Primary finding
Probable cause
A partial loss of engine power due to the failure of the No. 4 cylinder exhaust valve.
Investigator assessment
Analysis narrative
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.
Source record
Factual narrative
On October 21, 2020, about 1117 mountain standard time, a Piper PA-28-180 airplane, N7405W, was substantially damaged when it was involved in an accident near Show Low, Arizona. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations (CFR) Part 91 personal flight. 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 about 1,600 to 1,700 rpm. 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. 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.