ERA24LA271

The flight instructor reported that, during an instructional flight, while maneuvering about 2,000 ft above ground level, the engine began to run rough and lost partial power. The airplane could not maintain altitude and the instructor was unable to restore engine power. He subsequently performed a forced landing on a road, during which the left wing impacted a tree and the airplane came to rest inverted. Subsequent examination of the engine revealed that the No. 3 exhaust valve rocker was loose and did not depress the exhaust valve when the pushrod was fully extended. The rocker and stud were removed and remnants of threads from the cylinder head were observed between the threads of the stud, consistent with over-tightening of the stud during installation. Review of maintenance records revealed that the No. 3 cylinder was replaced due to low compression about 13 months and 564 engine tachometer hours before the accident. Although the maintenance procedures specified that the stud be tightened to 150 ft-lbs of torque, the stud was likely over-tighten during installation, as was evident by cylinder thread material being pulled into the stud threads, which resulted in its looseness and inability of the rocker to open the exhaust valve.

On June 21, 2024, about 1210 eastern daylight time, a Cessna 172N, N734XT, was substantially damaged when it was involved in an accident near Hernando, Florida. The flight instructor and commercial pilot were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor reported that the airplane departed about 20 minutes prior with 30 gallons of fuel. The commercial pilot and flight instructor were practicing commercial maneuvers at 2,000 ft mean sea level when they experienced extreme engine roughness and a loss of engine power. The flight instructor could not maintain altitude and could not correct the engine anomaly. He subsequently performed a forced landing on a road. During the landing, the left wing struck a tree and the airplane came to rest inverted. Examination of the wreckage by a Federal Aviation Administration inspector revealed substantial damage to the left wing and fuselage. The engine was further examined following wreckage recovery to a storage facility. During the examination, the propeller was rotated by hand and thumb compression was attained on all cylinders. Crankshaft, camshaft, and valvetrain continuity (with the exception of the No. 3 exhaust valve) was confirmed to the rear accessory section of the engine. The rocker covers were removed, and the No. 3 exhaust valve rocker was loose and did not depress the exhaust valve when the pushrod was fully extended. The rocker and stud were removed and remnants of threads from the cylinder head were observed between the threads of the stud, consistent with over-tightening of the stud during installation. All other rockers were secured in place and operated as expected. Review of maintenance records revealed that the No. 3 cylinder was replaced due to low compression on May 12, 2023, about 564 engine tachometer hours before the accident. According to the director of maintenance (DOM) of the repair station that performed the cylinder installation, he endorsed the maintenance logbook entries once the paperwork was complete and reviewed. The DOM added that a torque wrench was used to tighten the rocker stud to 150 ft-lbs per the Lycoming O-320 Operations Manual Maintenance Procedures, and he provided a copy of those procedures and the calibrated torque wrench serial number that was used to tighten the stud.