Cessna 172N N5204K accident near Rockford, Idaho

Probable cause

A partial loss of engine power during cruise flight due to the failure of the No. 3 cylinder exhaust valve for reasons that could not be determined due to the damage to the valve head and the missing valve stem. Contributing to the accident was the flight instructor’s improper approach to the runway, which resulted in an aborted landing and subsequent off-airport landing.

NTSB analysis

The flight instructor reported that, while the airplane was in level cruise flight, the engine began to vibrate, and the rpm dropped to about 1,900. The airplane could not maintain altitude, so the instructor took control of the airplane and chose to divert to a nearby airport. During the approach, the altitude and airspeed were too high to safely land on the runway, so the instructor aborted the landing. He subsequently made a forced landing to an open field. During the landing, the airplane nosed over and came to rest inverted, which resulted in substantial damage to the wings and tail section.

A postaccident examination of the engine revealed that the Nos. 2 and 3 cylinders had no compression. A large amount of metal dust and debris were found in the No. 2 cylinder's intake and exhaust ports. The No. 3 cylinder was removed, and the exhaust valve head was found embedded in the cylinder head. The piston head was severely damaged, and the valve stem was missing and had likely exited the engine out of the exhaust. The reason for the failure of the No. 3 cylinder exhaust valve could not be determined due to the damage to the exhaust valve head and the missing valve stem. Although the debris in the No.2 cylinder's ports would have prevented the valve from sealing properly and resulted in reduced compression, it would not have caused the partial loss of engine power as the No. 3 cylinder's failure would have.

Text from the NTSB. Open the NTSB docket for WPR12LA306 for the full record.