Summary
On 30 June 1966, a Piper J3C aircraft, registration HB-OAL, crashed near Sion. The flight departed from Ascona for Berne. The pilot intended to stop at Sion for refueling but experienced fuel exhaustion and performed a forced landing. Both occupants sustained injuries.
Investigation
The investigation was conducted by the Federal Commission of Inquiry into Aircraft Accidents. The judicial authority was the Investigating Judge of Sion district. The aircraft was a modified Piper J3C with a total fuel capacity of 84 liters, consisting of two 32-liter wing tanks and a 20-liter collector tank. The wing tanks had transparent tube gauges with approximate markings at 30, 25, and 0 liters. The fuel system was not compliant with the aircraft's country of origin standards, but the Swiss Federal Air Office had accepted it and issued a normal airworthiness certificate.
Narrative
The flight began at 0858 local time from Berne to Ascona, with about 75 liters of fuel according to the pilot's estimation from the gauges. The flight proceeded via the Aar and Kander valleys, crossing the Lötschen Pass and Simplon Pass, landing at Ascona at 1016.
At 1550, the pilot and passenger departed Ascona for Berne, with an estimated 50 liters of fuel. The pilot chose not to refuel to avoid overloading. After encountering turbulence in the Centovalli, the pilot diverted to the Maggia valley. At 1620, the wing tank gauges showed zero, leaving only the 20-liter reserve. The pilot estimated this would suffice for about one hour. He crossed the Nufenen Pass into the Rhône valley and headed toward the Grimsel. Seeing clouds on the north side, he decided to divert to Sion for refueling. At 1712, the engine stopped due to fuel exhaustion, 82 minutes after departure from Ascona. The pilot selected a field on the left bank of the Rhône but was unable to reach the Sion runway. During the approach, the aircraft struck a tree with the right wing, crashed, and came to rest under the tree.
Damage
Both occupants were seriously injured: the pilot suffered a fracture of the right foot and facial hematomas; the passenger had one shoulder fractured and the other dislocated. They were evacuated by helicopter to Sion hospital, treated, and discharged the next day for convalescence at home. The aircraft sustained substantial damage, estimated at 65%, affecting the fuselage, wings, and landing gear. There was no significant ground damage.
Findings
- The aircraft had been rebuilt after a fire in 1958 and fitted with an O-200A engine and a custom fuel system.
- At the time of the accident, the aircraft had flown 63 hours since its last overhaul on 4 March 1966.
- The maximum takeoff weight was 580 kg; actual weight was about 535 kg, and center of gravity was within limits.
- Using a typical consumption of 25 liters per hour, the pilot had about 67 liters at departure from Berne and 34 liters from Ascona.
- The forced landing field was about 200 meters long with two ditches, oriented at 060° azimuth, and the area was characterized by uneven fields, trees, and hidden ditches.
- The engine stop was confirmed to be due to fuel exhaustion.
- The pilot's evaluation of fuel quantity was inaccurate, likely due to the rudimentary gauges; the collector tank had no gauge, so after the wing tanks emptied, the pilot had no means to monitor remaining fuel.
- Multiple airfields were available along the Rhône valley but the pilot did not consider landing earlier.