Background
On 12 July 2015, a Piper PA-28-181 Archer III, registered as HB-PQS, was being operated by a private pilot and his wife, who also held a pilot license and acted as safety pilot. The flight was planned from Lommis (LSZT) to Lausanne (LSGL) and back. The aircraft had been fully fueled after the previous flight, providing an endurance of approximately five hours.
Flight
Outbound Leg
The aircraft departed Lommis at 13:33 LT. The pilot reported switching the fuel tank selector every 30 minutes to avoid reliance on fuel quantity gauges and to prevent imbalance. The flight to Lausanne took 1 hour and 19 minutes without incident. After a short break, the pilot and his wife prepared for the return flight, noting no anomalies. No fuel samples were taken before departure.
Return Leg and Engine Irregularities
Takeoff from Lausanne was at 15:41 LT. The flight proceeded at FL55. Between Gruyères and Thun (approximately 15:57 to 16:08), the pilot noticed a slight power loss and a drop in fuel pressure. He switched on an electric fuel pump, and engine performance normalized. Toggling the fuel selector had no effect. The pilot attributed the irregularity to vapor formation in the fuel system due to high temperatures and the use of unleaded automotive fuel. He decided to continue to Lommis with the electric pump on.
Final Phase
At 16:43, over Rickenpass, the fuel gauges indicated 15 US gallons right and 5 US gallons left. The pilot briefly turned off the electric pump; fuel pressure dropped, so he turned it back on. Believing he had sufficient fuel and two electric pumps, he continued toward Lommis. Near Wattwil, the right gauge showed maximum (considered erroneous), and left gauge dropped to zero. Shortly, engine power decreased; the pilot switched the selector to the right tank and power returned. Then the right gauge also reached zero, which the pilot again interpreted as a malfunction.
At 16:47, the aircraft was at 3500 ft near Bütschwil. At 16:49, the engine failed completely near Bazenheid. The pilot glided straight ahead toward Wil/SG without further fuel system adjustments.
Emergency Landing
The pilot executed an emergency landing on a rising meadow at the western edge of Wilen. Both occupants exited the aircraft uninjured. The aircraft sustained no damage, and there was no ground damage. The pilot reported that until landing he believed the aircraft still had 80 to 100 liters of fuel.
Cause
The serious incident was caused by engine failure due to fuel starvation, resulting in the emergency landing. Contributing factors included: an in-flight leak in the fuel filter housing due to loss of the drain valve, and the pilot's inappropriate analysis of the situation, which led him to continue toward the destination. A risk factor identified was that observations and complaints about fuel quantity indications in the aircraft logbook had not been addressed consistently.