On August 23, 2016, about 0745 mountain daylight time, a Hiller UH-12D helicopter, N97TH, impacted terrain near Cokeville, Wyoming. The pilot received minor injuries and the helicopter was substantially damaged during the accident. The helicopter was registered to and operated by Wyoming Helicopters, Inc., Boulder, Wyoming, under the provisions of 14 Code of Federal Regulations Part 137 as an agricultural flight. Visual meteorological conditions prevailed at the time.
The pilot reported that he started a spray application run and was about 15 ft agl (above ground level) and at 50 mph when he felt a "bump" in the cyclic control. The helicopter began to shake violently and the pilot tried to slow the helicopter down for a landing. However, the helicopter's skid caught on some bushes, and the helicopter rolled over, coming to rest on its side. During the accident, the tail boom and right skid tube were torn from the fuselage and the rotor head separated from the mast. An outboard section of the rotor cuff/paddle was located about 150 ft from the helicopter wreckage. The rotor cuff's spar had separated near a bolted part of a joint.
The separated section of the rotor cuff was sent to the NTSB materials laboratory in Washington D.C. for examination. The examination found fatigue cracks, starting at a bolt hole, that progressed around the rotor cuff spar tube.
The specialist's full materials laboratory factual report is located in the docket for this accident.
A review of the Federal Aviation Administration (FAA) airworthiness directives (AD) notes AD 97-10-16 applicable to the Hiller UH-12 helicopter. The AD requires (in part) that the control rotor blade spar tube be inspected "… for corrosion or cracks, or elongation, corrosion, burrs, pitting or fretting of the bolts holes…" "During the annual inspection, not to exceed 100 hours and every 100-hours, thereafter."
The AD also specifies for helicopters with cuff part number 36124:
(d) For cuffs, P/N 36124, without a complete prior service history, within the next 25 hours TIS, unless already accomplished within the last 25 hours TIS prior to the effective date of this AD, and at intervals not to exceed 50 hours TIS, perform a dye penetrant inspection of the cuff in accordance with paragraph G of the Accomplishment Instructions of Hiller Aviation Service Bulletin, No. 36-1, Revision 3, dated October 24, 1979. If a crack is discovered, remove the cracked cuff from service prior to further flight. A cuff for which the prior service history cannot be documented cannot be used as a replacement part. Remove from service all cuffs prior to the accumulation of 225 hours total TIS since April 7, 1977.
A review of the helicopter's maintenance records indicated that a rotor cuff (part number 36124) was installed on February 11, 2013 with a component total time of 988.7 hours, and 0 since overhaul. A component listing dated September 28, 2015, noted the helicopter's Hobbs time of 1,093.3 hours. At the time of the accident, the Hobbs meter read 1,225.2 hours; 131.9 hours had accumulated on the part since the September 2015 listing. The pilot reported that the helicopter was on an annual inspection program, and its last 100-hour inspection was done on August 5, 2016.