On March 26, 2022, at 1715 eastern daylight time, a Robinson R44 helicopter, N288L, was substantially damaged when it was involved in an accident near Myrtle Beach, South Carolina. The pilot and three passengers were not injured. The helicopter was operated as a Title 14 Code of Federal Regulations Part 91 sightseeing flight. The pilot reported that he had been flying tours all day on the day of the accident, about 40 flights total; all but two were in another company helicopter. The accident helicopter had also been flown about 40 flights by another company pilot on the day of the accident. After a shift change, the accident pilot performed a preflight inspection on the accident helicopter and then took a break due to the wind conditions. After the break, he completed one uneventful flight in the accident helicopter. On the accident flight, shortly after takeoff about 500-700 ft agl, the helicopter began vibrating. He described the vibration as “very violent, like a brick in a [clothes] dryer, but three times more violent.” The vibration seemed to come from “behind him.” It shook the airframe in all directions and did not seem to be isolated to any one of the helicopter’s controls. He noticed the erratic movement of the rpm on the tachometer and he noted that the helicopter was no longer climbing. The pilot subsequently entered an autorotation and confirmed a full split of the engine/rotor tachometer needles. The vibration continued throughout the autorotation all the way to the ground; however, the pilot reported no unusual resistance, binding, or other issues with the flight controls. During the approach, he flared the helicopter and pulled the collective to cushion the landing; the helicopter landed on its skids and slid on the pavement of a parking lot before impacting a mound of dirt and rolling on its left side. The pilot and passengers were not injured and the helicopter sustained substantial damage to the tail boom and fuselage. An initial examination of the airframe and engine by a Federal Aviation Administration inspector revealed that the tail boom was separated about 3 ft forward of the tail rotor gearbox. The tail rotor blades remained intact and attached to the gearbox with one blade bent about 8 inches from the tip. The tail rotor gearbox rotated smoothly; the chip detector was absent of debris and the drive shaft and tail rotor controls remained attached and intact aft and forward of a tail boom fracture. The tail rotor drive flex couplings were undamaged. The main rotor blades were bent slightly but mostly undamaged, with no evidence of delamination; the blade weights and tip caps remained attached. Rotation of the main gearbox revealed slight binding when rotated by hand and its mounts appeared undamaged. The main gearbox chip detector was absent of debris. The main rotor blade pitch change links were fractured; the ends remained attached to their respective attach points. The main rotor flight controls were intact from the cockpit controls to the fractured pitch change links. The upper and lower main drive sheaves and belts appeared undamaged. The examination of the engine revealed that its mounts were secure; the magnetos operated normally and all valves moved normally (as viewed with the valve covers removed while rotating the engine by hand). The cylinder compression was measured, which ranged from a low of 68 psi (No. 5) to a high of 79 (Nos. 3 and 4). A borescope examination of all cylinders revealed “normal wear and carbon buildup”. The engine was test run successfully with a “slight” vibration noted. Fuel samples taken from the main and auxiliary fuel tanks were unremarkable. A subsequent postaccident examination of the helicopter by the National Transportation Safety Board and representatives from the airframe and engine manufacturers revealed no preimpact anomalies or malfunctions that would have precluded normal operation. No airframe anomalies were noted. A postaccident engine examination was performed. The cylinder valve covers were removed and valve action was observed as the crankshaft was rotated by hand. Valve lift was visually observed to be smooth and consistent for all 12 valves. A borescope examination revealed no damage or excessive heat signatures on the piston domes, cylinder walls or valve heads. Debris was noted on the valve seats in the valve contact area in the left-side cylinders. The engine (which was disconnected from the transmission) was started and run for about 20 minutes. A slight roughness could be heard until the engine warmed up. Exhaust pipe temperatures were observed using a laser/infrared thermometer. All six cylinder exhaust pipes indicated between 650ºF to 750ºF. An investigator, sitting in the pilot seat and operating the controls, indicated that he could feel some roughness and observed small oscillation in the tachometer needles. An occasional “chuff” could be heard from the engine exhaust as the engine ran at high power. A magneto check was performed and a drop of about 12 percent rpm was noted from each magneto. The engine was shut off and allowed to heat soak for about 5 minutes. It was restarted and run for about 5 minutes and it ran smoother. It was shut off and allowed to heat soak for about 5 minutes and restarted and run another 3 minutes. It was again smoother than the first run. After the engine was shut down and moved into a hangar, a compression check was performed on each cylinder. The test values ranged from a low of 62 psi (No. 4) to a high of 78 psi (No. 1). A review of the engine maintenance records revealed that at the time of the accident the engine had accrued a total time of 3,318 hours, and 1,118 hours since overhaul. Since the overhaul, there were 3 maintenance entries that indicated the engine’s valves were inspected and maintained in accordance with a Lycoming mandatory service bulletin (SB-338C), which was prescribed as preventative maintenance to reduce the likelihood of stuck or broken valves due to excessive carbon build up between the valve guide and valve stem. All were performed within the 400-hour interval noted in the bulletin, the last of which occurred about 5 weeks and 85 flight hours before the accident.