What happened
On July 2, 2015, a Piper J3C-65 Cub, registration F-BFML, was conducting a local flight from Pontoise Aerodrome. After departing from runway 05, the pilot flew east for approximately ten minutes before returning to the airfield to join the landing pattern.
During the final turn of the circuit, the engine began to misfire, resulting in a significant drop in RPM and power. The pilot attempted to rectify the issue by adjusting the throttle and toggling the carburetor heat, but these actions failed to restore engine performance. Unable to reach the runway due to the low altitude at which the failure occurred, the pilot declared an intention to perform an emergency landing in a nearby field. The aircraft entered an unharvested wheat field, where it traveled approximately twenty meters before striking a utility pole and flipping onto its back. The aircraft sustained heavy damage.
The investigation
The investigation focused on the engine's performance and the use of the carburetor heat system. Technical examinations of the engine and fuel system revealed no mechanical malfunctions or defects.
Investigators examined the pilot's actions and the environmental conditions. At the time of the accident, the temperature was 30 °C. The investigation noted that while the club's checklist suggested activating carburetor heat during the downwind leg, the pilot had been trained to use the system systematically regardless of the weather. The investigators also reviewed the aerodynamic effects of carburetor heat, noting that activating the system increases the temperature of the incoming air, thereby decreasing air density and enriching the fuel-air mixture.
Findings
- The primary cause of the engine power loss was the excessive enrichment of the air-fuel mixture caused by the use of carburetor heat during high ambient temperatures.
- The use of carburetor heat in 30 °C weather led to an overly rich mixture, causing the engine to lose power.
- The unharvested state of the wheat field contributed to the aircraft's inability to avoid striking a utility pole during the landing roll.