Cessna 172N N4737D accident near Flat Rock, Virginia

Probable cause

The partial loss of engine power due to the failure of the exhaust valve spring seat for reasons that could not be determined.

NTSB analysis

According to the pilot, while in cruise flight, the engine began to lose power. He noticed a vibration in the throttle control and an “oily” smell. Engine power decreased to 1,500 rpm. The airplane was unable to maintain altitude, and the pilot elected to make an emergency landing in a field. During the landing roll, the airplane collided with an embankment and sustained substantial damage to the fuselage. An examination of the engine revealed that the No. 4 valve spring seat had broken. A review of the maintenance records revealed that the engine had accumulated 1865.34 hours since the last overhaul 6.5 years before the accident. According to Lycoming Service Instruction 1009AU, the recommended time between overhaul is 2,000 hours or 12 years, whichever occurs first. Further review of the maintenance records revealed compliance with Airworthiness Directive 80-04-03 R2, which covers the replacement of the upper exhaust valve spring seats and the exhaust hydraulic lifters. The exhaust valve seat failed 134.66 hours before the next recommended overhaul. The investigation was unable to determine the reason for the valve seat failure.

Text from the NTSB. Open the NTSB docket for ERA12LA191 for the full record.