HISTORY OF FLIGHTOn June 3, 2014, at 1903 Pacific daylight time, an experimental amateur-built Mark Emery (Rotorway International) 162F helicopter, N78291, landed hard following a loss of power to the tail rotor system near Merced Regional Airport/Macready Field, Merced, California. The helicopter was registered to, and operated by, the owner under the provisions of 14 Code of Federal Regulations Part 91. The private pilot and passenger were not injured. The helicopter sustained substantial damage during the accident sequence, and was destroyed by a post-impact fire. The local personal flight departed Merced at 1840. Visual meteorological conditions prevailed, and no flight plan had been filed. The pilot reported performing a preflight inspection, which included checking the tailrotor drive belt tension. After then performing a run-up, followed by a pick-up and set-down, he departed in the helicopter to an agricultural field just north of the airport to practice autorotations. The first autorotation was uneventful, but as he applied power during the flare at the conclusion of the second autorotation, the helicopter began to yaw to the left. He applied right antitorque pedal control, but the helicopter did not respond, and began to spin to the left. The helicopter spun about 1 1/2 rotations before the pilot reduced engine power and lowered the collective. The helicopter then descended, struck the ground, and rolled onto its right side. The two occupants exited as flames began to emerge from the engine compartment. The majority of the airframe structure was consumed by fire, with only the landing gear and steel frame components remaining intact. The tailboom aft of the second bulkhead remained integral, with the center and aft idler pulleys undamaged. The tailrotor drive was still attached to the tailboom, and one of the tailrotor blades had bent about 15 degrees at its root. Both blades were free of rotational damage signatures. Examination of the drive system at the accident site revealed that the aft tail rotor drive belt remained intact and connected between the tail rotor gearbox and aft pulley, the center belt had fractured, and the majority of the forward belt had been consumed by fire. The tailboom structure was intact between the two pulleys holding the center belt, and sustained only a dent to the lower skin. TESTS AND RESEARCHTailrotor Drive Design The tailrotor shaft was driven via the main transmission through three V-belts connected in series along the length of the tailboom assembly. The belts were interconnected through two idler pulleys, which hung from the tailboom bulkheads by scissor mounts. The recommended replacement interval for the belts was 250 hours. The pilot reported that the helicopter had previously experienced a premature tailrotor center drive belt failure during takeoff, 36.1 hours after it was installed. At that time, the helicopter was equipped with the factory-recommended Bando brand non-cogged V-belts. His intention was to ultimately retrofit the helicopter with a tailrotor drive shaft conversion; however, the Rotorway factory was temporarily closed during that period. Having lost confidence in the Bando belts, he decided to install Goodyear "HY-T Wedge" cogged belts under the guidance of a fellow Rotorway 162F owner, who was also the mechanic who performed condition inspections on the helicopter. He reported that the belts were installed using the steps outlined in the maintenance manual, and that prior to the accident they had accumulated about 2.6 hours of time, which included low-hover, air taxi, and a flight in the traffic pattern with belt checks and readjustments performed as needed during that period. He reported that at no time during the adjustment process did the belts stretch beyond their mandatory replacement limit. Drive Belts Examination of Goodyear and Bando drive belt documentation revealed that the Goodyear cogged belt was dimensionally equivalent to the factory-recommended Bando non-cogged belt. However, according to Goodyear, the cogged belt had slightly different tensioning requirements, and was able to dissipate heat more efficiently than the non-cogged belt. Additionally, the non-cogged belt was recommended for "drives where pulsation, shock loads, high tension, and long centers are involved." The failed belt, along with an exemplar cogged belt, were sent to the NTSB Office of Research and Engineering Materials Laboratory for examination. The fracture surface of the accident belt consisted of the failed rubber and the polyester fibers that had been twisted, elongated, and pulled out. The tips of the fractured fibers were examined utilizing a scanning electron microscope and generally exhibited either a flat, angled morphology, or terminated with a globular "mushroom-cap" shape. Both of these fiber fracture features were consistent with failure from tensile overstress. The rubber fracture surface was generally flat relative to the belt direction. The rubber portion was rough and tortuous, and exhibited a dull luster. There were no indications of river marks or hackles that would indicate crack initiation sites, and there were no indications of rubber embrittlement. No glazing, cracks, or wear marks were found on the accident belt contact surfaces or notches; however, the notches adjacent to the failure surface appeared to be 0.02 inches wider than the exemplar belt notches. The Goodyear V-Belt Troubleshooting Guide documented a series of failed belt examples with their associated corrective action. The guide indicated that the failed belt may have experienced a tensile break, which could be caused by "high shock loads, foreign object between the bottom of the sheave and the bottom of the belt or damage during installation." Corrective action included, "maintaining proper drive tension and installation procedures."