History of Flight
On June 3, 2014, at 1903 Pacific daylight time, an experimental amateur-built Mark Emery (Rotorway International) 162F helicopter, registration 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 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 tail rotor drive belt tension. After a run-up and a pick-up and set-down, he departed to an agricultural field just north of the airport to practice autorotations. The first autorotation was uneventful. During the flare at the conclusion of the second autorotation, as the pilot applied power, the helicopter began to yaw left. He applied right antitorque pedal control, but the helicopter did not respond and began to spin left. The helicopter spun about one and a half 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 tail rotor drive was still attached to the tailboom, and one tail rotor blade had bent about 15 degrees at its root. Both blades were free of rotational damage signatures.
Examination Findings
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.
The tail rotor shaft was driven via the main transmission through three V-belts connected in series along the tailboom assembly. The belts were interconnected through two idler pulleys hanging from 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 tail rotor center drive belt failure during takeoff, 36.1 hours after installation, with the factory-recommended Bando non-cogged V-belts. His intention was to retrofit with a tail rotor drive shaft conversion, but the Rotorway factory was temporarily closed. He then installed Goodyear "HY-T Wedge" cogged belts under the guidance of a fellow Rotorway 162F owner and mechanic. The belts were installed per the maintenance manual and had accumulated about 2.6 hours of time before the accident, including low-hover, air taxi, and traffic pattern flights with belt checks and readjustments as needed. The pilot reported that at no time did the belts stretch beyond their mandatory replacement limit.
Drive Belt Analysis
Examination of Goodyear and Bando drive belt documentation showed that the Goodyear cogged belt was dimensionally equivalent to the factory-recommended Bando non-cogged belt. According to Goodyear, the cogged belt had slightly different tensioning requirements and dissipated heat more efficiently. 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. The fracture surface of the accident belt consisted of failed rubber and polyester fibers that had been twisted, elongated, and pulled out. Scanning electron microscope examination of fiber tips showed either a flat, angled morphology or a globular "mushroom-cap" shape, both consistent with failure from tensile overstress. The rubber fracture surface was generally flat relative to the belt direction, rough and tortuous, with a dull luster. There were no indications of river marks, hackles, or rubber embrittlement. No glazing, cracks, or wear marks were found on the accident belt contact surfaces or notches, but notches adjacent to the failure surface appeared 0.02 inches wider than the exemplar belt notches.
The Goodyear V-Belt Troubleshooting Guide indicated that the failed belt may have experienced a tensile break, possibly 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."