Event CEN23LA399

Factual narrative

On September 1, 2023, about 1505 central daylight time, a Beech 35-B33 airplane, N9522Y, sustained substantial damage when it was involved in an accident near Ava, Missouri. The pilot and the passenger sustained no injury. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The airplane departed from the Lost Mine Airpark (MO56), Theodosia, Missouri, for the cross-country flight to the Floyd W. Jones Lebanon Airport (LBO), Lebanon, Missouri. About halfway through the flight, the pilot, who also was the aviation mechanic who recently worked on the airplane, noticed the engine produced a “very subtle vibration.” Several minutes later, the propeller speed increased from 2,400 rpm to 2,600 rpm. The pilot noticed that the engine oil pressure had decreased below the green indicator arc and shortly before the engine produced a “pretty severe vibration.” Smoke was observed to exit from the engine cowling and the engine sustained a total loss of engine power. The pilot performed a forced landing to a sloped grass field, during which the nose gear separated from the airplane. The airplane came to rest upright in a nose-down attitude. The pilot and the passenger egressed from the airplane without further incident. The airplane sustained substantial damage to an engine mount. A hole was observed on the top-left side of the engine crankcase. Additionally, engine oil was observed on the lower-right side of the engine cowling and on the underside of the fuselage. Postaccident examination and disassembly of the engine revealed a connecting rod failure consistent with a lack of lubrication. While connecting rod Nos. 1, 2, 3, and 4 were found damaged, connecting rod Nos. 1 and 2 were found separated from the crankshaft. No other anomalies were noted with the engine. The engine was found to be equipped with an Eaton Aerospace Aeroquip E-Z Drain Sump Valve (part number AE99950G, manufactured in October 1989); however, the valve was not equipped with a drainage hose, nor was it required to be per the manufacturer. The oil sump valve was found in the closed position. A review of the valve installation document showed that the part number corresponded to the engine make and model. The oil sump valve could be opened by turning the union nut (copper in color) counterclockwise, pushing the nut up along the valve body, and then turning it clockwise. This action pushes up an internal stem, moving a metal plate and an O-ring out of the sealed position and allowing oil to flow through the stem. A ball embedded into the union nut follows a ‘C’-shaped guide channel machined into the surface of the valve, and the valve is held open when the ball rests at the upper end of the C. When closed, the union nut is held in place by an internal spring that inhibits its rotational and axial motion. Postaccident examination of the guide channel on the oil sump valve showed damage to the surfaces around the channel. The channel was distorted at both ends, consistent with the ball impinging on the ends of the guide channel. These shapes of the impingement areas were consistent with the union nut (and ball) being forced open or closed without first turning the nut. It was found that the oil sump valve could be partially opened without turning the union nut if a force was applied to the bottom of the nut. When the force was released, the oil sump valve returned to a ‘closed’ position. It was unclear if the partial opening would result in a significant oil leak. A secondary groove, or false channel, was imprinted into the surface next to the guide channel, extending between the two impingement areas. The channel was approximately 0.02 inch deep and 0.06 inch wide at the center. Toward the lower end of the channel, just above the edge of the union nut, the channel widened to 0.08 inch. At this widening point, a tertiary groove (perpendicular to the guide channel and false channel) appeared to connect the two channels. This groove was approximately 0.06 inch wide. Tool marks were observed on opposite sides of the knurled portion of the union nut. Damage was noted on the safety wire guides at the top end of the valve. When functionally tested, the oil sump valve assembly still appeared to operate as normal. The union nut could be turned and pushed up along the guide channel. Additionally, X-ray imaging with computed tomography revealed that the spring and other internal structures remained intact. The oil sump valve was submitted to Eaton Aerospace for leak tests on their specialized testing equipment. The manufacturer verified that liquid could flow through the oil sump valve when it was open. The leak tests were conducted according to standard Eaton Aerospace procedures, first at 5-10 psi for a minimum of 30 seconds and then 300 psi (-0/+30 psi) for a minimum of 30 seconds. The oil sump valve was initially tested with the copper crush washer that was submitted