Piper PA-28-180 N7405W accident near Show Low, Arizona

Probable cause

A partial loss of engine power due to the failure of the No. 4 cylinder exhaust valve.

NTSB analysis

The pilot reported that, while in cruise flight about 8,000 ft mean sea level, there was a loud bang followed by severe engine vibration and the engine lost partial power. The pilot attempted to troubleshoot; however, the engine continued to produce less than full power. The pilot declared an emergency and performed a forced landing to a highway, during which the airplane’s right wing impacted a barbed wire fence, resulting in substantial damage.

Examination of the engine revealed a stuck exhaust valve on the No. 4 cylinder. The exhaust valve head had separated from the valve stem. The loose valve head was found in the cylinder’s combustion chamber; the combustion chamber and piston dome exhibited damage consistent with repeated contact with the valve head. The loss of engine power is consistent with the failure of the No. 4 cylinder exhaust valve; however, the reason for the valve failure could not be determined based on the available information.

Text from the NTSB. Open the NTSB docket for WPR21LA020 for the full record.