What happened
On October 17, 2018, a Sonex LL was conducting a series of four scheduled flight measurement missions between various airfields, including EPBY, EPMO, and EPSY. Following a successful initial leg, the pilot departed EPMO for EPSY at 11:20 LMT. Shortly after establishing contact with FIS Olsztyn, the aircraft began experiencing significant vibrations and oscillations.
The pilot notified air traffic control of the engine issues and expressed an intention to return to EPMelowo. However, as power continued to decrease and the aircraft became unable to maintain a constant altitude, the pilot decided to perform an emergency landing in an open field. While descending at approximately 85 knots over the area of Michałówek, the engine stopped working entirely. To clear nearby buildings while at an altitude of approximately 200 feet AGL, the pilot deployed the flaps to the takeoff position, which resulted in a near-stall speed. The aircraft landed in a field approximately 2 km north of Załuski, experiencing a stall just before touchdown. The pilot exited the cabin and performed a visual inspection, noting no immediate external damage.
The investigation
Investigators analyzed data from the Engine Monitoring System (EMS) SD card. The data revealed a rapid drop in RPM and intense fluctuations starting at the 5-minute 15-second mark of the flight. By the 9-minute mark, combustion had ceased in cylinders 3 and 5, and the engine finally shut down at 15 minutes and 40 seconds into the flight.
Upon physical examination of the Jabiru 3300A engine, investigators discovered that two through-bolts securing the cylinder 5 flange were missing. Additionally, the intake manifold fitting for cylinder 5 had been pushed out of the collector. The investigation also identified a leak in cylinder 3, evidence of high-temperature damage on the valve lever assembly of cylinder 3, and traces of exhaust gases within all intake channels.