What happened
On March 11, 1978, a Piper Arrow PA-28 R 200, registration HB-PAH, departed from Sion for a private sightseeing flight through the Alps. The aircraft was carrying a pilot and three passengers. During the climb, while performing a turn above the Hérémence area at an altitude of approximately 2,400 meters, the engine suddenly began to sputter and lost power.
As the pilot focused on troubleshooting the engine, the aircraft began descending into a small valley. Realizing there was insufficient altitude to return to the Rhone Valley and noting that the terrain near Evolène offered no suitable landing sites, the pilot executed a controlled forced landing on a north-facing, snow-covered slope near Vouasson. The impact, performed using a mountain landing technique, caused significant damage to the aircraft, including a collapsed nose gear and broken main landing gear.
The investigation
Investigators examined the aircraft's mechanical condition and the environmental factors present at the time of the accident. The engine, a Lycoming 10360-C1C, was removed and tested on a bench; it started immediately and showed normal compression and performance. A complete teardown of the engine revealed no identifiable defects.
Fuel analysis showed the 100LL aviation gasoline was within specifications, and the fuel system was found to be clean, with only a tiny droplet of water found in the filter bowl. Meteorological data indicated excellent visibility and low humidity, making carburetor icing highly improbable. The investigation also noted that the aircraft's weight and balance were within limits and the airframe was well-maintained.
Findings
- The primary cause of the accident was a loss of engine power occurring over mountainous terrain that was unsuitable for an emergency landing.
- The specific reason for the engine power loss could not be determined.
- The pilot's decision to fly into the Merdesson valley for climbing maneuvers reduced the available maneuvering space and time to troubleshoot the engine failure.
- The pilot's decision to land on the snowy slope was considered the most viable option, as returning to Sion via gliding was impossible and the terrain near the village of Evolène was too obstructed.