What happened
On 1 July 2024, a Tecnam P2008-JC, registration G-PFTE, was conducting a training flight at Oxford Airport. The flight involved a student pilot and an instructor performing circuits and touch-and-go maneuvers. After approximately one hour of incident-free flight, the aircraft was climbing from a touch-and-go at an altitude of roughly 300 ft agl when the instructor noticed a drop in engine power. As the aircraft began to sink, the instructor assumed control of the aircraft. Upon checking the engine settings, the instructor confirmed that the throttle was at full power, the choke was off, and the carburettor heat was also disengaged. Despite the engine running smoothly, the RPM had dropped to 2,000 and the engine note was lower than usual. The instructor decided to perform an immediate forced landing, maneuvering the aircraft into a field located approximately 10° left of the runway centerline. There were no injuries to the crew and the aircraft sustained no visible damage.
The investigation
The investigation focused on the mechanical cause of the power loss. Investigators examined the engine and found that the spring for the left carburettor throttle butterfly valve was missing. The aircraft's throttle system uses a cable-operated mechanism where the spring tension is essential to ensure that moving the hand control forward effectively opens the butterfly valve. Without this spring, the throttle cable had a tendency to buckle instead of moving the valve lever. To confirm this finding, an engine ground run was performed with the spring removed, which successfully replicated the power loss symptoms observed during the flight. It is suspected that the spring broke and detached while the aircraft was in operation.
Findings
- The primary cause of the power loss was a missing butterfly valve spring in the left carburettor.
- The absence of the spring caused the throttle cable to buckle rather than actuate the valve lever.
- This mechanical failure created an imbalance between the two carburettors, resulting in the observed reduction in engine performance.