On April 20, 2017, about 1030 eastern daylight time, a Robinson R44 II, N899GB, was substantially damaged during an autorotative landing near Orlando, Florida. The commercial pilot and two passengers were not injured. Visual meteorological conditions prevailed at the time, and no flight plan was filed for the local flight that originated about 5 minutes earlier from Air Orlando Heliport (2FD7), Orlando, Florida. The revenue sightseeing flight was operated under the provisions of Title 14 Code of Federal Regulations Part 91.During the 1st flight of the day while returning to 2FD7, the pilot stated that he was flying southbound and descending out of 1,000 feet, when at 600 feet, he heard and felt a loud "pop" that was accompanied by a yaw to the right, shaking and vibration, with engine roughness. At that time the rotor tachometer had, "dropped to zero", while the engine tachometer was "at about 60-70%." He immediately entered an autorotative descent to a median between two roads, but did not recall his airspeed during the maneuver, and landed, "…a little hard…." After touchdown the helicopter began to roll forward, which he attempted to correct with aft cyclic input. Once the helicopter was stable on the ground, he secured the engine, and then evacuated the passengers after the main rotor had stopped. Postaccident examination of the helicopter revealed the aft section of the tailboom was partially detached from the helicopter, and the leading edge of the main rotor blades exhibited slight damage. In the area of the tailboom separation point, the tail rotor drive shaft was fractured and the tail rotor push-pull tube assembly was deformed. Further examination of the tailboom revealed the arm associated with the damper assembly was bent about 90° and fractured. The damper remained connected to the support which was torn and deformed. No deformation was noted internally to structural members adjacent to the damper area. No other damage to the tail rotor drive system was noted. Examination of the engine compartment revealed one magnet assembly part number (P/N) B570-1 associated with the main rotor tachometer indicating system was separated from the Yoke Assembly, P/N C908-1, and was not recovered. The other magnet assembly remained secured to the yoke assembly. The yoke assembly was not damaged, but there was evidence of slight damage to 1 sender assembly, consistent with the magnet assembly separation. The lowest numeric marking on the dual tachometer associated with the main rotor is 50, with 2 graduated lines beneath. The engine was removed from the helicopter and taken to a repair facility where it was examined and then placed on a test stand and operated twice. The first and second engine runs lasted 15 and 25 minutes, respectively with no discrepancies noted. The engine was operated a third time, and about 10 minutes into the engine run, the left magneto failed the magneto check. The defective magneto was removed, and a replacement magneto was installed. The engine was started and operated normally. Robinson Helicopter Company personnel reported there have been instances of a magnet separating from a magnet assembly which prompted issuance of Service Bulletin (SB) 86; however, they report there have not been any previous instances of the magnet assembly separating from the yoke assembly. According to the Pilot's Operating Handbook, a caution in the emergency procedures section related to power failure specifies to not apply aft cyclic during touchdown or ground slide to prevent possible blade strike to the tailcone. According to the maintenance manual, the sensor for the main rotor tachometer is an electronic Hall Effect device which senses passage of two magnets attached to main rotor gearbox input yoke assembly. Robinson personnel reported that with only 1 magnet installed, the main rotor tachometer rpm would indicate about 50% of the actual rotor rpm.