Loss of engine power (partial): Cessna 172N Skyhawk, N734XT, accident occurred on June 21, 2024, near Hernando, Florida

• Location: Hernando, Florida 
• Accident Number: ERA24LA271 
• Date & Time: June 21, 2024, 12:10 Local 
• Registration: N734XT
• Aircraft: Cessna 172 
• Aircraft Damage: Substantial 
• Defining Event: Loss of engine power (partial) 
• Injuries: 2 None 
• Flight Conducted Under: Part 91: General aviation - Instructional

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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, 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.

- Probable Cause: The improper installation of a cylinder rocker stud by maintenance personnel, which resulted in a partial loss of engine power and subsequent forced landing.