What happened
On May 26, 1973, an American Trainer AA-1A, registration HB-UBM, was conducting training circuits at the Lommis airfield with a flight instructor and a student on board. During the sixth circuit, following a series of touch-and-go maneuvers, the engine failed while the aircraft was at approximately 50 meters altitude. The instructor immediately took control and applied power, which caused the engine to regain performance.
Deciding to abort the flight, the instructor performed an emergency landing on a meadow located ahead of the flight path. The aircraft traveled approximately 80 meters along the ground before striking a barbed-wire fence. The aircraft eventually came to a halt after an additional 20 meters. The crew remained uninjured, though the aircraft sustained minor damage and there was slight damage to third-party property.
The investigation
The investigation examined the fuel management procedures and the mechanical state of the aircraft. Prior to takeoff, the fuel selector valve had been set to the left wing tank. During the flight, the crew noted a significant discrepancy in fuel levels: the left tank indicator showed 1/8, while the right tank showed between 1/2 and 3/4. Despite this observation, the crew did not switch the fuel selector to the right tank.
Post-accident analysis revealed that only approximately 1 liter of fuel remained in the left tank. The investigation confirmed that the aircraft was within weight and balance limits and that there were no pre-existing mechanical defects or health issues affecting the crew. The electrical fuel pump was still operational at the time of the engine failure.
Findings
- The primary cause of the engine failure was the failure to switch the fuel selector valve to the tank with the higher fuel volume, as required by operating procedures.
- The crew failed to follow the manual's instruction to select the tank with more fuel during pre-takeoff and pre-landing checks.
- The engine failure could have been avoided, or the flight continued normally, had the pilot switched the selector to the right tank, as the fuel pump was still active.