What happened
On August 4, 1999, an S-6 Coyote II mod. ultralight aircraft, registration OH-U247, was performing a repositioning flight from Kannonkoski to Viitasaari. While approaching the grass runway at Viitasaari, the pilot encountered turbulence caused by morning winds and decided to abort the landing at an altitude of approximately two meters to perform a go-around.
As the pilot increased power to climb, the engine experienced a significant loss of power. The aircraft was unable to maintain altitude, climbing only about 100 feet before the pilot was forced to execute an emergency landing in a young forest approximately 150 meters beyond the airfield. The aircraft struck the ground at an estimated speed of 40 km/le, causing the aircraft to skid and rotate 100 degrees to the left. The pilot sustained minor injuries to the neck.
The investigation
The investigation examined the engine, the intake system, and the ignition components. Technical inspections revealed that the engine had undergone extensive modifications and overhauls using automotive parts without the required authorization from the aviation authority. Investigators examined the intake manifold, the carburetor, and the magneto timing. The investigation also reviewed the aircraft's maintenance documentation, which was found to be inconsistent and lacked proper signatures from the manufacturer or inspector.
Findings
Several technical deficiencies were identified that contributed to the engine failure. The most significant factor was the design of the intake manifold, which featured excessively long pipes (up to 100 cm) with sharp bends and uneven welds. These long, copper and aluminum pipes were prone to cooling, causing fuel to condense on the inner walls. When the pilot increased power, the sudden surge of airflow mobilized this accumulated fuel, creating an overly rich mixture that disrupted engine operation.
Other contributing factors included:
- Incorrect ignition timing: The magneto timing was set at 44-45° before top dead center, far exceeding the required 28-30°.
- Improper air filtration: The air filter housing was poorly constructed, allowing air to flow through only about 12% of the filter's surface area.
- Unauthorized modifications: The engine overhaul and modifications, including the use of a Dell'Orto DHLA 40 carburetor, were performed without official approval.
- Maintenance discrepancies: The aircraft's flight manual contained inaccurate information regarding the engine, propeller, and ignition procedures, and the aircraft had not been reweighed following engine modifications.