What happened
On February 21, 2009, a Piper PA-18A HB-PQJ, equipped with skis for winter operations, was conducting a private instructional flight near Valerette, Switzerland. The crew, consisting of an instructor and a student pilot, had been performing various mountain landings. During a simulated approach to a slope in Valerette, the instructor reported that the engine began to sputter when he attempted to apply power to maneuver.
Rather than abandoning the approach to glide back to the Bex airfield, the instructor elected to land on the snowy, 20-degree slope. Following the initial contact with the snow, the aircraft slid and eventually came to a stop. However, the aircraft then began to slide backward down the incline, traveling approximately 150 meters before striking a larch tree. The impact caused significant damage to the fuselage, left wing, and tail section.
During a subsequent attempt to recover the aircraft, a helicopter arrived at the scene. The downwash from the helicopter's rotor blades caused the stationary aircraft to slide further down the slope, eventually crashing into the edge of a forest, resulting in the total destruction of the airframe.
The investigation
SUST investigators examined the technical state of the engine and the operational decisions made by the crew. Technical testing of the Lycoming engine on a test bench revealed no mechanical anomalies or defects. While the possibility of carburetor icing could not be entirely ruled out, the ambient temperature and low humidity levels were not conducive to such an event.
Investigators also reviewed the suitability of the landing site. While the instructor had previously scouted nearby sites on skis, subsequent evaluations by experienced mountain instructors identified the Valerette site as inadequate for landing. Furthermore, the investigation noted that the aircraft's tail ski left no tracks in the snow, suggesting that sufficient engine power was actually being maintained during the approach, which would have allowed for a successful diversion to Bex.
Findings
- The primary cause of the accident was the decision to land on an unsuitable mountain site following a perceived engine malfunction.
- The instructor had the opportunity to abort the approach and reach the Bex airfield via a glide.
- The engine was found to be in good working order, with no mechanical failures identified during testing.
- The recovery operation was handled inadequately, as the helicopter's rotor wash was allowed to displace the damaged aircraft.
- The aircraft was at its maximum allowable weight for ski operations at the time of the incident.