Piper PA-18-150 N476SM accident near Joliet, Illinois

Probable cause

The failure of the No. 2 cylinder exhaust valve, which resulted in a total loss of engine power.

NTSB analysis

The pilot reported that he fueled the airplane and departed for a planned 3-hour banner tow flight. About two hours into the flight, he heard a “loud bang” from the engine, which resulted in strong vibrations and a gradual loss of engine power. Shortly after, the engine lost total power, and the pilot executed a forced landing to a field. During the landing roll, the airplane ground looped and impacted a fence, which resulted in substantial damage to the left elevator.

Postaccident examination of the engine revealed that the No. 2 cylinder exhaust valve failed. Most of the valve stem remained inside the valve guide while the valve head was destroyed inside the cylinder. There were no other engine mechanical malfunctions or anomalies that would have precluded normal operation.

Text from the NTSB. Open the NTSB docket for CEN25LA322 for the full record.