Buckland, AK, USA
N8019Z
CESSNA U206
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.
HISTORY OF FLIGHTOn September 9, 2022, about 1530 Alaska daylight time, a Cessna U206 airplane, N8019Z, was substantially damaged when it was involved in an accident near Buckland, Alaska. The commercial pilot and one passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 135 passenger flight. The purpose of the flight was to transport a passenger and his gear to a gravel bar on the Buckland River. During cruise flight, the engine began to shudder and lose power. The pilot made an immediate turn toward the airport in Buckland and maintained flying airspeed; however, the airplane would not maintain altitude. He attempted to restart the engine without success. The engine continued to run rough until there was a loud “clunk” and the propeller suddenly stopped. The pilot landed the airplane on rough terrain, and it nosed over during the landing. AIRCRAFT INFORMATIONAccording to the engine service and maintenance log, the engine had been overhauled and converted from an IO-520-F engine on September 10, 2021, in accordance with Vitatoe Aviation Supplemental Type Certification (STC) number SE03187AT. A deviation from the STC to install Continental cylinder and valve assemblies 658810A1 in place of assemblies 658592A1 was approved through Federal Aviation Administration Form 8110-3, dated September 20, 2021. The engine was installed on the airplane with zero time since overhaul on March 1, 2022. The last log entry was on September 8, 2022, at which time the engine had accumulated 505 hours of time in service (TIS) since overhaul. In that time, oil changes were performed at about 25-hour TIS intervals and the filter was changed at about 50-hour TIS intervals following the first two oil and filter changes that occurred after 11.5 hours and 32 hours TIS since overhaul. AIRPORT INFORMATIONAccording to the engine service and maintenance log, the engine had been overhauled and converted from an IO-520-F engine on September 10, 2021, in accordance with Vitatoe Aviation Supplemental Type Certification (STC) number SE03187AT. A deviation from the STC to install Continental cylinder and valve assemblies 658810A1 in place of assemblies 658592A1 was approved through Federal Aviation Administration Form 8110-3, dated September 20, 2021. The engine was installed on the airplane with zero time since overhaul on March 1, 2022. The last log entry was on September 8, 2022, at which time the engine had accumulated 505 hours of time in service (TIS) since overhaul. In that time, oil changes were performed at about 25-hour TIS intervals and the filter was changed at about 50-hour TIS intervals following the first two oil and filter changes that occurred after 11.5 hours and 32 hours TIS since overhaul. WRECKAGE AND IMPACT INFORMATIONA postaccident examination revealed the No. 1 cylinder had extensive internal damage. The No. 1 exhaust valve keeper was crushed. The exhaust valve rocker arms on cylinder Nos. 1 and 5 were deformed, along with the Nos. 1 and 5 valve stems. Multiple fragments of metal debris were found in the air intake and oil sump. Examination of components by the National Transportation Safety Board Materials Laboratory revealed the valves from the engine showed evidence of worn seat faces (valve recession) on the intake valve heads and substantial valve sticking damage on both intake and exhaust valve stems. Two of the exhaust valves and rocker arms showed severe contact wear at the rocker pad and valve tip, and the No. 1 cylinder exhaust valve was bent and fractured. The tip end of the No. 1 cylinder exhaust valve was severely worn to a shape resembling a top hat, and any identifying marks on the tip were obliterated by the wear. The shoulder at the stem side of the keeper slot was also worn away to a cone-shaped slope transitioning from the stem diameter to the keeper slot diameter. The corresponding contact pad on the exhaust rocker arm was recessed from wear in a shape mirroring the valve tip wear. A portion of the arm adjacent to the pad was also worn consistent with contact with the valve spring seat. The pad end of the valve was tinted consistent with high heat exposure, and the worn areas of the pad were oxidized. The No. 3 cylinder intake valve was intact. The valve tip end face had a normal circular contact wear pattern. Isolated scuffs and longitudinal scratch marks were observed on the valve stem. Near the valve tip, multiple semicircular indents were observed on the stem. The valve seat face had a concave shape consistent with substantial valve seat face wear. The extent of wear appeared relatively uniform around the circumference. The No. 3 cylinder exhaust valve was intact. The valve tip end face was stained dark and had a normal circular wear pattern. Near the middle of the stem, the surface appeared burnished in 5 distinct longitudinally-oriented patches distributed around the circumference. A light coating of gray combustion deposits was observed on the intake valve head face, and the exhaust valve had a thicker layer of reddish orange deposits. The No. 5 cylinder intake valve was intact. Extensive longitudinally-oriented dark streaks and galling damage were observed on the valve stem. Multiple rocker arm impact marks were observed on the end face. Oxidation and pits were also observed across the surface. The valve seat face had a concave shape consistent with substantial valve seat face wear. The No. 5 