What happened
On August 12, 2012, a pilot operating a Fisher USA FP 202 Koala ultralight aircraft departed from Soest for a sightseeing flight intended to visit Mönesee. While flying over the Arnsberg Forest, the pilot reduced the altitude from 3,000 ft to 1,500 ft near Arnsberg. As the pilot attempted to apply full power to proceed toward Brilon, the engine failed to respond and began stuttering.
Recognizing the engine failure, the pilot initiated an emergency landing on the A46 motorway. The pilot reported that no vehicle traffic was observed on a 1.5 km stretch of the highway in the direction of Meschede at the time of the incident. The aircraft landed on the center lane near the Arnsberg Ost exit, rolling out on the shoulder. The pilot then maneuvered the tail of the aircraft over the guardrail to park it perpendicular to the direction of travel. There were no injuries and no damage to third-party property.
The investigation
The BFU examined the aircraft and the fuel system following the incident. The investigation focused on the engine's failure to maintain power and the condition of the fuel components. The aircraft, a single-seat wooden construction ultralight, had recently undergone maintenance involving the removal and reinstallation of the fuel tank by the owner, during which the fuel filter was replaced. Additionally, welding work had been performed on the removed fuel tank by a certified welder prior to the flight.
Findings
Physical examination of the fuel system revealed contamination within the fuel filter, which contained black particulate matter. Furthermore, floating impurities and a small amount of water were detected in the fuel. The fuel selector valve also contained several white particles within the bore in the 'ON' position. These findings indicate that fuel contamination was the primary cause of the engine's loss of power.