Accident Narrative
On July 21, 2015, about 1446 mountain daylight time, a Cessna 182Q, registration N759LV, was substantially damaged during a forced landing near Prichard, Idaho. The airplane was operated by Interstate Aviation of Pullman, Washington, under contract with the United States Forest Service, conducting a local fire reconnaissance flight under 14 CFR Part 135. The commercial pilot and passenger were not injured. Visual meteorological conditions prevailed, and a company flight plan was filed. The flight originated from Coeur d'Alene, Idaho, about two hours prior to the accident.
The pilot reported hearing a loud pop during cruise flight, followed by oil covering the windscreen. Despite attempts, the engine would not restart, and the pilot initiated a forced landing to an open field. During the landing roll, the nose wheel landing gear separated, and the right wing impacted the ground.
Examination of Airframe and Engine
Examination by the pilot revealed substantial damage to the right wing. The wreckage was recovered for further examination. Inspection of the recovered airframe and engine showed 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. The upper portion of the number 6 piston remained in the cylinder and could be moved by hand. Portions of the lower piston, piston rings, connecting rod cap, and bolts were located in the oil sump. No evidence of overheat or thermal damage was observed on the crankshaft.
Materials Laboratory Examination
The remains of the number 6 piston, piston pin, and connecting rod were sent to the NTSB Materials Laboratory. Examination revealed that the underside of the piston crown had fractured into smaller pieces near the pin position. The fracture surface exhibited a rough, tortuous appearance and dull luster, with river lines and chevron marks consistent with fracture originating at the pin position. The features were consistent with overstress failure, with no pre-existing cracking or progressive failure observed on the piston.
One connecting rod bolt was fractured in two halves, exhibiting a 45-degree slant fracture surface with localized necking, consistent with tensile overstress. The other bolt was bent with a flat fracture surface along the threads, consistent with overstress in bending. The connecting rod was intact with no cracking or deformation. The connecting rod cap showed twisting and deformation from impacts, but no indications of failure prior to the bolt fractures.
The piston pin exhibited wear and impact marks. One tapered end had separated and deformed into a spherical shape. Most of the fracture surface had a shiny luster and flat shape, consistent with post-fracture batter. The middle had a raised, flattened feature. The periphery showed ridges consistent with ratchet marks, indicating multiple smaller cracks coalescing. A less damaged area exhibited crack arrest marks consistent with progressive failure. Scanning electron microscope examination revealed striations consistent with fatigue cracking. The fatigue crack initiation site showed no material or mechanical defects. The features were consistent with fatigue cracking initiating around the outside of the pin, with final fracture along the middle cross section.
The spherical deformed pin remnant had a circular impression, and its volume was consistent with an impression on the underside of the piston crown, occurring after deformation. Energy dispersive x-ray spectroscopy and x-ray fluorescence identified the pin material as AA 2024 aluminum alloy. Hardness and conductivity tests were consistent with a T6X or T8X temper.
Engine Logbook Review
Engine logbooks showed an overhaul on August 8, 2008, with new piston pins (part number SA 539467) installed. The most recent 100-hour inspection on July 14, 2015, recorded a time since major overhaul of 1,427.1 hours and HOBBS time of 2,344.6 hours. At the time of the accident, the engine had accumulated 1,455 hours since major overhaul.