Accident Overview
An airplane lost engine power and nosed over during a forced landing in a field. Post-accident examination revealed that the number 2 cylinder had separated from the engine. The engine had accumulated 900 hours since its last major overhaul, which occurred 13 years prior to the accident. The separated cylinder had 1,212 hours since its replacement in 1986; the hours on the cylinder before that time could not be determined.
Metallurgical Examination
A metallurgical examination of the cylinder found that the barrel failed due to a fatigue crack. The barrel fractured circumferentially between the 11th and 13th fin from the head. The fracture initiated approximately 1.8 inches below the head, starting from the inner diameter below a plated region. The initiation area was a small, flat, thumbnail-shaped region approximately 0.5-inch wide through the wall thickness, with signatures indicative of fatigue. Multiple crack initiation sites were observed in the origin area. From the initiation point, the fracture continued propagating in the circumferential direction with coarse arrest marks over approximately 45 percent of the barrel circumference.
Examination Findings
No material, manufacture, or plating defects were identified. The metallurgical examination could not determine why the cracking occurred at the given location; however, it occurred below the chrome plating. It was noted that the barrel may have exhausted its available fatigue life at that location, which was lowered by the presence of plating.