FTW00LA107

The flight instructor and private pilot receiving instruction were practicing basic maneuvers, during an instrument training flight, when the airplane began to 'vibrate violently.' They retarded the throttle, checked the magnetos, and then performed a precautionary landing to an open field. During the landing roll, the right horizontal stabilizer impacted a fence. An FAA inspector reported that the #3 exhaust rocker arm retaining stud separated. A section of stud was found lodged beneath the intake rocker arm, and the remaining portion of the stud remained seated in the cylinder head assembly. Metallurgical examination revealed that the stud failed due to fatigue that originated at the thread root from multiple origins.

On March 24, 2000, at 1615 central standard time, a Cessna 172N airplane, N75958, was substantially damaged during a precautionary landing near Maud, Oklahoma. The airplane was registered to and operated by Airman Flight School, Inc., of Norman, Oklahoma. The flight instructor (CFI) and private pilot receiving instruction were not injured. Visual meteorological conditions prevailed and no flight plan was filed for the 14 Code of Federal Regulations Part 91 instructional flight. The local flight originated from the University of Oklahoma Westheimer Airport, Norman, Oklahoma, at 1540.

According to the CFI and private pilot, the flight departed on an "instrument training flight," during which the student was "practicing basic attitudes." They reported that during the flight the "airplane started to vibrate violently." They retarded the throttle and checked the magnetos; however, the vibration intensified. Subsequently, a precautionary landing was executed to an open field, where the airplane impacted a fence during the landing roll.

According to an FAA inspector, who examined the airplane, the right horizontal stabilizer was bent upward and the propeller blades were damaged. He examined the Lycoming O-320-H2AD engine and determined that the #3 cylinder's exhaust rocker arm retaining stud had separated. A portion of the separated stud was found lodged beneath the intake rocker arm, and the remaining portion of the stud remained seated in the cylinder head assembly.

The cylinder head assembly and the separated portion of the exhaust rocker arm retaining stud were sent to the Textron Lycoming Materials Laboratory, Williamsport, Pennsylvania, for further examination. Macroscopic examination of the stud fracture surface revealed the presence of beach marks and ratchet marks on the fracture surface indicating that the stud failed in fatigue, which originated at the thread root from multiple origins. A hardness test reveled that the stud met the manufacturer's specifications and there were no additional defects noted with the stud's material.

The airplane and engine underwent their most recent annual and 100-hour inspections, respectively, on February 8, 2000, at which time they had accumulated a total of 7,010.7 hours and 1,350.2 hours since overhaul, respectively.