Cessna 172R accident near Redding, California, November 2, 2017

The NTSB's narrative final · quoted from the NTSB record

Two student pilots and the flight instructor were conducting a cross-country instructional flight. The instructor reported that he told the student who was flying to climb to 3,500 ft above ground level (agl). As the airplane approached 3,100 ft agl, the student began leaning the mixture. Subsequently, the engine began to sputter, followed by a loss of all power. The instructor then saw sparks near the propeller area, took the flight controls, and turned back toward the departure airport. During short final, due to fire near the propeller area and the smell of smoke in the cockpit, the instructor turned the airplane 360° to try to lose altitude; however, the airplane was still too high to safely land on the runway. Subsequently, he conducted a forced landing to a field just past the runway, during which the airplane struck a fence, which resulted in substantial damage to the wing and firewall. Examination of the engine revealed that the crankshaft had separated at the propeller hub assembly and that a portion of it remained in the assembly. No holes were found in the engine case. Metallurgical examination revealed that the crankshaft had fractured into two halves at the main bearing journal’s forward side. The fracture consisted of two primary mating crack faces, one of which was circumferential and the other of which was a spiral through a lubrication hole to the inside of the crankshaft journal. The lubrication hole exhibited crack arrest marks that originated from the inside portion of the hole wall. The mating fracture surfaces of the circumferential and spiral fractures exhibited crack arrest marks consistent with progressive crack growth due to fatigue. The areas outside of the circumferential fracture exhibited features consistent with subsequent overstress fracture. Maintenance records revealed that at the time of the accident the engine had accumulated a total time of 3,907.1 hours and 1,153.7 hours since major overhaul. Based on the evidence, it is likely that the crankshaft fractured due to a fatigue crack that had initiated at a lubrication hole on the main bearing journal and then progressed in an outward spiral direction toward the inside surface of the crankshaft journal. The fatigue crack then progressed circumferentially until the crankshaft fractured into two halves due to overstress, which resulted in a subsequent total loss of engine power and off-airport landing.

The complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.