WPR24LA248

The pilot was conducting a cross-country flight when he heard a “loud metal sound” followed by a partial loss of engine power. While troubleshooting the rough-running engine, he diverted to a local airport. The airplane then had a total loss of engine power, and the pilot initiated an off-airport landing to an open field. During the landing roll, the nose landing gear collapsed and the airplane impacted terrain, which resulted in substantial damage to the fuselage and both wings. Postaccident examination of the engine revealed mechanical damage to the No. 2 cylinder’s combustion dome and piston face. The keeper halves for the exhaust valve were missing, and the keeper shoulder on the valve stem showed fretting contact damage consistent with relative movement between the keeper halves and the valve stem. Contact wear patterns were also observed on the exhaust valve spring seat. The keepers likely became dislodged during the accident flight, allowing the valve to drop into the combustion chamber, where it was repeatedly impacted by the piston. The valve was then pushed back into the exhaust cavity, where the valve stem impacted the head adjacent to the exhaust valve guide. The fragmentation of the piston and exhaust valve likely led to the loss of engine power. Possible causes for dislodged keepers include improper installation or weak valve springs. The height of the intake and exhaust valve springs were similar, so weak springs are unlikely. Thus, the keepers were likely not installed properly before the accident. With improper installation, failure can be expected relatively soon after installation. Although the last record of work that would require disassembly of the valve to complete (a top overhaul) was dated about 36 years before the accident flight, the engine was reported to have only accrued about 320 hours since repair of the cylinders. It is likely that the keeper halves were not properly seated on the exhaust valve shoulder when the valve was installed. This incorrect installation allowed the keepers to eventually work their way out of position, allowing the valve to drop into the combustion chamber.

On July 15, 2024, about 1145 Pacific daylight time, a Cessna 150M, N714PW, was substantially damaged when it was involved in an accident near Ritzville, Washington. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that, while en route from Spokane, Washington, to Yakima, Washington, he heard a loud metal sound followed by an immediate reduction in engine rpm and a rough-running engine. While troubleshooting the rough-running engine, he diverted to the Pru Field Airport (33S), Ritzville, Washington. While en route a loss of engine oil pressure occurred followed by a total loss of engine power. The pilot initiated an off-airport landing to an open field. During the landing roll, the nose landing gear collapsed and the airplane impacted terrain. Postaccident examination of the airplane revealed that the left wing and forward fuselage were substantially damaged. Subsequent lighted borescope examination of the airplane’s engine revealed mechanical damage to the No. 2 cylinder’s combustion dome and piston face. The No. 2 cylinder was removed and the exhaust valve stem had exited the cylinder valve guide and was impaled into the cylinder head. The exhaust valve was fracture-separated from the valve stem and not observed within the cylinder or engine. Metallic debris was observed within the No. 2 cylinder. Remnants of the piston were observed within the cylinder and oil sump. The No. 2 cylinder, piston, and remnants of the No. 2 exhaust valve were sent to the NTSB Materials Laboratory, Washington, DC, for examination. Examination revealed the cylinder head damage was consistent with impact with the exhaust valve tip after it had been displaced out of the exhaust valve guide. The exhaust rocker arm exhibited grinding marks on the rocker pad. Marks corresponding to contact with the spring seat were also observed on the arm adjacent to the rocker pad. Wear marks corresponding to contact with the exhaust valve keeper halves were also observed on the center hole bore. The exhaust valve stem was fractured near the valve head, and the stem was bent into an “S” shape. The outside diameter of the valve stem measured .4343 inch near the tip end of the valve stem, which was slightly above the maximum outside diameter of a new exhaust valve stem. Fretting damage from contact with the keeper halves was observed at the shoulder near the valve tip. The fracture surfaces of the valve stem exhibited features consistent with overstress fracture. According to engine maintenance logs, the engine was last overhauled on June 11, 1981. All 4 cylinders were subsequently removed, repaired, and reinstalled on July 24, 1988. The maintenance record entry documenting this work included the statement, “ground and laped [sic] valves and seats,” which was the most recent entry that would require disassembly of the valves to complete. In the last maintenance entry, dated July 10, 2024, the enginewas reported to have 1,232.45 hours since major overhaul and 320.25 hours since repair of the cylinders. Continental Aerospace Technologies, Publication M-0, Standard Practice Maintenance Manual, dated August 2024, states: “Replace or overhaul the engine upon accumulating the operating hours specified in Table 6-1, or twelve (12) years after being placed in service, whichever occurs first.”