What happened
On August 10, 1966, a pilot and a passenger departed from Sion airport for a local pleasure flight over the Alps in a Piper J3C, registration HB-OUI. The flight path included crossing the Sanetsch Pass and descending through the Simmental and Kander valleys, with plans to fly over the Gemmi Pass.
During the return leg, near the Gemmi Pass, the pilot noticed that fuel levels had reached a critical state; the left tank gauge ball was no longer visible, and the right tank gauge was at the red limit. Attempting to conserve fuel, the pilot reduced power and headed back toward Sion. However, approximately 9 km from the airfield, the engine began to sputter and failed due to fuel exhaustion.
While attempting an emergency landing in a mown field near Uvisting/St-Léonard, the aircraft was too low on speed to clear a row of apple trees. The left wing struck a tree, causing the aircraft to spin and crash into the ground. Both occupants survived the accident without injury.
The investigation
Investigators examined the wreckage of the Piper J3C and analyzed the aircraft's fuel system. The investigation focused on the discrepancy between the fuel levels recorded at takeoff and the actual amount of fuel remaining in the tanks.
Technical examination of the fuel tanks revealed that the aircraft had been fitted with a non-standard wing tank installation during a major overhaul in 1961. This system replaced the original central tank with two wing tanks. The investigation specifically looked at the calibration of the fuel gauges and the impact of aircraft attitude on fuel level readings.
Findings
- The primary cause of the accident was fuel exhaustion resulting from the pilot starting the flight with insufficient fuel and failing to divert when critical levels were first identified.
- The pilot was misled by inaccurate fuel gauge readings at the start of the flight. Because the gauges were mounted too far aft of the actual fuel mass, readings taken while the aircraft was stationary on the ground (in a three-point stance) were significantly higher than the actual volume.
- Reconstructions showed that the gauges on the ground indicated approximately 10 liters more fuel than was actually present.
- The fuel gauge markings did not specify whether the readings were valid for a level flight attitude or a three-point ground position.
- Although the gauges provided an erroneous initial reading, the pilot failed to take corrective action, such as landing at a nearby airfield, once the gauges indicated the fuel had reached a critical level during flight.