Piper PA-28-151 N57SH accident near Granville, Massachusetts

Probable cause

Deformation of the No. 1 cylinder's intake valve, which resulted in a partial loss of engine power and a subsequent off-airport precautionary landing.

NTSB analysis

About 20 minutes into a visual flight rules cross-country flight, the airplane's engine began running roughly and shortly thereafter began losing power. The pilot initially attempted to divert to an airport about 20 miles away, but as the engine's power output continued to gradually decrease, the pilot instead decided to perform a precautionary landing to a farm field. During the landing, the airplane touched down hard resulting in substantial damage to the airframe.

Postaccident examination of the engine revealed that the engine's No. 1 cylinder compression was intermittently low, that the head of the intake valve of that cylinder was deformed, and that the valve was not seating properly. The deformation of the intake valve may have been the result of high valve seating forces that occurred as the result of improperly adjusted valve spring tension. However, the airplane's maintenance records did not note any maintenance actions to the cylinder or its internal components between the last engine overhaul and the accident. While a maintenance log entry documenting the engine overhaul indicated that new cylinder assemblies were installed at that time, it did not indicate when the spring tension for the valve was last set.

Text from the NTSB. Open the NTSB docket for ERA15LA015 for the full record.