Aircraft and Pilot Information
The aircraft involved was a Piper J-3C-65L-4 (65 hp), registered HB-OFK. It was privately owned and operated. The pilot, a Swiss citizen born in 1949, held a private pilot license and had logged 201 hours 41 minutes total flight time, with 40 hours 3 minutes on the accident type. In the preceding 90 days, he had flown 30 hours.
Flight History
On 4 August 1997 at 1740 local time, the pilot departed from Triengen alone on a private round flight intending to tour the Napf region, heading towards Entlebuch. According to his statements, the fuel quantity was approximately 30 liters. While over Willisau/Schülen at about 3000 feet, the engine lost power and began to stutter. The pilot attempted to restore power by applying carburetor heat and adjusting the throttle, but the engine continued to lose power and eventually stopped completely. Because the aircraft lacked an electric starter, the pilot prepared for a forced landing and made no further attempts to restart. He flew towards the Mettenberg area, a difficult terrain, in a westerly direction. In the Chanelgraben valley, a relatively flat section of the valley floor offered some potential for a landing. He crossed the low crest of the hill between Mettenberg and Mörisegg at low altitude. Shortly after, in gently descending terrain, the aircraft's right wing contacted the ground in a grain field. The aircraft then rotated approximately 180 degrees and came to a stop facing opposite the landing direction. The pilot remained uninjured.
Investigation Findings
An examination of the aircraft two days after the accident revealed no pre-existing damage that could have contributed to the event. The fuel system still contained gasoline; no contamination or water was found in the filter housing. The spark plugs indicated normal combustion. Both magnets were tested and functioned properly at various temperatures and speeds. The Stromberg carburetor was examined as far as its damage level allowed and was found to be in working order. The fuel filter was clean and in good condition. The aircraft was certified for VFR day operations. Mass and balance were within prescribed limits. The airframe had been overhauled on 27 January 1997 at 2716 hours. The engine underwent a major overhaul on 9 November 1985, was subsequently preserved, and inspected for long-term damage on 8 November 1996. A test run was conducted on 25 January 1997, and an oil change was performed on 7 June 1997 after 25 operating hours since installation.
Weather Conditions
At the time of the accident, the weather was slightly cloudy with visibility exceeding 10 km. Wind was from 50 degrees at 4 knots, gusting to 8 knots. Ground temperature was 26°C with a dew point of 16°C. At 1000 meters above sea level, the temperature was about 18°C and the dew point about 10°C.
Cause
The accident was attributed to a forced landing following an engine failure, likely caused by carburetor icing. Other technical or operational reasons for the engine failure could not be determined.