East St. Louis, IL, USA
N82694
AERONCA 7AC
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.
On June 11, 2016, about 1200 central daylight time, an Aeronca 7AC airplane, N82694, experienced a complete loss of engine power and made an emergency landing on a closed section of newly constructed highway in East St. Louis, Illinois. The airplane sustained substantial damage to the tail section and wing struts during impact with a construction barricade. The pilot and passenger were not injured. The airplane was privately registered and operated under the provisions of Title 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed for the flight, which departed without a flight plan from Sackman Field Airport (H49), Columbia, Illinois, about 1145. According to the pilot, about 15 minutes into the flight, he heard "a loud noise, a rattle" and the engine "seized." He decided to conduct a forced landing on a section of closed highway, however, the airplane was going too fast after touchdown to avoid the barricades at the end of the road closure. The wingtip impacted a barricade and caused a ground loop. After the airplane was recovered, a Federal Aviation Administration (FAA) inspector and representative from Continental Motors examined the A-65-8 engine. There were 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. The piston crown was still present inside the cylinder with a fragment of the connecting rod still attached by the piston pin. The longer portion of the connecting rod that was still connected to the crankshaft, extended outside the crank case hole. The No. 2 cylinder, including the piston crown and fragment of connecting rod, the longer portion of the connecting rod still attached to the crankshaft, and piston pin were examined at the NTSB Materials Laboratory, Washington, DC. The materials examination found the fracture surface on the arm side of the connecting rod had been mostly obliterated, consistent with post-fracture impact damage. However, the piston crown side of the connecting rod had not been similarly damaged. This fracture surface of the rod exhibited features, such as crack arrest and ratchet marks, consistent with progressive cracking, later determined to be consistent with fatigue. The nature of the fatigue cracking features was consistent with stresses from reverse bending, originating in four distinct initiation sites at the connecting rod flanges. These initiation sites showed ratchet marks of varying sizes, consistent with smaller fatigue cracks that coalesced as they propagated inward. These cracks grew inward until the remaining rod cross section succumbed to overstress. The relatively large percentage of fatigue features on the fracture surface was suggestive of low-load or high-cycle fatigue. The piston pin had fractured inside the connecting rod bore and was only discovered upon disassembly. The piston pin fracture location was between 1.37 and 1.59 inches from the shorter stub side. Most of the mating fracture surfaces exhibited smearing damage, consistent with continual rubbing while in contact inside the connecting rod bore. The fracture surfaces consisted of steps, generally oriented perpendicular to the pin forging direction. The surfaces were relatively flat, particularly around the outer regions. Examination of the fracture surface using a scanning electron microscope found two distinct regions of features on the fracture surface. The outer region, which extended to a depth of approximately 314 µm (0.012 inch), exhibited an intergranular fracture. The inner regions of the pin fracture surface exhibited dimple rupture, consistent with tensile overstress. The pin microstructure was consistent with a resulfurized carbon steel that had been surface case hardened, such by carburizing or carbonitriding. The case hardened layer exhibited a depth of approximately 0.02 inch, consistent with the depth of the intergranular fracture features observed on the pin fracture surface. The FAA issued Airworthiness Directive (AD) 46-36-01 which applied to all Continental Motors model A-65 engines. The AD outlined "a certain percentage of piston pins installed in engines of the above numbers and distributed as replacement parts are subject to failure without warning. The weakness of these pins cannot be detected by normal inspection methods. Piston pin breakage can result in complete engine failure." The AD required immediate compliance, if possible, but not later than 50 hours of engine operation after August 27, 1946. According to an engine logbook entry dated May 30, 2007, this AD was complied with on December 6, 1994. A second logbook entry, dated June 20, 2012, noted AD 46-36-01 was complied with "by piston pin replacement and piston inspection." The last annual inspection of the engine was completed on November 2, 2015, and noted the engine was "found to be in an airworthy condition for return to service."
A total loss of engine power due to fracture of the piston pin, which led to fatigue cracking and eventual fracture of the adjacent connecting rod.
Source: NTSB Aviation Accident Database
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