CEN16LA306

As the private pilot was descending the airplane for landing, he heard a loud "bang," and the engine subsequently began running rough. Unable to maintain altitude, the pilot conducted a forced landing to a soybean field, during which the airplane impacted a ditch and nosed over. Examination of the engine revealed that the No. 2 cylinder was completely separated between the flange and the head. Examination of the cylinder revealed a fatigue crack that initiated at a cooling fin valley on the exterior surface. The fatigue crack grew around 40% of the circumference of the cylinder, and overstress led to the eventual cylinder fracture. A metallographic cross-section of the cylinder revealed corrosion pits under the paint and primer. The alloy is not necessarily susceptible to pitting, but crevice corrosion near an unpainted area or exposure to salt environments can lead to similar corrosion features.

On August 4, 2016, at 2004 eastern daylight time, the pilot of a Cessna 150L, N10770, impacted terrain in a soybean field near Russellville, Ohio, after a loss of engine power. The private pilot on board sustained a minor injury and the pilot-rated passenger was not injured. The airplane was substantially damaged. The airplane was registered to and operated by a private individual under the provisions of 14 Code of Federal Regulations Part 91 as a personal flight. Visual meteorological conditions prevailed at the time of the accident, and no flight plan had been filed. The local flight originated from Brown County Airport (KGEO), Georgetown, Ohio, about 1945.The pilots told an FAA inspector that while they were descending for landing, they heard a loud "bang" and the engine began running rough. Unable to maintain altitude, they made a forced landing in a soybean field. The airplane struck a ditch and nosed over. Examination of the engine revealed the number 2 cylinder had completely separated between the flange and the head. The pilot was instructed to ship the cylinder halves to the National Transportation Safety Board's (NTSB) Materials Laboratory for examination. According to the Materials Laboratory's Factual Report (16-110), the cylinder had fractured about the circumference of the seventh cooling fin valley through approximately 40% of the cross section. The fracture was relatively flat, exhibited a reflective luster, and was oriented in the direction of piston movement. The fracture surface revealed the presence of crack arrest marks, which were consistent with progressive cracking due to fatigue. Additionally, several cooling fin flange surfaces exhibited small circular features consistent with pitting. An initial thumbnail crack was present adjacent to the crack initiation site, with radial marks and crack arrest marks propagating outward. Fatigue striations were present, consistent with fatigue crack propagation. The crack initiation site on the head side of the fracture surface consisted of three stepped features containing ratchet marks between the steps, consistent with multiple crack initiation sites that had coalesced as the fatigue cracks grew and propagated inward. The mating (open piston side) fracture surface exhibited three ridge shapes, consistent with the previously observed ratchet marks on the head side fracture surface. According to the engine maintenance records, cylinder number 2 (serial number 0049), manufactured by Superior Air Parts, Coppell, Texas, was overhauled on April 14, 2000. The overhaul included grounding the intake and exhaust valve seats, grounding the intake valve, replacing the intake and exhaust guides, and replacing the exhaust valve. The cylinder was then pressure checked and returned to service.