Summary
On 10 August 1966, a Piper J3C aircraft (registration HB-OUI) operated by the Valais section of the Aero Club of Switzerland was forced to land in a field near St-Léonard, Valais, after its engine stopped due to fuel starvation. The pilot and his passenger were unharmed, but the aircraft sustained substantial damage.
Pilot and Passenger
The pilot, born in 1942, held a private pilot license issued on 27 June 1961, valid until 25 July 1967. He had accumulated 124 flight hours and 564 landings, including 63 hours and 235 landings on Piper J3C and PA-18 Super Cub aircraft. His qualifications were unremarkable, and no prior accidents or violations were recorded. The passenger, born in 1941, had no aeronautical background.
Aircraft
The Piper J3C was a single-engine, high-wing monoplane with a mixed construction, two seats in tandem, fixed landing gear, and a tailwheel. It was powered by a Rolls-Royce 0-200 A engine rated at 100 hp, driving a fixed-pitch metal two-blade Mc Cauley propeller. Airworthiness certificates were issued on 30 March 1963. The airframe had accumulated about 1384 hours since its last major overhaul in July 1961; the engine had 184 hours since installation on 10 March 1966. The last periodic inspection was on 25 June 1965, and the last 33-hour check was on 21 July 1966.
Accident Narrative
On the morning of the accident, the pilot intended to rent a Piper Super Cub from Air-Glacier at Sion airport, but none were available. He then arranged to use the club Piper J3C HB-OUI. After checking the fuel gauges, he noted 15 liters in the left wing tank and approximately 20 liters in the right, totaling 35 liters. He planned a local flight of at most 60 minutes and deemed this sufficient, deciding not to refuel.
Takeoff was at 0934 LT. The pilot climbed, crossed the Sanetsch Pass, descended into the Simmental valley, then flew up the Kander valley, intending to return via the Gemmi Pass and Montana. However, before crossing the Gemmi Pass, he noticed the fuel level was critical: the left gauge ball was no longer visible, and the right gauge ball was at the limit of the red sector (5 liters). The air was somewhat turbulent, preventing an exact reading. Believing he could still reach his departure point, the pilot crossed the pass, began descent, and reduced power to conserve fuel.
Approximately 9 km from the airport, at about 700 m AGL, the engine began to misfire. Forced to land off-field, he spotted a cut meadow about 200 x 30 m, oriented east-west, near the cantonal road west of St-Léonard. With adequate height, he performed a reconnaissance turn, then turned left base for a westbound landing. In the final turn, he realized he was too short. He pushed the stick forward to gain speed, but when he tried to pull up to clear some apple trees before the intended landing spot, the aircraft was too slow and did not respond. About 50 m before the start of the field, the left wing struck a tree. The aircraft yawed left and crashed to the ground.
Damage
The occupants exited the aircraft unharmed. The aircraft suffered significant damage, particularly to the left wing and fuselage; the engine was recoverable, but the airframe was 70% destroyed. Ground damage included one damaged apple tree and flattened grass.
Findings
Examination of the wreckage confirmed that the engine stopped due to fuel starvation. The last refueling (68 liters) had occurred approximately 24 hours prior. After that, the aircraft had flown a series of flights totaling 2 hours 11 minutes. Assuming a consumption of 24 L/h at 75% power, only about 20 liters remained (about 50 minutes endurance) when the pilot began his flight.
The aircraft's fuel system had been modified in 1961: two wing tanks (36 liters each) were installed, each with a ball-type gauge and a graduated placard showing 5, 10, 15, and 20 liters, and a useful capacity of 34 liters. The placard also stated: "Attention, close the cap; take off and climb only above red," but did not specify whether the calibration was valid for level or three-point attitude. A subsequent reconstruction revealed that the indicated quantity in three-point attitude (on the ground) was 10 liters (5 per tank) more than the actual fuel on board at the time of departure.
Discussion
The time between takeoff and the accident confirmed that the fuel quantity at departure was insufficient. The difference between actual and indicated fuel on the ground (three-point attitude, about 15° nose-up) was due to the gauge placement: mounted too far aft in the wing profile, they gave errors up to 50% of the actual fuel when on the ground or in climb attitude. The pilot, flying this aircraft for the first time, was unaware of this characteristic. This would not have been consequential if the system had dual calibration (as on the original Piper Super Cub) or a placard warning that the reading is valid only in level flight.
Although misled initially, the pilot could have avoided being caught by fuel starvation if he had monitored the gauges more carefully during the flight, or at least landed at the nearest (military) airstrip (Tourtemagne or Heichenbach) when he noticed one gauge ball had disappeared and the other was in the critical sector.
Conclusion
The fuel starvation resulted from the pilot starting the flight with insufficient fuel, misled by erroneous gauge indications, and failing to interrupt the flight upon realizing the low fuel level.