Cessna 172 N5550J accident near Flower Mound, Texas

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.

NTSB analysis

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.

Text from the NTSB. Open the NTSB docket for CEN23LA188 for the full record.