NTSB Factual Report

On October 25, 2001, at 1940 central daylight time, a Cessna 152 single-engine airplane, N67797, was substantially damaged when it impacted a fence during a forced landing following a loss of engine power while in cruise flight near Bethany, Oklahoma. The airplane was registered to and operated by Airone Inc., of Oklahoma City, Oklahoma. The private pilot, who was the sole occupant, was not injured. Night visual meteorological conditions prevailed and a flight plan was not filed for the 14 Code of Federal Regulations Part 91 solo instructional flight. The cross-country flight originated from the Wiley Post Airport, Bethany, Oklahoma, at 1800, flew to Duncan, Oklahoma, and was returning to the Wiley Post Airport when the accident occurred.

According to the operator, the pilot reported that the engine began to rumble and lose power. The pilot could not maintain altitude to reach the airport, and therefore, elected to land in a field. The pilot landed the airplane in a field where it impacted a barbed wire fence. The pilot reported that he could not see the fence since it was dark. According to the operator, the airplane sustained structural damage to the horizontal stabilizer, and the right elevator was torn from its hinges.

An FAA airworthiness inspector examined the Lycoming O-235-L2C engine with the assistance of an aircraft mechanic. According to the operator, the #2 cylinder's push rod housing was bent. The FAA inspector reported that they removed the #2 cylinder's rocker arm cover and noted that "the exhaust valve adjuster 'lock nut' had loosened and come completely off."

Review of a maintenance record entry dated May 21, 2000, revealed that the #2 cylinder had been removed, repaired, and reinstalled at an aircraft total time of 825.0 hours. On August 13, 2001, the airplane and engine underwent their most recent 100-hour inspection at an aircraft and engine total time of 1,077.0 hours. The aircraft total time at the time of the accident is unknown.

According to the Lycoming Operator's Manual, persons installing pushrods for a cylinder are instructed to "be sure that the piston is at top center compression stroke and that both valves are closed. Assemble adjusting screw in valve rockers and using clearance gage adjust the clearance between the valve tip and valve rocker. Adjust in the following manner: Set tappets with .007 in. - .009 in. clearance cold. Recheck tappet clearance after run-in, clearance should be from .006 in. to .012 in. After adjusting, lock the adjusting screw in place with the locknut."

Numerous attempts to contact the owner/operator and obtain a completed Pilot/Operator Aircraft Accident Report were unsuccessful.

NTSB Probable Cause

the mechanic's failure to properly lock the exhaust valve's clearance adjusting screw locknut, which resulted in a loose nut, disabling of the exhaust valve pushrod, and a partial loss of engine power while in cruise flight. A contributing factor was the night light conditions for the forced landing.