On August 2, 2018, about 0920 mountain daylight time, a Bell 47G 3B helicopter, N347BH, was substantially damaged when it was involved in an accident near Custer, South Dakota. The pilot and passenger were not injured; the flight instructor received minor injuries. The airplane was operated as a Title 14 Code of Federal Regulations Part 91 instructional flight. The commercial-rated pilot and flight instructor completed several training flights in the accident helicopter before the accident flight to familiarize the pilot with Bell 47 helicopters. On the morning of the accident, the pilot completed hover practice and three circuits in the pattern at Crazy Horse Heliport (0SD9), Custer, South Dakota. Because the instructor did not intervene during the pattern work, the dual flight controls were removed to allow for additional occupants and for the accident pilot to practice flying with a heavier load like the pilot would encounter during future tour flight operations. After picking up a passenger, the pilot and flight instructor practiced hovering and a low-rpm recovery. The helicopter departed 0SD9 and followed a left traffic pattern. On the approach to land, about 100 feet agl, the pilot stated she noticed the rotor rpms drop below the normal operating range. She increased throttle but was unable to recover rotor rpms within normal range. The flight instructor stated that the helicopter lost a lot of altitude quickly and there was no time to recover. He also explained that the turbocharger on the engine had “some lag, the rotor rpm does not come back up with only opening the throttle but lowering the collective at that time was not an option due to the low altitude.” The helicopter continued to descend toward the ground with oncoming traffic on the road. The pilot executed a forced landing in a grassy area on the side of a highway. However, the right skid caught on uneven ground and the helicopter rolled over to the right on the sloping terrain, which substantially damaged the tail section and tail rotor. The flight instructor stated that the engine did not “shutter,” “kick,” or demonstrate another anomaly before or during the event and that the engine “performed well.” He also recalled that the engine did not have issues on any of the previous flights.