Cessna U206 accident near Buckland, Alaska, September 9, 2022

The NTSB's narrative final · quoted from the NTSB record

The pilot reported that during cruise flight the engine began to shudder and lose power. The engine continued to run until there was a loud clunk and the propeller stopped. Unable to maintain altitude or restore power, the pilot performed a forced landing and the airplane came to rest inverted. Postaccident examination of the engine components revealed obsolete rocker arms that were incompatible with the installed single-piece bushings, which were improperly positioned in the rocker arms. Five exhaust rocker arms and four intake rocker arms had a smooth rocker bore consistent with rocker arms designed for use with dual bushings that are no longer available for installation on the accident engine. As indicated in warnings in the engine manufacturer’s maintenance manual, use of single-piece bushings in arms designed for dual bushings may restrict or block oil flow to the rocker shaft and valve guides, resulting in possible engine failure. The examination also revealed the oil holes in the bushings were positioned incorrectly at the outboard side of the housing, and in some cases the holes were offset to the wrong side of the arm’s longitudinal centerline. The incorrect positioning of oil holes in the bushings (as observed in this engine) will result in a loss of rocker arm shaft lubrication, severe wear of the rocker arm bushing, shaft, and valve guide and possible engine failure. The overhauled engine had been installed in the airplane about 6 months, and just over 500 hours, before the accident. The use of obsolete rocker arms and the incorrect positioning of the bushing holes likely led to insufficient oil spray to the valve tips and rocker pads. The insufficient oil spray from both intake and exhaust rocker arms likely led to inadequate cooling as evidenced by heat tinting on the exhaust valve tips, exhaust valve springs, and exhaust rocker arms and the eventual overstress fracture of the No. 1 exhaust valve from impact with the piston while stuck open. The engine likely failed due to the improper installation of 5 obsolete exhaust rocker arms and 4 obsolete intake rocker arms when the engine was last overhauled. The obsolete rocker arms are incompatible with the current bushings that were installed in all of the rocker arms, which led to inadequate lubrication to the corresponding intake and exhaust valve tips and the subsequent fracture of the No. 1 exhaust valve.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.