Stinson 108-1 N8656K accident near Sterling, Alaska

Probable cause

Insufficient available power as a result of low compression in three engine cylinders. Contributing to the accident was the pilot’s decision to takeoff with a slight quartering tailwind.

NTSB analysis

The pilot was conducting a local flight with friends onboard. He estimated the airplane’s takeoff weight to be about 76 lbs under maximum gross weight. The pilot stated that during the initial climb, the engine did not feel like it was producing full power and the airplane descended into trees, which resulted in substantial damage to the fuselage and wings.

A video of the takeoff and initial climb revealed that the sound of the engine was consistent throughout the length of the video with no audible anomalies noted. A windsock that was visible in the video showed a slight right-quartering tailwind at the time of the takeoff.

A postaccident examination revealed that the top spark plug from the No. 6 cylinder was mostly unscrewed and exhibited external signatures consistent with exposure to high heat, which indicated hot gas was leaking out of the cylinder through the spark plug hole. When the cylinders were checked for compression using the thumb check method, the Nos. 2 and 3 cylinders produced low compression.

It is likely that the loose spark plug in combination with the two weak cylinders resulted in the engine producing less than full power. Additionally, the pilot’s decision to takeoff with a slight right-quartering tailwind likely contributed to the airplane’s lack of climb performance.

Text from the NTSB. Open the NTSB docket for ANC21LA068 for the full record.