What happened
On July 21, 1968, an AVIA-Starlet FL 3, registration HB-UOL, was conducting a flight from Schupfart near Lake Neuchâtel. Approximately 90 minutes into the flight, the engine began to malfunction, experiencing severe misfires and a significant drop in RPM from roughly 2250 to 1600. The pilot attempted to switch to the fuel reserve, but the engine instability persisted, forcing a decision to land at the nearby Olten/Gheid airfield.
During the approach to Olten, the pilot performed several circuits to check engine parameters. The approach was executed at an excessive speed and altitude. Fearing the aircraft would overshoot the runway and plunge into a nearby gravel pit located just 150 meters past the runway end, the pilot attempted a go-around. While the engine initially responded to full throttle, the power failed completely during a left-hand turn at an altitude of approximately 20 meters. The aircraft lost lift, stalled, and struck the ground, resulting in two serious injuries and the total destruction of the aircraft.
The investigation
Investigators examined the fuel system and the engine components to determine the cause of the power loss. The investigation focused on the fuel flow from the tank to the carburetor. While the engine and ignition systems showed no mechanical defects, the fuel tap and filter were found to have unusually small dimensions, significantly smaller than those found on similar aircraft like the Piper Cub.
Analysis of the fuel filter revealed a white, swellable residue consisting primarily of aluminum and zinc oxides. These substances were identified as corrosion products from the aircraft's fuel tank walls. The investigation noted that the aircraft had been sitting unused for long periods, which likely facilitated this internal corrosion.
Findings
- The primary cause of the engine failure was insufficient fuel supply to the carburetor.
- The fuel system's extremely small flow diameters made it highly susceptible to blockages.
- Corrosion products (aluminum and zinc oxides) from the tank walls had accumulated in the fuel filter, obstructing the flow of fuel.
- The engine power loss was most critical during the go-around because the increased fuel demand quickly exhausted the small amount of fuel that had accumulated in the carburetor bowl during the descent.
- The landing attempt at Olten was unsuccessful due to an improper approach technique, characterized by excessive speed and altitude.