Accident Overview
On September 1, 2023, at about 1505 central daylight time, a Beech 35-B33 airplane, registration N9522Y, sustained substantial damage during an accident near Ava, Missouri. The pilot and passenger were not injured. The flight was operating under Title 14 Code of Federal Regulations Part 91 as a personal flight.
The airplane departed from Lost Mine Airpark (MO56) in Theodosia, Missouri, en route to Floyd W. Jones Lebanon Airport (LBO) in Lebanon, Missouri.
Sequence of Events
Approximately halfway through the flight, the pilot—who was also the aviation mechanic that had recently worked on the airplane—noticed a "very subtle vibration" from the engine. Several minutes later, propeller speed increased from 2,400 rpm to 2,600 rpm, and engine oil pressure dropped below the green indicator arc. Shortly thereafter, the engine produced a "pretty severe vibration," smoke was seen exiting the engine cowling, and total loss of engine power occurred.
The pilot executed a forced landing to a sloped grass field. During the landing, the nose gear separated, and the airplane came to rest upright in a nose-down attitude. The pilot and passenger egressed without further incident.
Damage and Engine Examination
The airplane sustained substantial damage to an engine mount. A hole was observed on the top-left side of the engine crankcase, and engine oil was present on the lower-right side of the engine cowling and the underside of the fuselage.
Postaccident disassembly of the engine revealed a connecting rod failure consistent with a lack of lubrication. Connecting rods Nos. 1, 2, 3, and 4 were damaged; rods Nos. 1 and 2 were separated from the crankshaft. No other anomalies were found in the engine.
Oil Sump Valve Analysis
The engine was equipped with an Eaton Aerospace Aeroquip E-Z Drain Sump Valve (part number AE99950G, manufactured October 1989). The valve was not equipped with a drainage hose, and such a hose was not required per the manufacturer. The valve was found in the closed position.
Examination of the valve's guide channel showed damage and distortion at both ends, consistent with the ball impinging on the ends of the channel. These impingement shapes indicated that the union nut (and ball) had been forced open or closed without first turning the nut. The valve could be partially opened without turning the union nut if force was applied to the bottom of the nut; when released, it returned to the closed position. It was unclear if partial opening would result in a significant oil leak.
A secondary false channel was imprinted next to the guide channel, with tool marks on the union nut and damage to the safety wire guides. However, when functionally tested, the oil sump valve assembly still operated normally, and X-ray imaging showed the spring and internal structures were intact.
The valve was sent to Eaton Aerospace for leak tests. With the submitted copper crush washer, a leak was observed at the threaded joint at both low (5-10 psi) and high (300 psi) pressures. When tested with a rubber O-ring (the standard configuration), no leaks were observed.
Maintenance History
According to engine maintenance records, the last oil change was on January 1, 2023, at 1,204 hours tachometer time. A 100-hour/annual inspection was completed on August 1, 2023 (one month before the accident) at 1,210.12 hours tachometer time (airframe total time 5,586.57 hours, engine total time 3,775.73 hours, engine time since major overhaul 674.02 hours). The inspection noted an oil change was not performed, but the oil screen was examined for metal content with none found, and cylinders were inspected with a borescope with no anomalies. The engine was test run with no anomalies. At the accident time, the tachometer indicated 1,213.04 hours, meaning 9 hours since the last oil change and 2.92 hours since the inspection.
Records did not show when the oil sump valve was installed, and the owner did not recall who installed it. The owner performed most oil changes and reported no previous issues with the valve, noting he did not use any tooling with the valve during oil changes. The owner had purchased the airplane on January 26, 1976.
Probable Cause
The investigation did not state a probable cause.