What happened
On September 23, 1978, a private flight involving a Piper PA-2-8R-201 HB-PBN and three passengers departed from La Chaux-de-Fonds for Sion. After a brief stop in Sion, the pilot departed for the return leg, initially intending to follow a route via Martigny and Lausanne. However, the pilot decided to deviate from the original flight plan, heading north toward the Sanetsch Pass via the Morge Valley.
While navigating the valley at a relatively low altitude, the aircraft narrowly cleared a high-voltage power line. Shortly thereafter, the aircraft struck rising terrain at an elevation of 2,230 meters. The impact occurred in two stages: the nose of the fuselage first struck a rocky prominence, followed by the lower cabin area hitting the ground 24 meters further along the flight path. The aircraft was destroyed in the collision.
The investigation
Investigators examined the aircraft's mechanical condition and the pilot's flight history. The Piper PA-28R-201 was found to be in a serviceable condition, with no technical defects contributing to the accident. The aircraft was operating near its maximum weight limit, with a load of 1,240 kg against a maximum of 1,240 kg.
The investigation also reviewed the meteorological conditions, which included strong northwesterly winds and moderate turbulence in the region. The pilot's experience was noted, specifically that while the pilot held a valid private license, his experience with mountain flying was limited, having completed only five Alpine crossings in his career.
Findings
- The pilot's decision to deviate from the planned route led to the aircraft entering the Morge Valley at an altitude too low to safely clear the terrain.
- The pilot utilized an inadequate flight tactic by flying at an indicated airspeed of 80 to 90 knots, which was below the manufacturer's recommended cruise speed of 104 knots. This reduced the speed margin necessary for maneuvering.
- An optical illusion likely contributed to the accident, as pilots flying at low altitudes over rising terrain often unconsciously increase the angle of attack, perceiving the horizon to be higher than it actually is.
- The pilot failed to maintain the required safety altitude, which according to Swiss aeronautical regulations should have been 1,000 feet AGL.