Accident Overview
On July 15, 2024, at about 1145 Pacific daylight time, a Cessna 150M, registration N714PW, was substantially damaged during an off-airport landing near Ritzville, Washington. The pilot was not injured. The flight was conducted under Title 14 Code of Federal Regulations Part 91 as a personal flight.
Pilot Actions
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 rough engine operation. He diverted to Pru Field Airport (33S) in Ritzville. During the diversion, engine oil pressure was lost, followed by a total loss of engine power. The pilot initiated an off-airport landing in an open field. During the landing roll, the nose landing gear collapsed and the airplane struck the terrain.
Postaccident Examination
Postaccident examination revealed substantial damage to the left wing and forward fuselage. A lighted borescope inspection of the engine found mechanical damage to the No. 2 cylinder's combustion dome and piston face. The No. 2 cylinder was removed, revealing that the exhaust valve stem had exited the cylinder valve guide and was embedded in the cylinder head. The exhaust valve was fractured and separated from the valve stem; the valve head was not found within the cylinder or engine. Metallic debris was present in the No. 2 cylinder, and remnants of the piston were found in the cylinder and oil sump.
The No. 2 cylinder, piston, and exhaust valve remnants were sent to the NTSB Materials Laboratory for examination. The cylinder head damage was consistent with impact from the exhaust valve tip after it had displaced from the valve guide. The exhaust rocker arm had grinding marks on the rocker pad, and adjacent marks from contact with the spring seat. Wear marks on the center hole bore corresponded to contact with the exhaust valve keeper halves. The valve stem was fractured near the valve head and bent into an "S" shape. The stem's outside diameter near the tip measured 0.4343 inches, slightly above the maximum for 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 exhibited features consistent with overstress fracture.
Maintenance History
According to engine maintenance logs, the engine was last overhauled on June 11, 1981. All four cylinders were removed, repaired, and reinstalled on July 24, 1988. The maintenance record entry for that work stated, "ground and laped [sic] valves and seats," which was the most recent entry requiring valve disassembly. The last maintenance entry, dated July 10, 2024, reported the engine had accumulated 1,232.45 hours since major overhaul and 320.25 hours since cylinder repair. Continental Aerospace Technologies Publication M-0, Standard Practice Maintenance Manual (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."