What happened
On the afternoon of November 21, 1968, a Piper Cherokee PA-28-140, registration HB-OWG, departed from Bern for a short flight to Neuchâtel/Colombier. During the flight, the pilot encountered deteriorating weather conditions characterized by low cloud ceilings and reduced visibility. While flying near the Lyss-Schüpfen area, the aircraft experienced a significant loss of engine power.
As the engine RPM dropped from approximately 2300 to 1600, the pilot realized that returning to Bern was no longer possible. After evaluating potential landing sites and finding several obstructed by power lines, the pilot performed an emergency landing in an area consisting of meadows and orchards on the eastern edge of Rapperswil/BE. During the landing sequence, the aircraft struck a wire fence, causing the nose gear to collapse, and the left wing subsequently struck a tree trunk.
The investigation
The investigation focused on the cause of the engine power loss and the flight conditions. Investigators examined the fuel system, including the fuel tanks, lines, and filters, finding sufficient fuel levels and no signs of contamination or blockages. A subsequent test run of the engine following repairs showed that the powerplant functioned correctly.
Findings
- The primary cause of the engine malfunction was likely carburetor icing.
- Atmospheric conditions, including temperatures near 0°C and high humidity, were highly conducive to ice formation.
- Although the pilot had partially engaged the carburetor heat, the temperature gauge indicated a setting near the boundary of the green and yellow zones (approximately +5°C). This partial application of heat may have caused ice to melt in the front of the carburetor only to refreeze further downstream.
- The pilot's decision to reduce airspeed due to poor visibility may have further increased the tendency for icing to occur.
- All three occupants escaped the accident uninjured, though the aircraft sustained significant damage to the propeller, engine, nose gear, and left wing.