Accident Details
On May 16, 2023, at about 1912 central daylight time, a Cessna 172N, registration N5550J, was substantially damaged when it was involved in an accident near Flower Mound, Texas. The airplane was operated as an instructional flight under Title 14 Code of Federal Regulations Part 91. The flight instructor and the student pilot reported no injuries.
Sequence of Events
The instructor reported that, while descending from cruise flight, the engine began to run rough and experienced a partial loss of power with a corresponding drop in rpm. Unable to restore engine power, the instructor executed a forced landing to a field. During the landing, the airplane collided with a barbed wire fence and trees, resulting in substantial damage to the wings and fuselage.
Engine Examination
The engine was examined under the auspices of Federal Aviation Administration inspectors. The examination revealed that one of the rocker arms (part number LW-15014) had failed. The failed component was sent to the National Transportation Safety Board Materials Laboratory for further analysis.
Metallurgical Findings
At the laboratory, the pushrod was removed from the rocker arm. A portion of the rocker arm adjacent to the oil squirt hole was pressed inward in the direction the pushrod drives the rocker arm. A fracture surface was visible on the inner surface of the rocker arm, extending circumferentially around the hole. The circumferential fracture surface was relatively flat with visible crack arrest marks consistent with fatigue. The crack arrest marks indicated that fatigue originated from the outboard side of the rocker arm.
Three radial fractures branched from the circumferential fracture: one nearly aligned with the centerline of the outboard curved geometry of the rocker arm, one diametrically opposite the first, and the last about halfway between the other two. The macro fracture surface features were consistent with fatigue cracking progressing circumferentially from that same location, and the radial crack occurred before the circumferential cracking.
Service Bulletin Information
According to the manufacturer, rocker arms with similar damage have been observed previously. Lycoming Service Bulletin No. 639 (dated January 17, 2020) details the anticipated damage to the rocker arm oil squirt hole due to contact with the mating pushrod and provides a methodology to inspect for it. The bulletin states that radial cracks are warning signs that damage is occurring and that once observed, the rocker arm should be replaced.
In a telephone conversation with a representative of the airplane owners, the representative was under the impression that Lycoming Service Bulletin No. 639 applied only to engines overhauled by Lycoming. However, the service bulletin specifies that the affected models are “O-320-H2AD, O, LO, TO, LTO-360-E1A6D new, rebuilt, or overhauled engines shipped from Lycoming since June 2017 with less than 500 hours of operation or engines with less than 500 hours of operation since rocker arm replacement.” The bulletin calls for compliance “at next scheduled oil change, then every 50 hours until 500 hours of engine operation.”
The engine had accrued about 173 hours since the overhaul during which the rocker arms were replaced.