What happened
On March 11, 2023, a C-T206H aircraft, registration HL1113, was conducting an aerial survey mission over the airspace of Hoengseong-gun, Gangwon-do. While cruising at approximately 10,500 feet, the engine suddenly emitted a loud bang, followed by a significant drop in manifold pressure and RPM.
The pilot attempted to stabilize the aircraft by setting the power to idle and monitoring the instruments. However, upon attempting to increase power, the propeller came to a complete stop. Despite six unsuccessful attempts to restart the engine, the pilot was forced to glide toward the nearest available landing site. Because the distance to Wonju Airport exceeded the aircraft's remaining gliding range, the pilot executed an emergency landing in a pasture located within the Livestock Research Institute. The aircraft was destroyed upon impact, though the pilot and photographer onboard escaped without injury.
The investigation
Investigators from the ARAIB examined the wreckage and performed a detailed analysis of the Lycoming TIO-540-AJ1A3 engine. The investigation included a microscopic analysis of engine components and a chemical examination of the engine oil and fuel.
Laboratory tests of the engine oil revealed significant amounts of copper and aluminum alloys. These metallic residues, found in the oil filter and oil box, were identified as remnants of the cylinder 6 connecting rod small end bushing. The investigation also reviewed maintenance records and the applicability of existing service bulletins (SB) from the engine manufacturer.
Findings
Technical analysis established that the engine failure resulted from cascade damages starting with the failure of the cylinder 6 connecting rod small end bushing. The investigation determined that the bushing had worn down—potentially due to improper installation or non-conforming specifications—creating a gap between the piston pin and the connecting rod.
This gap allowed for repetitive impact loads that eventually caused the small end of the connecting rod to fracture. Once the rod disconnected from the piston, the resulting uncontrolled motion caused the rod to strike the piston and the camshaft. This sequence of events led to the seizure of the piston and caused secondary damage to the camshaft, crankcase, and air intake duct, ultimately forcing the engine to stop.
A contributing factor was the manufacturer's insufficient service bulletin preparation, as the engine had not been subject to specific inspections (SB No. 632B or SB No. 645) regarding connecting rod bushing integrity.
Safety action
Following the findings, the ARAIB issued safety recommendations to domestic operators using similar Lycoming engines to inspect engine oil filters for copper alloy traces. Additionally, a recommendation was made to Lycoming Engines to reappraise and improve their related service bulletins.