On July 2, 2018, about 1130 central daylight time, an amateur-built Safari Helicopters 400 helicopter, N326RW, impacted the ground following a loss of control near Aguilares, Texas. The private pilot sustained serious injuries, and the helicopter sustained substantial damage. The helicopter was owned by Panda Ventures, LLC, and was operated by the pilot under the provisions of Title 14 Code of Federal Regulations Part 91. Visual meteorological conditions prevailed and no Federal Aviation Administration (FAA) flight plan had been filed for the flight. The personal flight departed Slator Ranch, near Aguilares, Texas, about 1000.According to a statement given to the responding FAA inspector, the pilot was working on the Phase I flight test requirements following the experimental certification of his helicopter. The helicopter had about 25 hours total time since it was built from a commercial kit. The helicopter was in straight and level flight, between 20 and 30 ft above ground level when it started to shudder. The pilot started to look for a place to land; he then heard a "pop" and the helicopter immediately lost tail rotor authority. The helicopter started to rotate counterclockwise, and the pilot attempted to stop the rotation without success. The helicopter impacted the ground and rolled onto its left side. The fuselage was crushed and buckled, and both main rotor blades were substantially damaged. A teardown and examination of the transmission revealed that the tail rotor pinion had failed. The tail rotor pinion was located inside of the transmission. The exterior of the transmission exhibited impact damage at the mast but was otherwise unremarkable. There was no damage to the tail rotor blades or the tail rotor driveshaft. There were no other mechanical anomalies noted with the helicopter or its systems that would have precluded normal operations. The tail rotor pinion was shipped to the National Transportation Safety Board's Materials Laboratory for further examination. Examination revealed that the tail rotor pinion shaft fractured between the portion carrying a beveled gear and that containing a helical gear. Both halves of the pinion shaft fracture surface exhibited mechanical damage by recontact resulting in rubbing of the fracture surfaces. No evidence of a preexisting crack was observed. The overall fracture macro-morphology was consistent with bending overstress. Microhardness measurement values were consistent with the specifications for the pinion shaft. Details of this examination are contained in a specialist report in the public docket for this accident. According to a representative with Safari Helicopters, prior to the accident, Safari Helicopters found that a batch of pinions and gears were not hardened properly. The pinion in the accident helicopter was not part of the batch with the hardening issue.