What happened
On April 15, 1968, a Pilatus PC-6 Porter, registration HB-FAV, was operating a flight near La Croix de Coeur, Valais. The pilot had previously completed a flight from Bex to drop off passengers and was preparing to depart with a new group of tourists bound for the Trient Glacier. During the initial takeoff attempt, the aircraft veered due to an unblocked rear ski, necessitating a restart.
During the subsequent engine start, a carburetor fire ignited, though it was quickly extinguished by the pilot and bystanders. After inspecting the engine, the pilot decided to abort the mission and return to Bex. However, shortly after taking off for the return leg, the aircraft reached an altitude of approximately 20 meters when the engine suffered a total loss of power. The pilot executed a sharp left turn and performed a forced landing on a steep, snowy slope.
Following the touchdown, the aircraft began sliding backward down the slope for approximately 450 meters. The pilot evacuated the aircraft as it moved toward the tree line. The aircraft eventually struck trees, resulting in severe damage.
The investigation
The investigation focused on the engine's sudden power loss and the mechanical state of the fuel system. Investigators examined the fuel tanks, which were configured with two 200-liter tanks. At the time of the accident, the right tank contained approximately 75 liters, while the left tank held 120 liters. The fuel selector valve was set to the right tank.
Technical inspections of the engine and fuel lines revealed that while the carburetor fire had left some traces on the air filter and pressure pipes, it had not caused structural damage to the engine. The investigation also noted that the fuel selector valve assembly did not meet specific service bulletin standards, as the valve key was seated too deeply, restricting the passage to only 60% of its intended capacity.
Findings
- The primary cause of the engine failure was a deficiency in the fuel supply.
- The position of the fuel pickup point at the rear of the tank, combined with the aircraft's nose-low pitch and the -10° slope of the terrain, likely caused fuel starvation as the fuel level dropped.
- The investigation noted that the fuel selector valve was not functioning according to the required specifications, although this alone was not deemed sufficient to cause the total power loss.
- The engine briefly regained power during the landing maneuver due to the windmilling effect of the propeller, but failed permanently once the rotation stopped.