On September 6, 2020, about 0900 eastern daylight time, a Robinson R22 Mariner helicopter, N194HC, was involved in an accident near Fort Myers, Florida. The commercial pilot and his airline transport-rated pilot receiving instruction were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The instructor and pilot receiving instruction each submitted written statements, and their versions of events were consistent throughout. The preflight and engine-start procedures were completed with no anomalies noted. As the engine accelerated to near its operating rpm, both pilots heard a loud “bang.” The pilot receiving instruction leaned out his door and announced that “the tail was hanging off the back of the aircraft.” The pilots shut down the helicopter and reported the event to the helicopter’s owner and his mechanic. In a telephone interview, the pilot receiving instruction said that, after the helicopter was shut down, he observed no evidence that an unsecured aircraft cover, clothing item, or bird had struck the tailrotor. He added that the only debris surrounding the helicopter on the ramp was aircraft wreckage. Examination of photographs revealed damage to the tailrotor driveshaft, tailrotor gearbox, tailrotor blades, and the tailcone’s section forward of the tailrotor gearbox. These areas and components were sectioned and forwarded to the NTSB Materials Laboratory in Washington, DC for examination. Examination of all the submitted components revealed signatures consistent with overstress fracture on all the fracture surfaces. There was no evidence of fatigue. There was no evidence (mechanical/paint transfer/biological) of foreign object damage (FOD). The fractures and impact signatures were consistent with the fracture of the tailrotor gearbox mount structure (bulkhead) of the tailcone, followed by impact signatures of the rotating tailrotor assembly against the vertical and horizontal stabilizers.