Accident Overview
On the evening of 22 July 1990, a Socata TB 10 (registration HB-EQP) operated by the Fluggruppe Oberwallis departed from Raron aerodrome for a short flight to Sion. The pilot, a Swiss private pilot born in 1955 with 220 total flight hours, was tasked with retrieving a club member. Due to time pressure and a prior airshow, the pilot did not perform a visual inspection of the fuel tanks and relied solely on the fuel gauges, which indicated approximately one quarter full.
Flight Sequence
The aircraft departed Raron at 1912 local time and landed in Sion at 1924. After a brief stop, the pilot and passenger departed Sion at 2007. The pilot again checked fuel only by the gauges, which still showed at least one quarter. About eight minutes later, while flying over the Bois de Finges at 4000 feet, the engine lost all power. The pilot switched the fuel selector to the left tank and attempted to restart, but the engine did not respond. He then aimed for the aerodrome at Turtmann, approximately 5 km away, but lost too much altitude and was forced to land on a soft, uneven meadow about 1300 meters short of the runway.
Findings
The aircraft, a Socata TB 10 with tanks holding 105 liters each (102 usable), was found with the right tank empty and intact, and the left tank empty but damaged, likely leaking any residual fuel. The fuel gauge for the right tank was tested and showed readings higher than actual content, especially around the quarter-tank mark where it could indicate 20-25 liters when only 10 were present. The pilot reported that the gauges had been complained about by multiple people, but no clear maintenance instruction was given. The aircraft’s weight and balance were within limits, and there was no evidence of a mechanical defect in the fuel system.
Weather
At the accident site, in Turtmann, weather was fair: wind 270/5-8 knots, visibility 20-25 km, scattered cumulus at 2500-2800 m and altocumulus at 4000 m, temperature 29°C, and light turbulence near the ground.
Cause and Contributing Factors
The investigation concluded that the accident resulted from an emergency landing in terrain following an engine failure caused by fuel shortage. Contributing factors included: insufficient fuel reserve, failure to perform a visual fuel level check, inaccurate fuel gauge indications, and time pressure.