Primary finding
Probable cause
The fatigue failure of the rocker arm which resulted in a partial loss of engine power. Contributing was the airplane operator’s failure to inspect the rocker arm in accordance with the engine manufacturer’s service bulletin.
Investigator assessment
Analysis narrative
The flight instructor reported the engine began to run rough and lose power as they descended from cruise flight. Unable to restore full engine power, the instructor performed 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. A postaccident examination of the engine found that one of the rocker arms (p/n LW-15014) had failed. A metallurgical examination revealed a portion of the rocker arm adjacent to the oil squirt hole was pressed inwards in the direction the pushrod drives the rocker arm. Fatigue cracks emanated from the circumference of the oil squirt hole. Rocker arms with similar damage have been observed previously by the engine manufacturer. Lycoming Service Bulletin No. 639 (dated January 17, 2020) details the damage anticipated to the rocker arm oil squirt hole due to contact with the mating pushrod and provides an inspection methodology. Radial cracks are highlighted as warning signs that damage is occurring and once observed, the rocker arm should be replaced. In the subject rocker arm, the missed and/or unaddressed radial cracks likely led to the circumferential fatigue cracking and subsequent failure of the rocker arm and pushrod assembly. The airplane operator was under the impression that the service bulletin only applied to engines overhauled by Lycoming. The service bulletin states that the models affected 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 service bulletin should then be complied with “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.
Source record
Factual narrative
On May 16, 2023, about 1912 central daylight time, a Cessna 172N, N5550J, was substantially damaged when it was involved in an accident near Flower Mound, Texas. The instructor and student pilot were not injured. The airplane was operated under the provisions of Title 14 Code of Federal Regulations Part 91 as an instructional flight. The instructor reported that, while descending from cruise flight, the engine started to run rough. The engine sustained a partial loss of engine power with an associated drop in rpms. Unable to restore engine power, the instructor performed 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. The engine was examined under the auspices of inspectors of the Federal Aviation Administration. The examination found that one of the rocker arms (p/n LW-15014) had failed. The part was examined by the National Transportation Safety Board Materials Laboratory. The pushrod was removed from the rocker arm in the laboratory. A portion of the rocker arm adjacent to the oil squirt hole was pressed inwards 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 the fatigue emanated from the outboard side of the rocker arm. There were three radial fractures branching 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 the fatigue cracking progressing circumferentially from that same location. This is consistent with the radial crack occurring before the circumferential cracking. Rocker arms with similar damage have been observed previously by the manufacturer. Lycoming Service Bulletin No. 639 (dated January 17, 2020) details the damage anticipated to the rocker arm oil squirt hole due to contact with the mating pushrod and methodology to inspect for it. Radial cracks are highlighted as warning signs that damage is occurring and 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 only applied to engines overhauled by Lycoming. The service bulletin states that the models affected 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 service bulletin should then be complied with “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.