Accident Overview
On June 11, 2016, about 1200 central daylight time, an Aeronca 7AC airplane, registration 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 14 CFR Part 91 as a personal flight. Visual meteorological conditions prevailed, and the flight departed without a flight plan from Sackman Field Airport (H49), Columbia, Illinois, about 1145.
Pilot Account
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 the closed highway, but 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.
Engine Examination
After recovery, a Federal Aviation Administration (FAA) inspector and a representative from Continental Motors examined the A-65-8 engine. Two holes were found through the top of the crankcase. Cylinders No. 1 and No. 2 were separated from their mounts on the crankcase. Inside the No. 2 cylinder, the connecting rod was fractured just below the piston head. The piston crown was still inside the cylinder with a fragment of the connecting rod attached by the piston pin. The longer portion of the connecting rod, still connected to the crankshaft, extended outside the crankcase hole. The No. 2 cylinder, piston crown, and fragments were sent to the NTSB Materials Laboratory for examination.
Materials Analysis
At the materials laboratory, the fracture surface on the arm side of the connecting rod was mostly obliterated from post-fracture impact. However, the piston crown side exhibited features consistent with progressive cracking, determined to be fatigue. The fatigue features indicated stresses from reverse bending, originating from four initiation sites at the connecting rod flanges. Smaller fatigue cracks coalesced as they propagated inward until the remaining cross section succumbed to overstress. The large percentage of fatigue features suggested low-load or high-cycle fatigue.
The piston pin had fractured inside the connecting rod bore (discovered upon disassembly). The fracture location was between 1.37 and 1.59 inches from the shorter stub side. Most mating surfaces had smearing damage from continual rubbing. Fracture surfaces consisted of steps generally perpendicular to the pin forging direction, with relatively flat outer regions. Scanning electron microscope examination revealed two distinct regions: an outer region (depth approximately 314 µm) with intergranular fracture, and inner regions with dimple rupture consistent with tensile overstress.
The pin microstructure was consistent with resulfurized carbon steel surface case hardened by carburizing or carbonitriding. The case hardened layer depth (about 0.02 inch) matched the depth of intergranular fracture features.
Compliance with Airworthiness Directive
The FAA issued Airworthiness Directive (AD) 46-36-01 for all Continental Motors model A-65 engines. The AD stated that "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 compliance within 50 hours after August 27, 1946. An engine logbook entry dated May 30, 2007, noted compliance on December 6, 1994. A second entry dated June 20, 2012, noted compliance "by piston pin replacement and piston inspection." The last annual inspection, completed on November 2, 2015, found the engine airworthy for return to service.