along with it. In this configuration, a leak was observed at the threaded joint at both low and high pressures. In a second round of testing, Eaton Aerospace replaced the crush washer with a rubber O-ring, as this is the standard configuration in which the oil sump valves are sold. No leaks were observed when the oil sump valve was tested in this configuration. According to the engine maintenance records, the last oil change was completed by the owner on January 1, 2023, at 1,204 hours tachometer time. The final maintenance entry was for a 100-hour/annual inspection completed on August 1, 2023, which was one month before the accident date. No oil leak was documented during this inspection. At that time, the tachometer time was 1,210.12 hours and the airframe total time was 5,586.57 hours. The engine total time was 3,775.73 hours and the engine time since major overhaul was 674.02 hours. The entry stated that an oil change was not performed; however, it stated, “examined screen for metal content – none found.” All cylinders were inspected with a borescope and no anomalies were noted. The engine was test run with no anomalies noted. The airplane tachometer indicated 1,213.04 hours at the time of the accident. The airplane had accumulated 9 hours since the last engine oil change and 2.92 hours since the last 100-hour/annual inspection. A review of the engine maintenance records did not reveal when the oil sump valve was installed on the engine. The owner did not remember when it was installed, nor who performed the installation work. The owner reported that he performed most of the oil changes and that he did not have any previous issues with the oil sump valve. The owner reported that he did not use any tooling with the oil sump valve during the oil changes he completed. A review of Federal Aviation Administration registration records showed that the owner had purchased the airplane on January 26, 1976. About halfway through a cross-country flight, the pilot, who also was the aviation mechanic who recently worked on the airplane, noticed that the engine produced a “very subtle vibration.” Several minutes later, the propeller speed increased from 2,400 rpm to 2,600 rpm. The pilot noticed that the engine oil pressure had decreased below the green indicator arc shortly before the engine produced a “pretty severe vibration.” Smoke was observed to exit from the engine cowling, and the engine sustained a total loss of engine power. The pilot performed a forced landing to a sloped grass field, during which the nose gear separated from the airframe. The airplane came to rest upright in a nose-down attitude. The airplane sustained substantial damage to the engine mount. A hole was observed on the top left side of the engine case. Engine oil was observed on the lower right side of the engine cowling and on the underside of the fuselage. Postaccident examination and disassembly of the engine revealed two connecting rod failures due to a lack of lubrication. No other anomalies were noted with the engine. The engine was found to be equipped with a quick drain oil sump valve which was found in the closed position. Postaccident examination revealed the oil sump valve was installed on the engine with a copper crush washer instead of the rubber O-ring that was specified/supplied by the oil sump valve manufacturer. An oil leak was observed at the threaded portion of the oil sump valve when it was bench tested with the copper crush washer installed. However, there was no oil leak observed when the copper crush washer was replaced with the manufacturer-specified rubber O-ring. The investigation was unable to determine if the use of the copper crush washer instead of a rubber O-ring contributed to the oil leak during the accident flight. The airplane had accumulated 9 hours since the last engine oil change and 2.92 hours since the last 100-hour/annual inspection. According to the engine maintenance documentation, the engine oil was not replaced during the last 100-hour/annual inspection. No oil leak was documented during this inspection. Given that the loss of engine oil event appeared to have occurred in the 2.92 hours since the last inspection, it was likely a sudden sequence, and not a long-term slow leak. Based on the available evidence, the investigation was unable to conclusively determine what caused the loss of engine oil. Source: NTSB Aviation Accident Database Retrieved: 2026-02-12

NTSB Findings

Hierarchical cause / factor breakdown from the FAA bulk avdata database. Each finding tagged C (Cause) or F (Factor).

• — Aircraft-Aircraft power plant-Engine (reciprocating)-Recip eng oil sys-Failure
• — Aircraft-Aircraft power plant-Engine (reciprocating)-Recip eng oil sys-Damaged/degraded
• — Aircraft-Aircraft power plant-Engine (reciprocating)-Recip eng oil sys-Unknown/Not determined
• — Not determined-Not determined-(general)-(general)-Unknown/Not determined