History of Flight
On September 9, 2022, about 1530 Alaska daylight time, a Cessna U206 airplane, registered N8019Z, was substantially damaged during a forced landing near Buckland, Alaska. The commercial pilot and one passenger were not injured. The flight was conducted under 14 Code of Federal Regulations Part 135 as a passenger flight. The purpose was to transport a passenger and gear to a gravel bar on the Buckland River. During cruise, the engine began to shudder and lose power. The pilot turned toward Buckland Airport and maintained airspeed but could not maintain altitude. An attempt to restart the engine was unsuccessful; the engine continued running rough until a loud "clunk" was heard and the propeller stopped. The pilot landed on rough terrain, and the airplane nosed over.
Engine and Maintenance
According to engine maintenance records, the engine had been overhauled and converted from an IO-520-F on September 10, 2021, under Vitatoe Aviation Supplemental Type Certification (STC) SE03187AT. A deviation to install Continental cylinder and valve assemblies 658810A1 instead of 658592A1 was approved via FAA Form 8110-3 on September 20, 2021. The engine was installed on the airplane with zero time since overhaul on March 1, 2022. The last log entry on September 8, 2022, showed 505 hours of time in service (TIS) since overhaul. Oil changes were performed at approximately 25-hour TIS intervals, and oil filter changes at approximately 50-hour TIS intervals, after initial changes at 11.5 and 32 hours TIS.
Postaccident Examination
A postaccident examination revealed extensive internal damage to the No. 1 cylinder. The No. 1 exhaust valve keeper was crushed, and exhaust valve rocker arms on cylinders No. 1 and 5 were deformed, along with their valve stems. Multiple metal fragments were found in the air intake and oil sump.
Examination by the National Transportation Safety Board Materials Laboratory showed worn seat faces (valve recession) on intake valve heads and substantial valve sticking damage on both intake and exhaust valve stems. Two exhaust valves and rocker arms exhibited severe contact wear at the rocker pad and valve tip. The No. 1 cylinder exhaust valve was bent and fractured, its tip end severely worn to a top-hat shape, with identifying marks obliterated. The shoulder at the stem side of the keeper slot was worn away to a cone-shaped slope. The corresponding exhaust rocker arm had a recessed contact pad mirroring the valve tip wear, and a portion of the arm adjacent to the pad was worn consistent with contact with the valve spring seat. The pad end was tinted from high heat exposure and oxidized.
The No. 3 cylinder intake valve was intact with normal circular wear on the tip face, but the stem had isolated scuffs and longitudinal scratches, and near the tip there were multiple semicircular indents. The valve seat face had substantial concave wear, relatively uniform around the circumference. The No. 3 exhaust valve was intact with a dark stained tip face and normal circular wear; the stem had five longitudinally-oriented burnished patches. Gray combustion deposits were light on the intake valve head, while the exhaust valve had a thicker layer of reddish orange deposits.
The No. 5 cylinder intake valve had extensive longitudinal dark streaks and galling on the stem, rocker arm impact marks on the end face, and oxidation and pits. The valve seat face had substantial concave wear. The No. 5 exhaust valve and rocker arm showed similar longitudinal dark streaks and galling, with the tip worn to a top-hat shape. The corresponding rocker arm pad was recessed and the adjacent area worn, with high heat tinting and oxidation.
Two exhaust rocker arms not associated with specific cylinders had pads tinted dark from heat, coked oil deposits, and depressions consistent with contact and wear from valve tips, along with pitting.
Procedures for rocker arm inspection and bushing replacement from the Continental Standard Practice Maintenance Manual (SPMM) were reviewed. The SPMM specifies exhaust and intake rocker arm assemblies part numbers 652130 and 652131 for Continental IO-550-series engines. The drawing for intake rocker arm (652131) shows a "bump" on the inboard side, while the exhaust drawing (652130) does not. The SPMM warns that dual bushing rocker arms have been discontinued and that installing single piece bushings in rocker arms designed for two piece bushings may restrict oil flow, leading to possible engine failure. Correct bushing installation requires oil passage holes to be positioned at specific locations; incorrect positioning can result in loss of lubrication, severe wear, and possible engine failure.
Examination of the accident engine's rocker arms showed that a bump was present on the inboard side of 5 of the 6 exhaust rocker arms, inconsistent with the current exhaust rocker arm drawing. None of the oil holes were located per SPMM specifications. In 5 exhaust rocker arms, including those from cylinders No. 1 and 5, both oil holes were outboard of the bushing centerline; in the remaining arm, one hole was outboard and one inboard but offset incorrectly. One rocker arm from another cylinder had the bushing oil holes offset to the wrong side relative to the arm's longitudinal axis. After pressing out bushings, smooth rocker bores were observed in 5 of the 6 exhaust arms, and none of the oil hole passages were positioned correctly. All 6 intake rocker arms had oil holes similarly located outboard of the bushing centerline, and 4 had holes offset to the wrong side. Two of the six intake arms had a machined oil feed groove in the bore, while four had smooth bores.