Stinson 108 N6642M accident near Klamath Falls, Oregon

Probable cause

The mechanic’s failure to identify an engine performance issue, which led to a partial loss of engine power. Contributing to the accident was the pilot’s decision to operate the airplane with a known mechanical deficiency.

NTSB analysis

The pilot had departed on a cross-country flight. About 1,000 ft above ground level, the engine lost partial power. The pilot made a forced landing in an open field, during which the left wing sustained substantial damage as it contacted the ground.

The airplane had a recurring engine performance deficiency that the pilot was aware of, and he used a temporary fix to work around the issue. The mechanic who performed maintenance on the engine before the accident thought that the issue involved the carburetor. Postaccident examination of the carburetor revealed no malfunctions that would have precluded normal operation.

Postaccident examination of the engine revealed multiple incorrectly set valve clearances and multiple cylinders with lower-than-acceptable compression. Thus, the mechanic who performed maintenance on the engine failed to identify the engine’s out-of-adjustment valves or the low-cylinder compression.

Text from the NTSB. Open the NTSB docket for WPR19LA207 for the full record.