What happened
On August 24, 2011, an ultralight aircraft, a B & F Technik FK9 MK 2, departed from Jesenwang airfield for a planned flight to Tannheim. Shortly after takeoff from runway 07, the pilot reported engine issues via radio. During the initial climb, the pilot notified the flight controller that the engine had caught fire and the aircraft was burning.
Witnesses observed black smoke emanating from the lower engine cowling as the aircraft performed a right turn. As the aircraft flew toward the village of Babenried, the pilot activated the onboard emergency parachute system. The aircraft remained suspended under the parachute for approximately two to three seconds before impacting a gravel pit. The pilot, who sustained serious injuries, managed to extricate himself from the burning wreckage. The aircraft was destroyed by the fire.
The investigation
The BFU investigation focused on the engine's condition and the cause of the fire. Investigators examined the engine, which had been retrofitted with an Ecofly/Mercedes M160/1 engine. The investigation noted that the engine's documentation regarding the conversion was incomplete, lacking serial numbers and operating hours.
Physical examination of the engine revealed a broken exhaust bracket, which was determined to have failed due to fatigue from vibrations. Furthermore, testing conducted with the engine manufacturer revealed that the oil filter housing was susceptible to failure if over-tightened. Specifically, applying torque exceeding 100 Nm caused the ring-shaped stop on the oil filter cap to break, allowing the filter to loosen easily by hand. The engine manual provided no specific torque specifications for the oil filter.
Findings
- The fire likely originated near the turbocharger and oil filter area.
- A maintenance error involving the oil filter housing, combined with engine vibrations, likely caused the oil filter to loosen.
- Escaping engine oil ignited upon contact with the turbocharger, leading to a fire that spread to the aircraft structure.
- The pilot had previously replaced the exhaust due to vibrations, using a self-made bracket that did not match the manufacturer's original configuration.