HISTORY OF FLIGHTOn December 9, 2013, about 0630 Pacific standard time, a Hiller UH 12E, N36MV, landed hard near Chowchilla, California, following a loss of drive to the rotor system. The helicopter was owned and operated by S and S Helicopters under the provisions of 14 Code of Federal Regulations Part 91, as a frost control flight. The commercial pilot was not injured, and the helicopter sustained substantial damage during the accident sequence. The local flight departed from the operator's facility in Chowchilla, about 0400. Visual meteorological conditions prevailed, and no flight plan had been filed.
The pilot/owner stated that he was performing frost control over a citrus grove at about 100 feet above ground level when the helicopter's engine began to overspeed, with an accompanying decrease in rotor speed. He initiated an autorotation and the helicopter stuck trees in an adjoining orchard. The owner removed the helicopter immediately following the accident, and began both disassembling it, and sending components out for repair prior to the arrival of a Federal Aviation Administration (FAA) inspector 2 days following the accident. The FAA inspector subsequently examined the helicopter and observed that both the elastomeric ring and lower drive lugs (engine side) of the main transmission torsional coupling had sheared.
Despite multiple correspondences, the owner did not make the torsional coupling available for detailed examination by the NTSB investigator-in-charge. He additionally stated to the FAA inspector that the helicopter's maintenance logbooks had been misplaced.
The helicopter was involved in another accident on February 2, 2010. The NTSB determined the probable cause to be, "A partial loss of engine power due to a failure of the throttle interconnect hardware, which resulted in a hard landing." During that accident the helicopter sustained substantial damage to the forward fuselage and tailboom, which had become crushed and separated from the aft bulkhead. Additionally, both main rotor blades were destroyed after making contact with the ground while rotating. Despite the significant nature of the damage, no FAA records were located indicating that an FAA Form 337 (Major Repair and Alteration - Airframe, Powerplant, or Appliance) had been submitted. According to maintenance records submitted during that investigation, as of February 25, 2009, the helicopter had accrued 6,815 total flight hours. The owner reported that during the period between that inspection and the last, dated September 20, 2013, the helicopter had accrued an additional 1,154 hours of flight time. TESTS AND RESEARCHThe torsional coupling and clutch consisted of upper and lower flanged plates bonded to either side of a disk shaped section of elastomeric material. The elastomeric component provided a torsionally elastic cushion between the engine and transmission. The assembly also contained secondary drive lugs, which during normal operation do not contact each other. However, in the event of a failure of the elastomeric material, the secondary drive lugs would continue to drive the main transmission.
The coupling was manufactured by Lord Mechanical Products, who defined a finite service life for the part. In 1998 Hiller Aviation issued Service Bulletin 21-4 Revision 1, specifying a series of 300 and 1,200 hours inspection intervals, along with adherence to the components finite service life in accordance with Lord service data as follows:
"Prior to installation in the aircraft, parts should be stored in accordance with MIL-STD-1523 for a maximum of sixty (60) months. Couplings stored for more than twenty-four (24) months shall be inspected for ozone/flex cracks in accordance with Section I.A.2 (b) of this Service Data. After installation in the aircraft, the J-10114 series coupling operates on a FINITE SERVICE LIFE basis: 5,000 hours maximum or five (5) years maximum from date of installation or until the limits of the maintenance manual are met, whichever occurs first."
The FAA inspector who examined the helicopter reported that the coupling was stamped with a manufacture date of 2007.
The helicopter's owner held an FAA airframe and powerplant owner mechanics certificate with inspection authorization. Based on maintenance records from the previous accident, he appeared to be the primary mechanic for the helicopter. He initially reported to the FAA inspector that he inspected the coupling about 6-8 months prior to the accident, and that he had most likely replaced it with a spare component from another helicopter. In a follow-up conversation he reported that he had no recollection of ever replacing the coupling.