GRAFTON, OH, USA
N812WC
Hiller UH-12C
About 500 feet above ground level, and 10 minutes before landing, the pilot noticed a change in main rotor noise. The engine rpm increased, and rotor rpm decreased. The pilot reduced collective and increased throttle, but engine rpm continued to increase, and rotor rpm continued to decrease. The pilot entered an autorotation, the helicopter impacted a wire, and then came to rest upright. Inspection of the mercury clutch revealed a 'minor' leak from the transmission sun gear seal, but no oil was discovered on the clutch shoes. In addition, the intermediate drive shaft teeth were 'severely' worn, and three of the teeth had broken corners. The clutch shoes were worn beyond limits, and were glazed consistent with overheating. One clutch return spring was broken. The driver slots in the clutch side plates were 'severely' worn, and the driver lugs were also 'severely' worn. All the clutch attaching hardware were 'improperly' safetied, using .031 inch safety wire, and 'numerous' fasteners were safetied backwards. The clutch was a conditional item. Since the last 100 hour inspection on March 12, 1999, the helicopter flew 100 hours.
On May 15, 1999, about 2045, eastern daylight time, a Hiller UH-12C helicopter, N812WC, was substantially damaged after contacting a wire while executing an autorotation to an open area near Grafton, Ohio. The certificated commercial pilot and passenger were not injured. Visual meteorological conditions prevailed. No flight plan was filed for the personal cross-country flight conducted under 14 CFR Part 91, that departed Findlay, Ohio, destined for the Columbia Airport, Columbia, Ohio. The day before the accident, the pilot and passenger flew to Findlay, Ohio, and landed at the local fair grounds. After coordinating with fair organizers, and conducting a flight to the Findlay Airport to coordinate fuel, the pilot started giving helicopter rides. After several rides, he stopped because of weather. In addition, the pilot felt the weather was to sever to return to Columbia, so he and the passenger checked into a hotel. The next day, the pilot and passenger arrived at the fair grounds, about 1000. The pilot started giving rides about 1300, and continued until about 1745. He then flew back to the Findlay Airport, paid his fuel bill, and returned to the fair grounds. About 1930, the pilot preformed a second preflight for the return flight to Columbia. The pilot and passenger boarded, and the pilot started the helicopter's engine. Once the engine oil temperature went above 40 degrees Celsius, and the main rotor rpm and engine rpm matched, the pilot increased engine rpm to 2,700. He checked both the left and right magnetos, and noted no change in engine rpm. He then stabilized the engine rpm at 3,100, "cut" the throttle, and checked the sprag clutch. No anomalies were noted. After reestablishing 3,100 engine rpm, the pilot increased collective, and executed a crosswind takeoff to the northeast. He stopped the climb at 500 feet agl, and established the helicopter on course for Columbia. Ten minutes before landing at Columbia, the pilot noticed a change in main rotor noise. He also noticed the engine rpm increasing and rotor rpm decreasing. He reduced collective and increased throttle, but engine rpm continued to increase, and rotor rpm continued to decrease. At this point, the helicopter started to shake, and the pilot felt he was loosing control. He estimated that main rotor rpm decreased to 250. The pilot entered an autorotation, and started a right turn to regain main rotor rpm. The main rotor rpm increased, and the helicopter stop shaking. While in the turn the pilot located an open area, and maneuvered to land. On final for the open area, and about 75 feet agl, the pilot identified a wire below his descent path running parallel to a road. Prior to passing over the wire the pilot identified a second wire above the first. The helicopter contacted the second wire, and the pilot heard a loud noise, and saw several intense flashes of light. The helicopter then rotated to the left several times while traveling backwards. The helicopter impacted the ground in a level attitude. The tail rotor made contact with rising terrain destroying the tailrotor blades, and shearing the tailrotor drive shaft. With the main rotor blades still rotating, the pilot secured the ignition system, electrical master, and fuel, before he and the passenger exited the helicopter without injury. The pilot added that the helicopter consumed 1 quart of oil for every 2 to 2 1/2 hours of flight time. In addition, the pilot had 5,400 hours of total flight experience, 5,100 hours in helicopters, and 80 hours in helicopters in the last 90 days. On May 26, 1999, the mercury clutch was examined at a maintenance facility under the supervision of a Federal Aviation Administration Inspector. During the examination, the Inspector discovered a "minor" leak from the transmission sun gear seal, but no oil was discovered on the clutch shoes. In addition, the intermediate drive shaft teeth were "severely" worn, and three of the teeth had broken corners. The clutch shoes were worn to the bottom of the wear groves, and were glazed consistent with overheating. One clutch return spring was broken. The driver slots in the clutch side plates were "severely" worn, and the driver lugs were also "severely" worn. All the clutch attaching hardware were "improperly" safetied, using .031 inch safety wire, and "numerous" fasteners were safetied backwards. The Inspector added that the clutch shoes were worn beyond limits. According to the FAA Inspector, the clutch was a conditional item, and the last 100 hour inspection was completed on March 12, 1999. Since the inspection, the helicopter flew 100 hours.
Inadequate maintenance which resulted in the failure of the mercury clutch assembly.
Source: NTSB Aviation Accident Database
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