What happened
On July 12, 2015, a Piper PA-28-181 Archer III, registration HB-PQS, was performing a private VFR flight from Lausanne to Lommis. During the cruise phase at flight level 55, the pilot noticed a drop in engine power and fuel pressure. To rectify the issue, the pilot activated an electric fuel pump, which temporarily normalized engine performance.
As the flight progressed, the fuel gauges began to show erratic readings, eventually dropping to zero. The pilot interpreted these fluctuations as faulty gauge readings rather than a loss of actual fuel. Shortly after passing the area of Bazenheid, the engine failed completely due to fuel exhaustion. The pilot executed an emergency landing on an upward-sloping meadow on the western edge of Wilen. There were no injuries to the two occupants, and the aircraft sustained no damage.
The investigation
SUST investigators examined the aircraft's fuel system and the pilot's actions. The investigation focused on why the fuel supply was depleted despite the aircraft having been fully fueled for a flight of much shorter duration. Physical inspection of the HB-PQS revealed that the drain valve on the gascolator (fuel strainer) was missing. This loss of the valve created a leak in the fuel filter housing, allowing fuel to escape the system during flight.
Findings
- The engine failure was caused by fuel exhaustion.
- A leak developed in the fuel filter housing because the drain valve had become detached during flight, likely due to insufficient torque during installation.
- The pilot performed an incorrect situational analysis, mistakenly believing that the engine power fluctuations were caused by vapor lock and that the dropping fuel gauges were simply malfunctioning.
- The pilot's habit of switching fuel tanks every 30 minutes to balance levels contributed to the late detection of the fuel depletion.
- While not a direct cause of the incident, investigators noted that previous pilot entries in the logbook regarding fuel gauge discrepancies had not been consistently addressed, representing a risk factor.