On February 17, 2012, about 1652 eastern standard time, a Cessna 172N, N4737D, made a force land following a partial loss of engine power near Flat Rock, Virginia. The airplane sustained substantial damaged and the private pilot was not injured. The airplane was registered to Green Hat Aviation LLC, and operated by an individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed and no flight plan was filed. The flight originated from Manassas Regional Airport/Harry P. Davis Field (HEF), Manassas, Virginia at 1549.
According to the pilot, he was at an altitude of 4,500 feet when the engine began losing power. He noticed a strong vibration in the throttle control and began to notice an "oily smell." The engine power decreased to 1,500 rpm but continued to run. The pilot contacted the air traffic controller (ATC) and declared an emergency. He advised ATC of his intention to land in a field, and ATC suggested a private grass strip. The pilot advised ATC that he did not have enough altitude to make it to the strip and continued towards the field. The pilot made a forced landing in the field, and during the landing roll the airplane collided with an embankment.
Examination of the airplane by a Federal Aviation Administration inspector confirmed structural damage to the firewall, fuselage and both wings. During an examination of the engine, it was noted that the No. 4 cylinder exhaust valve spring seat had broken. As a result of the broken valve spring seat, the push rod became dislodged.
A review of the maintenance records revealed that airworthiness directive (AD 80-04-03 R2) was performed, which instructs the replacement of the upper exhaust valve spring seats and the exhaust hydraulic lifters. According to the Lycoming Service Instruction 1009AU, the recommended time between overhaul (TBO) periods is 2,000 hours or 12 years, whichever comes first. The last engine overhaul was conducted on August 29, 2005, and 134.66 hours remained on the engine before the next recommended overhaul.