cylinder exhaust valve and rocker arm showed extensive longitudinally-oriented dark streaks and galling damage were observed on the valve stem. The tip of the exhaust valve was worn to a shape resembling a top hat, and any identifying marks were obliterated by the wear. The corresponding contact pad on the exhaust rocker arm was recessed from wear in a shape mirroring the valve tip. A portion of the arm adjacent to the pad was also worn consistent with contact with the valve spring seat. The pad end of the valve was tinted consistent with high heat exposure, and the worn areas of the pad were oxidized. The valve heads had a uniform heat signature pattern around the circumference. Both valve faces had limited deposits of combustion products. Two of the exhaust rocker arms were not associated with a particular cylinder. The rocker pads for both rocker arms were tinted dark consistent with high heat exposure, and coked oil deposits were observed on the rocker arm. A depression was observed in each pad consistent with impact and wear damage from contact with the corresponding valve tip. Pits were also observed on the pad surfaces within the contact areas. Procedures for rocker arm inspection and bushing replacement for the accident engine are described in the Continental Standard Practice Maintenance Manual (SPMM), and a copy of relevant pages from the SPMM revision dated January 6, 2023, was provided by a representative from Continental Aerospace Technologies. A table in the SPMM lists exhaust and intake rocker arm assembly part numbers 652130 and 652131, respectively, for use in engines including the Continental IO-550-A, B, and C engines. A drawing of these rocker arms as illustrated in the SPMM was included in the documentation. Since part numbers can fade or become illegible over time, the SPMM provides diagrams for identifying distinguishing features associated with parts currently in use. The drawing for the intake rocker arm (part number 652131) notes a “bump” on the inboard side of the valve arm. In contrast, the drawing for the exhaust rocker arm (part number 652130) does not show a bump. The SPMM notes that rocker arm bushings must be replaced at engine overhaul. The SPMM includes a warning that dual bushing rocker arms have been discontinued for engines including the IO-550-series engines, and dual bushings for installation in these older rocker arms are no longer available. The obsolete rocker arms (rocker arms with a smooth rocker arm bore designed for use with the dual bushings) should be discarded and replaced with rocker arms that have an oil feed groove machined into the rocker arm bore for use with the current single bushing installation. The warning in the SPMM further states, “Installation of single piece rocker bushings in rocker arms designed for two piece bushings may restrict or block oil flow to the rocker shaft and valve guides, resulting in possible engine failure.” The single bushings for the part numbers 652130 and 652131 intake and exhaust rocker arms contained two oil passage holes. The bushings are to be installed with the oil passage holes located at the inboard side of the rocker arm at specific angles relative to the transverse plane. Additionally, the hole centers are offset from the midpoint between the two end faces of the bushing, and the SPMM provides a diagram of the rocker arms viewed looking inboard showing correct positioning of the offset relative to each rocker arm’s longitudinal axis. A warning in the SPMM states, “Incorrectly positioned bushing oil passages 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.” A bump was present on the inboard side of the valve arm on 5 of the 6 exhaust rocker arms from the accident engine, which was inconsistent with the drawing for the current exhaust rocker arm for the Continental IO-550-series engine as illustrated in the SPMM. None of the oil holes were located in the positions described in the SPMM. In 5 of the 6 exhaust rocker arms, including those from cylinder Nos. 1 and 5, both oil holes were located outboard of the bushing centerline. In the remaining exhaust rocker arm, one hole was located outboard of the bushing centerline, and the other hole was located directly inboard of the bushing centerline. In one of the rocker arms removed from a cylinder other than No. 1 or 5, the bushing was also installed with the oil holes offset to the incorrect side relative to the arm’s longitudinal axis. The bushings were pressed out of the rocker bore in each of the exhaust rocker arms. Smooth rocker bores were observed in 5 of 6 exhaust rocker arms, including those from the Nos. 1 and 5 cylinders after the bushings were pressed out. None of the oil hole passages were located in the positions described in the SPMM. The oil holes in all 6 intake rocker arms were similarly located outboard of the bushing centerline. Additionally, the oil hole positions were offset to the incorrect side of the longitudinal centerline, as compared to the SPMM, in 4 of the 6 arms. Bushings were pressed out of the rocker bore in each of the intake rocker arms. Two of the six intake rocker arms had a machined oil feed groove in the rocker bore while the remaining four intake rocker arms had a smooth rocker bore.
The installation of incompatible rocker arms and rocker arm bushings during an engine overhaul, which resulted in a complete loss of engine power.
Source: NTSB Aviation Accident Database
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