WPR18LA181

While maneuvering during a local instructional flight, the engine began running rough, and the instructor decided to return to the airport. The engine subsequently lost total power and the propeller stopped rotating. The instructor performed a forced landing to a clearing where the airplane nosed over, resulting in substantial damage. Examination of the engine revealed that the crankshaft was fractured at the No. 4 connecting rod journal. The fracture surface was flat and crack arrest lines were consistent with a fatigue fracture which initiated subsurface. No flaws or embedded particles were observed at the site of the fatigue initiation. The engine was overhauled about 3 years before the accident and had accumulated 2,798 hours of operation since the overhaul.

On June 26, 2018, about 1020 mountain standard time, a Cessna 172SP airplane, N154ME, was substantially damaged when it was involved in an accident near Phoenix, Arizona. The pilot received minor injuries; the student pilot and one student pilot passenger were not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The flight instructor reported that, while maneuvering about 5,000 ft mean sea level (msl), the vacuum annunciator light illuminated, and he immediately experienced engine roughness. The pilot decided to depart the practice area and return to the airport but did not declare an emergency or report the engine roughness to the tower controller, because he "didn’t anticipate" an engine failure. The instructor positioned the fuel mixture to full rich and turned on the fuel boost pump switch; however, the engine continued to run rough during the return flight. About 8 miles northwest of the airport, the tower controller instructed the pilot to perform a left 360° turn. After completing the turn, the engine lost all power and the propeller stopped rotating. The instructor configured the airplane for best glide and located a forced landing site. During the landing roll, the airplane impacted rocks and vegetation and nosed over. An examination of the engine revealed that the crankshaft fractured separated at the No. 4 connecting rod journal. The No. 4 connecting rod was separated at the I-beam near the piston attachment and moved freely on the crankshaft. The fractured crankshaft was examined at the NTSB Materials Laboratory. The fracture surface was relatively flat, with crack arrest lines extending from close to the outer diameter of the journal on the web side. The crack arrest lines propagated from a subsurface location about 0.05 inches inboard of the outer diameter. The flat fracture surface and crack arrest lines were consistent with fatigue fracture. No flaws or embedded particles were observed at the site of the fatigue initiation. The engine was overhauled and installed on the accident airplane in May 2015. At the time of the accident, the engine had 2,798 hours of operation since the overhaul.