What happened
On July 28, 1973, a Beagle Pup B-121, registration HB-NAU, departed from Porrentruy for a flight toward Berne. The aircraft was carrying a pilot and three passengers, including a five-year-old child. Shortly after takeoff, the engine began to vibrate and the RPM dropped significantly. Although the pilot attempted to use the carburetor heat to restore power, the engine's performance remained unstable.
As the aircraft climbed through a valley near Chevenez, the engine power dropped abruptly. Due to the narrow terrain, the pilot was unable to perform a 180-degree turn at low altitude and continued flying straight to avoid hitting obstacles. While attempting a wide turn to avoid flying over the village of Chevenolog, the aircraft's speed decreased, and the engine began to misfire heavily, producing loud, irregular explosions. Realizing a landing was necessary, the pilot attempted to reach an open pasture. However, the aircraft entered a stall at approximately 15 meters above the ground and struck the ground in a slight dive.
The investigation
Following the accident, investigators examined the wreckage and the engine components. The aircraft was destroyed upon impact, and the incident resulted in one injury among the occupants. The investigation focused on the engine's performance and the fuel used during the flight.
Technical analysis of the engine revealed traces of deposits on the spark plugs and pistons. Laboratory testing of the spark plugs identified significant amounts of lead and nickel. Furthermore, the investigation looked into the fuel specifications, noting that the aircraft had been refueled with 100L aviation gasoline, which contains a much higher lead content than the 80/87 grade for which the engine was originally designed.
Findings
- The primary cause of the engine failure was a short circuit in one or more spark plugs caused by conductive combustion residues.
- These residues were primarily composed of lead, stemming from the use of 100L fuel, which contains approximately four times the amount of tetraethyl lead compared to the 80/87 grade.
- High engine temperatures during the low-speed, high-power climb phase likely facilitated the formation of these conductive deposits.
- The pilot's decision to maintain a straight flight path through the narrow valley was considered appropriate given the low altitude and terrain constraints.
Safety action
- The investigation noted an increase in engine malfunctions in engines designed for 80/87 fuel following the widespread introduction of 100L fuel.
- It was recommended that engines originally designed for lower-lead fuel should continue to use 80/87 grade gasoline to mitigate the risk of spark plug fouling and reduce environmental impact.