Aeronca 7AC accident near East St. Louis, Illinois, June 11, 2016

The NTSB's narrative final · quoted from the NTSB record

The airline transport pilot and passenger departed on a local flight from an uncontrolled airport. The pilot flew for about 15 minutes when he heard "a loud noise, a rattle," and the engine lost total power. He chose to conduct a forced landing on a section of closed highway; however, the pilot was unable to slow the airplane and impacted barricades at the end of the road closure. Postaccident examination of the engine revealed two holes through the top of the crankcase, and cylinders No. 1 and No. 2 were separated from their mounts on the crankcase. Inside the No. 2 cylinder, the connecting rod was found fractured just below the piston head. Upon further examination of the connecting rod, the piston pin, which was manufactured from resulfurized steel, was also found fractured inside the connecting rod bore. The No. 2 cylinder failed due to a fractured piston pin that retained the connecting rod inside the piston crown underside. The fracture of the piston pin led to misalignment of the connecting rod, which eventually failed from fatigue in reverse bending. The fatigue stresses on the connecting rod were relatively low, consistent with most of the fracture surface exhibiting fatigue propagating from multiple crack initiation sites. The piston pin likely fractured first, in part exacerbated by intergranular cracking in the outer, case-hardened layer. Resulfurized steels, designed for better machinability, exhibit reduced notch toughness and ductility. Carburizing or carbonitriding of these steels is generally not recommended, as the surface treatment can increase susceptibility to intergranular fracture in the hardened condition. In addition, the degree of the surface case hardening was deeper than typical for this alloy. It is likely that a stress high enough to fracture the outer layer would lead to through-fracture of the pin. However, the location of the pin fracture was such that immediate failure of the engine did not occur until the connecting rod subsequently fractured from fatigue. The pin had been partially embrittled by the surface case hardening, which sufficiently lowered the fracture toughness to cause the pin to fracture prematurely.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.