the area northeast of the Hartebeespoort dam within the Johannesburg Special Rules Area.
As the aircraft was descending from 7,000 ft to 6,000 ft near Atteridgeville, the student pilot attempted to level off and increase power. At this moment, the engine began running heavily and lost significant power. The instructor assumed control of the aircraft and performed several troubleshooting steps, including checking the magnetos, fuel pump, and carburettor heat, but the engine continued to run poorly at low power levels.
Due to the loss of power, the instructor initiated a forced landing in an open field located approximately 4nm west of FASK, near the Erasmia neighborhood. During the landing roll on rocky and uneven ground, the nose gear and main undercarriage collapsed. While the student pilot escaped without harm, the instructor sustained back and neck injuries.
The investigation
Investigators examined the engine and induction system to determine the cause of the power loss. The investigation focused on the engine's performance during the transition from descent to level flight when the throttle was advanced.
Findings
- The primary cause of the engine malfunction was a steel washer found lodged inside the carburettor inlet.
- This obstruction restricted airflow into the carburettor, creating an excessively rich fuel-to-air ratio whenever the throttle was moved beyond the 1/4 position.
- It is suspected that the washer may have been introduced into the induction system during a routine maintenance inspection while the air box was open, though the exact source remains unconfirmed.