WPR15LA220

The commercial pilot reported that, during cruise on the fire reconnaissance flight, he heard a loud pop, followed by oil covering the windscreen; the engine subsequently experienced a total loss of power. Despite the pilot's attempts, the engine would not restart, and he initiated a forced landing to an open field. During the landing roll, the nose landing gear separated from the airplane and the right wing impacted the ground. Postaccident examination of the engine revealed that the bottom portion of the No. 6 cylinder piston, piston pin, and connecting rod cap were separated. Metallurgical examination of the piston pin revealed evidence of a fatigue fracture originating from the outside of the pin that progressed inward until the final fracture along the middle of the pin cross-section. The fatigue crack initiation site did not reveal any material or mechanical defects, such as pits, voids, or inclusions. Examination of the piston, connecting rod bolts, and connecting rod cap revealed fractures consistent with overstress. No additional anomalies were noted with the engine.

On July 21, 2015, about 1446 mountain daylight time, a Cessna 182Q, N759LV, was substantially damaged during a forced landing near Prichard, Idaho. The airplane was registered to and operated by Interstate Aviation, Pullman, Washington, under contract with the United States Forest Service. The local fire reconnaissance flight was operated under the provisions of 14 Code of Federal Regulations (CFR) Part 135. The commercial pilot and his passenger were not injured. Visual meteorological conditions prevailed and a company flight plan was filed for aerial observation flight. The local flight originated from Coeur d'Alene, Idaho, about 2 hours prior to the accident.The pilot reported that during cruise flight, he heard a loud pop, followed by oil covering the windscreen. Despite the pilot's attempts, the engine would not restart and he initiated a forced landing to an open field. Subsequently, during the landing roll, the nose wheel landing gear separated from the airplane and the right wing impacted the ground. Examination of the airplane by the pilot revealed that the right wing was substantially damaged. The wreckage was recovered to a secure location for further examination. Examination of the recovered airframe and engine revealed a large hole in the crankcase above cylinder number 6. Several pieces of the engine case and the number 6 piston pin were found outside the engine case lying between the cylinders. The upper portion of the number 6 piston was still in the cylinder. The piston could be moved by hand within the cylinder. Portions of the lower part of the piston, piston rings, the connecting rod cap and bolts were located within the oil sump. No evidence of any overheat or thermal damage was observed on the crankshaft. The remains of the number 6 piston, piston pin, and connecting rod were sent to the NTSB Materials Laboratory, Washington, DC, for further examination. According to a materials engineer, the examination revealed that the underside of the piston crown was fractured into smaller pieces near the piston pin position. The fracture surface exhibited a rough, tortuous appearance along with a dull luster. The fracture surface of the piston crown exhibited river lines and chevron marks consistent with fracture originating at the pin position. The fracture features of the piston crown underside were consistent with overstress failure. No indications of pre-existing cracking or progressive failure were observed on the piston fracture surfaces. One of the two connecting rod bolts was fractured in two halves, exhibiting a 45 degree slant fracture surface with localized necking adjacent to the fractures. The bolt fracture surface showed a dull luster and rough surface. All of these features were consistent with failure from tensile overstress. Only half of the other bolt was found, and was bent over, with a relatively flat fracture surface along the bolt threads. This fracture surface also exhibited dull, gray, and rough features. Thesecharacteristics were consistent with failure from overstress in bending. The connecting rod was generally intact with no indication of cracking, fracture, twisting or gross deformation along the rod. The connecting rod cap exhibited twisting and deformation consistent with batter from impacts with an adjacent object. However, there were no indications consistent with the connecting rod or cap having failed prior to the bolts fracturing. The piston pin exhibited some wear and impact marks along the barrel. Of note was that one of the tapered piston pin ends had separated and had become deformed into a spherical shape. Most of the fracture surface exhibited a shiny luster and general flat shape, consistent with post-fracture batter. The middle of the pin fracture had a raised, flattened feature. Around the periphery of the fracture surface, adjacent to the barrel of the pin, the fracture surface exhibited ridges consistent with ratchet marks. The ratchet marks are consistent with multiple smaller cracks that coalesce to form a larger crack. A less damaged area of the fracture surface, exhibited crack arrest marks consistent with progressive failure. Closer examination of the visible progressive features using a scanning electron microscope (SEM), revealed striations consistent with fatigue cracking. Examination of the fatigue crack initiation site did not reveal any material or mechanical defects, such as pits, voids, or inclusions. All of the features of the pin fracture surface were consistent with fatigue cracking initiating around the outside of the pin, with the final fracture along the middle of the pin cross section. The spherical deformed separated portion of the pin exhibited severe damage which obscured the features that would conclude fracture information. A circular impression was visible on the spherical fragment. The volume of the spherical pin remnant was consistent with a spherical impression on the underside of the piston crown and is consistent with it occurring after the severe deformation of the now spherical pin fragment. The composition of the pin fracture surface was examined using energy dispersive x-ray spectroscopy (EDS). The composition was consistent with AA 2000 series aluminum alloy. Further inspection using x-ray fluorescence (XRF) found the pin material to be consistent with AA 2024 aluminum. The hardness was inspected per ASTM E18.2 The hardness of the section averaged 81 HRBW. The electrical conductivity of the part was inspected per ASTM E1004.3 The conductivity averaged 39.0% IACS. These data were consistent with a T6X or T8X temper for this alloy. Review of the engine logbooks revealed that the engine was overhauled on August 8, 2008. At this time, new piston pins, part number SA 539467 were installed. The most recent logbook entry, dated July 14, 2015, was a 100-hour inspection, at a time since major overhaul (TSMOH) of 1,427.1 hours and HOBBS time of 2,344.6 hours. At the time of the accident, the engine hadaccumulated 1,455 hours TSMOH.