Accident Overview
On February 17, 2012, about 1652 eastern standard time, a Cessna 172N, registration N4737D, made a forced landing following a partial loss of engine power near Flat Rock, Virginia. The airplane was registered to Green Hat Aviation LLC and operated by an individual under 14 CFR 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.
Pilot Actions
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 an oily smell. Engine power decreased to 1,500 rpm but continued to run. The pilot contacted air traffic control (ATC) and declared an emergency, stating his intention to land in a field. ATC suggested a private grass strip, but the pilot reported insufficient altitude to reach it and continued toward the field. The pilot made a forced landing in the field, and during the landing roll the airplane collided with an embankment.
Aircraft Damage and Examination
The airplane sustained substantial damage. A Federal Aviation Administration (FAA) inspector confirmed structural damage to the firewall, fuselage, and both wings. Examination of the engine revealed 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.
Maintenance Records
A review of maintenance records indicated that airworthiness directive AD 80-04-03 R2 had been performed, which requires replacement of the upper exhaust valve spring seats and exhaust hydraulic lifters. According to Lycoming Service Instruction 1009AU, the recommended time between overhaul (TBO) 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 before the next recommended overhaul.