Accident Overview
On June 26, 2018, about 1020 mountain standard time, a Cessna 172SP airplane, registered as N154ME, was substantially damaged during a forced landing near Phoenix, Arizona. The flight instructor received minor injuries; the student pilot and one student pilot passenger were not injured. The airplane was being operated as a Title 14 Code of Federal Regulations Part 91 instructional flight.
Sequence of Events
The flight instructor reported that while maneuvering at about 5,000 ft mean sea level, the vacuum annunciator light illuminated, and he immediately experienced engine roughness. He 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, as he did not 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 identified a forced landing site. During the landing roll, the airplane impacted rocks and vegetation and nosed over.
Engine Examination
An examination of the engine revealed that the crankshaft fractured and 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 analyzed 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 had been overhauled and installed on the accident airplane in May 2015. At the time of the accident, the engine had accumulated 2,798 hours of operation since the overhaul.