What happened
On August 1, 1973, a private VFR flight departed from Buttwil, Switzerland, in a Bolkov-209, registration HB-UEN. The flight, carrying one passenger, was intended to include a landing at the Speck-Fehraltorf airfield. During the flight, the pilot noted a significant imbalance between the fuel tanks, with the left tank at three-quarters full and the right tank at one-quarter full.
While flying over Pfäffikon, the engine experienced a sudden loss of power. Although the engine regained full power after the pilot increased the throttle, a second engine failure occurred shortly thereafter. Believing the issue was a mechanical malfunction rather than a fuel supply problem, the pilot opted for an immediate emergency landing at the grass strip in Speck instead of following standard approach procedures. The aircraft touched down in the final quarter of the 600-meter runway and subsequently overturned in a grain field approximately 25 meters past the runway end. There were no injuries to the occupants, though the aircraft sustained heavy damage.
The investigation
Investigators examined the aircraft's fuel system and engine performance following the accident. The aircraft was found to be airworthy, and no mechanical defects were discovered in the engine, oil, or fuel systems. Post-accident inspections revealed that the fuel selector valve was set to the right tank. The remaining fuel levels were recorded as 500 ml in the right tank and 3.2 liters in the left tank. While the fuel gauges showed inaccuracies, these were within the expected range for the instrument type.
Crucially, the investigation looked into the pilot's actions regarding fuel management. The aircraft's operating manual mandates that the pilot switch to the fuller tank prior to landing. However, the pilot admitted to a psychological reluctance to perform fuel tank switches while in flight, a practice he had maintained throughout his flying career.
Findings
- The pilot incorrectly assumed the engine power loss was due to a mechanical engine failure, despite all engine monitoring instruments showing normal parameters.
- The pilot failed to execute the most logical corrective action—switching to the fuller fuel tank—due to a personal aversion to manipulating the fuel selector during flight.
- Inaccurate fuel gauge readings may have contributed to the pilot's false belief that the fuel supply was sufficient.