What happened
On December 16, 2006, a pilot was conducting a local flight in a Rans Coyote S-6 ES ultralight aircraft from a private airfield in Esperce, France. During the initial climb, while reducing engine RPM from 6,500 to 6,000, the engine and propeller suddenly stopped. The pilot performed an emergency landing in a nearby field.
Following the first landing, the pilot and the aircraft owner inspected the engine and noticed a loose battery terminal. After tightening the terminal, the engine was restarted and ran normally for several minutes. Believing the loose connection was the sole cause of the failure, the pilot decided to attempt a second takeoff from the edge of the field to maximize the available runway length, flying solo to reduce the aircraft's weight. During this second climb, at an altitude of approximately 300 feet, the engine and propeller failed a second time. The pilot executed a turn to return to the field for another landing. During the landing roll, the nose gear struck a rut, causing it to break, and the aircraft came to a halt on its propeller.
The investigation
The investigation focused on the cause of the dual engine failures and the validity of the pilot's initial diagnosis. Investigators examined the Rotax 582 engine and found evidence of two successive engine seizures. While the cylinder dimensions were within manufacturer specifications and the oil pump was functional, the investigation looked into fuel delivery issues. The owner noted that the carburetor had not been cleaned since the aircraft was acquired a year prior. Investigators identified that chemical reactions within the fuel can create deposits in the coldest part of the carburetor, specifically the needle valve well. Such deposits can restrict fuel flow, leading to a lean mixture and subsequent overheating.
Findings
- The primary cause of the engine failures was an engine seizure caused by a lean fuel mixture.
- Insufficient knowledge of the fuel system led the pilot to misdiagnose the initial failure as being caused by a loose battery terminal.
- The loose battery terminal was determined to be an irrelevant factor, as it could not have caused the engine to stop.
- The aircraft sustained heavy damage during the second landing attempt.