CEN23LA399 · Beech 35-B33

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.

Verbatim NTSB analysis and probable cause from the NTSB dataset