History of Flight
On December 9, 2013, at about 0630 Pacific standard time, a Hiller UH 12E helicopter, registration N36MV, experienced a loss of drive to the rotor system while conducting a frost control flight over a citrus grove near Chowchilla, California. The helicopter was owned and operated by S and S Helicopters under 14 CFR Part 91. The commercial pilot, who was also the owner, reported that the engine began to overspeed with a corresponding decrease in rotor speed while at about 100 feet above ground level. He initiated an autorotation, during which the helicopter struck trees in an adjoining orchard. The pilot was not injured, and the helicopter sustained substantial damage.
Post-Accident Actions and Examination
The pilot/owner removed the helicopter from the accident site immediately and began disassembling it and sending components out for repair before the arrival of a Federal Aviation Administration (FAA) inspector two days later. The FAA inspector examined the helicopter and observed that both the elastomeric ring and the lower drive lugs on the engine side of the main transmission torsional coupling had sheared. Despite multiple requests, the owner did not make the torsional coupling available for detailed examination by the National Transportation Safety Board (NTSB) investigator-in-charge. The owner also stated that the helicopter's maintenance logbooks had been misplaced.
Torsional Coupling Details
The torsional coupling and clutch assembly consisted of upper and lower flanged plates bonded to either side of a disk-shaped elastomeric material, providing a torsionally elastic cushion between the engine and transmission. The assembly included secondary drive lugs that, under normal operation, did not contact each other but would engage if the elastomeric material failed, thereby continuing to drive the main transmission. The coupling was manufactured by Lord Mechanical Products, which defined a finite service life. In 1998, Hiller Aviation issued Service Bulletin 21-4 Revision 1, specifying inspection intervals at 300 and 1,200 hours and adherence to a finite service life of 5,000 hours or five years from installation, whichever occurred first. The coupling installed on the accident helicopter was stamped with a manufacture date of 2007.
Maintenance and Previous Accident
The owner held an FAA airframe and powerplant mechanic certificate with inspection authorization and appeared to be the primary mechanic for the helicopter based on previous maintenance records. He initially told the FAA inspector that he had inspected the coupling about six to eight months prior to the accident and most likely replaced it with a spare from another helicopter. In a later conversation, he reported having no recollection of ever replacing the coupling. The helicopter had been involved in a previous accident on February 2, 2010, for which the NTSB determined the probable cause to be a partial loss of engine power due to a failure of throttle interconnect hardware, resulting in a hard landing. That accident caused substantial damage to the forward fuselage and tailboom, and both main rotor blades were destroyed. No FAA Form 337 (Major Repair and Alteration) was found on file for that damage. Maintenance records from the previous investigation showed the helicopter had accrued 6,815 total flight hours as of February 25, 2009; the owner reported an additional 1,154 hours by September 20, 2013.