What happened
On July 29, 2004, a Piper PA-18-180M, registered as HB-POD, was performing glider towing operations at Zweisimmen airfield. During the morning preparations, the pilot noted an abnormal RPM drop on the right magneto during a engine run-up. After a mechanic inspected the engine and cleared it for flight, the pilot proceeded with the takeoff.
Initially, the takeoff appeared normal, with engine RPM increasing as expected. However, approximately 50 meters above ground level, just after passing the Kleine Simme river, the engine RPM plummeted from takeoff power to 1400 RPM. Recognizing the engine failure, the pilot notified the glider pilot via radio to release the tow cable. The pilot then executed an emergency landing in a field north of the confluence of the Kleine Simme and Simme rivers. The aircraft sustained damage to the left main landing gear attachment due to the descent rate and a slight lateral tilt upon impact, but no injuries were reported.
The investigation
Investigators examined the aircraft and the engine performance. While the engine started normally during ground tests, a significant RPM drop was observed when switching from the 'both' position to the right magneto. Further technical analysis of the fuel selector valve revealed that when positioned between the left and right tanks (at an angle of 30 to 45 degrees), the fuel flow cross-section is severely restricted, reducing fuel delivery by approximately 30%.
Testing confirmed that maintaining the fuel selector in this intermediate position for one to two minutes under full power resulted in a sustained engine power drop to 1400–1500 RPM. The pilot later recalled that the fuel selector was found in a middle position between the left and right tanks following the accident, rather than on the right tank as initially observed by investigators.