Accident Overview
On June 21, 2024, at approximately 1210 eastern daylight time, a Cessna 172N, registration N734XT, sustained substantial damage during an accident near Hernando, Florida. The aircraft was being used for an instructional flight under Title 14 Code of Federal Regulations Part 91. The onboard flight instructor and commercial pilot were uninjured.
Forced Landing and Damage
The airplane had departed about 20 minutes prior with 30 gallons of fuel. While practicing commercial maneuvers at an altitude of 2,000 feet mean sea level, the engine began to run extremely rough and lost power. The flight instructor was unable to maintain altitude or remedy the engine problem and executed a forced landing on a roadway. During the landing, the left wing struck a tree, and the aircraft came to rest inverted. Examination of the wreckage by a Federal Aviation Administration inspector revealed substantial damage to the left wing and fuselage.
Engine Examination
Following recovery to a storage facility, the engine underwent further examination. Hand rotation of the propeller resulted in thumb compression on all cylinders. Continuity of the crankshaft, camshaft, and valvetrain was confirmed up to the rear accessory section of the engine, except for the No. 3 exhaust valve. Removal of the rocker covers showed that the No. 3 exhaust valve rocker was loose and did not depress the valve when the pushrod was fully extended. The rocker and its stud were removed, and remnants of threads from the cylinder head were found between the stud threads, a condition consistent with over-tightening of the stud during installation. All other rockers were secure and operated normally.
Maintenance History
Review of maintenance records indicated that the No. 3 cylinder had been replaced on May 12, 2023, due to low compression. This occurred about 564 engine tachometer hours before the accident. The director of maintenance (DOM) of the repair station that performed the cylinder installation stated that he endorsed the maintenance logbook entries after the paperwork was complete and reviewed. He reported that a torque wrench was used to tighten the rocker stud to 150 foot-pounds, as specified in the Lycoming O-320 Operations Manual Maintenance Procedures, and he provided a copy of the procedures and the serial number of the calibrated torque wrench used.