What happened
On August 20, 2017, a Piper PA-28-161, registration HB-PTL, departed Sion Regional Aerodrome for a flight toward Bienne-Kappelen. The aircraft was carrying the pilot and two passengers under Visual Flight Rules (VFR). After departing from runway 25, the aircraft climbed through 4,500 ft QNH near Savièse and turned toward the Sanetsch Pass.
As the aircraft entered the valley, several ground witnesses observed the plane flying at dangerously low altitudes. In the vicinity of Dilogne, the pilot attempted a climbing turn, likely realizing the aircraft's performance was insufficient to maintain a direct path to the pass. However, the aircraft failed to reach the necessary altitude and continued toward the terrain. At approximately 13:09, the aircraft struck a slope at an elevation of roughly 2,100 meters, resulting in a post-impact fire. All 3 occupants of the aircraft died in the accident.
The investigation
The Swiss Transportation Safety Investigation Board (SUST) examined radar data, radio communications, and witness statements. The investigation established that the aircraft's technical condition was sound, with the weight and center of gravity within prescribed limits. The investigation also reviewed meteorological conditions, noting a light north wind and good visibility, though local terrain-induced winds were present near the pass.
Witnesses provided critical evidence regarding the aircraft's flight path. One professional pilot and several other observers noted the aircraft was flying significantly below the recommended minimum altitudes for the terrain. Specifically, while a recommended altitude of 8,400 ft AMSL is advised for crossing the Sanetsch Pass, the aircraft was observed at much lower levels, even appearing as low as 100 meters above the ground in its final moments.
Findings
The investigation concluded that the accident was caused by inappropriate mountain flight tactics. Several contributing factors led to the pilot entering an inescapable situation:
- An insufficient evaluation of the Piper PA-28-161's climb performance.
- A short-term modification to the planned flight route.
- An inappropriate choice of flight path relative to the terrain.
- Failure to adhere to the recommended minimum safe flying altitude for mountain operations.