What happened
On October 17, 2020, a flight instructor and a student pilot were conducting a training flight near the La Victoria de Chacabuco Aerodrome (SCVH) in the Metropolitan Region of Chile. While performing a left-hand traffic pattern for runway 21 in a Pipistrel Alpha Trainer, the crew noticed a significant engine malfunction. As the instructor attempted to reduce power to idle, the engine failed to respond, remaining at approximately 3,600 RPM.
Recognizing the inability to control engine power, the instructor took control of the aircraft. Once the runway was secured, the instructor intentionally shut down the engine to perform a controlled landing. The aircraft landed safely on runway 21 without structural damage, and both occupants remained uninjured. Following the landing, the engine was restarted, and the aircraft taxied back to the operator's apron.
The investigation
The DGAC Chile investigation focused on the mechanical state of the engine and the maintenance history of the aircraft. The aircraft, a Pipistrel Alpha Trainer, had undergone a major inspection just five hours prior to the incident, which had not identified any discrepancies in the engine or its systems. Maintenance records indicated that the operator was in full compliance with approved maintenance programs.
Investigators examined the engine's carburetor assembly and the throttle control mechanism. Physical inspection of the left carburetor revealed that the spring responsible for returning the throttle valve lever to the idle position had become detached. The investigation focused on why this component failed despite the recent inspection and pre-flight checks.
Findings
- The investigation identified structural failures in the plastic bushing and the metal support of the throttle valve lever on the left carburetor.
- These fractures, which included wear-related damage to the metal support and the plastic bushing, were caused by mechanical friction between metal parts.
- This structural failure allowed the return spring to slip out of its anchor point, leaving the throttle valve stuck in an accelerated position.
- The loss of the spring caused a desynchronization between the two carburetors, resulting in an uneven engine running condition and the inability to reach idle RPM.
Safety action
- The investigation recommends that all operators of aircraft utilizing Rotax 912 UL2-01 engines update their inspection programs to specifically include a verification of the condition of the throttle valve return spring supports on the carburetors.