On January 6, 2020, about 1309 Pacific standard time, a Cessna 172H airplane, N1658F, was substantially damaged when it was involved in an accident near El Monte, California. The pilot was not injured. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 personal flight. The pilot reported that the purpose of the flight was to break in new engine cylinders while circling the city of El Monte at 3,500 ft mean sea level. During the flight, the airplane suddenly vibrated severely followed by a sudden loss of engine power and the appearance of oil on the windscreen. The pilot noticed that the propeller was not rotating and there was visible damage to the engine cowling. The pilot initiated a forced landing to the San Gabriel Valley Airport (EMT) and landed uneventfully. Examination of the airplane by a Federal Aviation Administration inspector revealed that the engine mount was bent and one of the propeller blades was separated about 1 ft outboard of the blade root. The propeller was retained for further examination. Examination of the propeller by the National Transportation Safety Board Materials Laboratory revealed that the propeller exhibited a chordwise fracture across the propeller approximately 12 inches outboard of the center of the propeller hub. The fracture surface was relatively flat in the area, including the leading edge of the propeller, and transitioned to a slant type fracture toward the trailing edge. The leading edge exhibited an area of darker coloration and smoother surface texture, which was evident when exposed to side lighting (see figure). Figure: View of the fracture area of the propeller blade. The leading edge exhibited an area of post-fracture impact damage when examined using the scanning electron microscope (SEM), however, further examination with the SEM revealed fracture features consistent with progressive crack growth on the fracture surface in the area described as having a darker and smoother appearance, consistent with a fatigue fracture originating at the leading edge of the propeller. Review of the propeller logbooks revealed that the propeller was overhauled on December 4, 2014. The last 100-hour inspection was completed on December 4, 2019. It could not be determined how much total time since new or time since overhaul the propeller had accrued at the time of the accident.