Accident Summary
On 15 April 1968 at approximately 1420 hours Central European Time, a Pilatus PC-6 Porter, registration HB-FAV, equipped with skis, was departing from La Croix de Coeur, Valais, Switzerland. The pilot was alone on board, returning to Bex after aborting a tourist flight due to a carburetor fire during engine start. Shortly after takeoff, at a height of about 20 meters, the engine experienced a total loss of power, with the propeller windmilling. The pilot performed a forced landing in a steep left turn into the snow-covered slope. The maneuver was successful, but the aircraft then began to slide backward down the slope. The pilot evacuated through the main door and was uninjured. The aircraft slid approximately 470 meters before being stopped by a tree, resulting in severe damage.
Aircraft and Pilot Information
The aircraft was a Pilatus PC-6 Porter (serial number 345, built 1960), powered by a Lycoming GS0-480-B1-A6 engine. It had been purchased second-hand and registered to its new owner on 25 October 1967. After a 100-hour inspection revealed metal particles in the oil, the engine underwent a major overhaul, including installation of thicker cylinder liners. The engine was bench-tested and a flight check was conducted on 10 April 1968. At the time of the accident, the engine had accumulated 1 hour and 45 minutes since the overhaul. The fuel system consisted of two non-communicating 200-liter tanks in the wing roots, each with a single fuel outlet (pre-series configuration), a three-way selector valve, filter, check valve, engine-driven pump, carburetor, and an electric auxiliary pump. The pilot, born in 1930, held a professional pilot license with mountain and glacier landing endorsements, and had 1,252 total flight hours, including 21 on the Pilatus Porter. He was in good health at the time of the accident.
Sequence of Events
The pilot departed Bex at 1307 hours with three guests, landing at La Croix de Coeur. After dropping the guests, he took on board tourists destined for the Trient glacier. During the takeoff attempt, the rear ski was not locked, causing a swerve, and the takeoff was aborted. The pilot then performed a circuit alone to reposition. After boarding passengers again, engine start was very difficult and a carburetor fire occurred, which was quickly extinguished by witnesses and the pilot. Deciding to abort the mission, the pilot prepared to return to Bex alone. After starting the engine and letting it run for 2-3 minutes, he applied power and began the takeoff. At an altitude of about 20 meters, the engine lost all power. The pilot executed a forced landing in a left turn against the slope. During the landing, the engine momentarily regained power. The aircraft came to a stop but then began sliding backward. The pilot tried to turn the aircraft sideways but could not control the slide, so he evacuated. The aircraft slid 470 meters before hitting a tree. The pilot shut off the electrical system and secured the aircraft with the help of witnesses.
Examination Findings
Fuel quantities at departure from Bex were 120 liters in the left tank and 135 liters in the right tank. At the accident site, the left tank still contained about 120 liters, while the right tank held approximately 75 liters, indicating consumption of about 60 liters in 35 minutes of flight. The cabin showed the throttle forward, fuel selector on "right", auxiliary pump switch on, right fuel gauge at 1/3 to 1/2, flaps in takeoff position, propeller in low pitch, skis down, and altimeter set to 6500 ft QNH. The aircraft was tilted to the right; draining the right tank produced a regular flow, while the left tank had ruptured and leaked a large quantity of fuel. The carburetor, air filter, and fuel pressure line showed fire damage. An expert examination by Pilatus Flugzeugwerke AG found that the carburetor fire had not caused damage. The fuel system and combustion system were otherwise in order, except for cracked fuselage hoses due to age and a slightly dirty carburetor filter. The three-way fuel selector valve was found out of specification: the key was 3.5 mm too deep (Service Bulletin No. 36 specifies a maximum of 2 mm), resulting in the valve opening being only 60%. The priming valve lever was stuck when pushed, and the valve could remain open, delivering two-thirds of normal flow—this was the case at the time of the accident. After purging fuel from the intake pipes and compressor, the engine was tested to full static run and found to be in order. The engine would stop after fuel selector closure within specific times; with the priming valve leak and auxiliary pump on, stopping times increased.
Discussion and Probable Cause
The investigation concluded that the airframe and powerplant were in order, and the cause of the accident was a deficiency in the fuel supply system. The defective priming valve could have caused the carburetor fire but was unlikely to have produced an abrupt power loss at high engine speed. The influence of the carburetor fire on the subsequent power loss through gas formation in the lines was practically excluded by the use of the electric auxiliary pump. The fuel selector was set to the right tank, which still contained about 75 liters, and the left tank was three-fifths full. If the selector had been closed, the engine would have stopped before departure. The non-standard selector operation did not explain the power loss. Therefore, the probable cause is a deficiency in the fuel supply.