What happened
On June 20, 1961, a pilot operating a Piper Cub HB-OIS departed from Bern for a round-trip flight to Kloten, carrying his elderly father and a five-year-old niece. During the outbound leg, visibility deteriorated near the Sempach and Hallwiler lakes. Fearing the visibility had dropped below safe limits, the pilot decided to turn back toward Bern. During the return flight, the pilot became disoriented, and due to the passengers' physical distress—including the child becoming ill—the pilot opted to perform an emergency landing in an open field near Huttwil.
After landing safely on a field, the pilot intended to return to Bern immediately. Before departing, he inspected the available takeoff area on foot, estimating a required distance of 120 meters. At approximately 15:30 local time, the pilot attempted a takeoff. However, the aircraft reached the end of the usable strip and entered a grain field, which caused significant deceleration. Rather than aborting the attempt, the pilot maintained full throttle. The aircraft continued rolling across various fields until the landing gear struck a terrain depression, causing the aircraft to flip onto its back.
The investigation
The investigation examined the aircraft's weight, the pilot's flight planning, and the physical conditions of the takeoff site. It was established that the Piper Cub was overloaded for the takeoff, with a weight of approximately 593 kg, exceeding the maximum allowable takeoff weight of 580 kg. The investigation also revealed that the aircraft's braking system had been modified with non-standard parts that caused increased rolling resistance, though this was not the primary cause of the accident.
Furthermore, the investigation noted that the pilot had not consulted weather services prior to the flight and was not proficient in using the aircraft's radio equipment to request special VFR clearances. The takeoff surface itself was identified as unsuitable, characterized by uneven, soft ground and a 1.5% downgrade slope.
Findings
- The aircraft was operating significantly above its maximum takeoff weight.
- The pilot attempted to force a takeoff from an unsuitable, short, and uneven field.
- The pilot underestimated the required takeoff distance, failing to account for the high ambient temperature and the heavy load.
- The pilot's decision to continue the takeoff despite the aircraft losing ground contact in the grain field contributed to the accident.