Incident Overview
On 17 October 2018, a Sonex LL aircraft, operated by the Instytut Techniczny Wojsk Lotniczych, experienced an engine-related incident during a measurement flight. The aircraft was performing a series of flights between airports EPBY, EPMO, and EPSY. After departing EPMO at 11:20 LMT for a flight to EPSY, the pilot reported vibrations and power loss. He declared an intention to return to EPMO but was forced to perform a precautionary landing in a field approximately 2 km north of Załuski. The landing occurred at about 11:36 LMT after the engine ceased operation at an altitude of around 200 feet AGL. The pilot landed with a stall just above the ground. Post-landing, the pilot exited the aircraft, found no visible damage, and informed air traffic control and the police.
Flight and Engine Issues
During the flight, approximately 5 minutes and 15 seconds after takeoff, data from the engine monitoring system (EMS) showed a sudden drop in RPM and intense fluctuations. By the 9th minute, cylinders 3 and 5 ceased firing, and the engine stopped completely at 15 minutes and 40 seconds. The pilot had previously reported vibrations and power loss to flight information services (FIS Olsztyn) and the tower at EPMO.
Investigation Findings
An inspection of the Jabiru 3300A engine revealed:
- Missing through-bolts securing the flange of cylinder 5.
- A dislocated intake stub for cylinder 5, pulled out from the intake manifold.
- Leakage in cylinder 3.
- Signs of high temperature on the valve train of cylinder 3.
- Traces of exhaust gases in the intake ports of all cylinder heads.
Probable Cause
The probable cause, as determined by the investigation, was the occurrence in quick succession of the following factors: 1. Flame entering the intake system through a leaky intake valve in cylinder 3, igniting fresh mixture in the intake and causing an extreme and rapid drop in RPM. 2. The sudden RPM drop and loss of power generated high inertial forces that sheared the through-bolts of cylinder 5. 3. The sheared bolts caused loss of rigidity of cylinder 5 and dislocation of its intake stub, leading to an unmetered air leak, lean mixture, and eventual engine shutdown.
No preventive actions or safety recommendations were formulated by the investigating body.