Introduction
On August 23, 2016, about 0745 mountain daylight time, a Hiller UH-12D helicopter, registration N97TH, impacted terrain near Cokeville, Wyoming. The helicopter was registered to and operated by Wyoming Helicopters, Inc., based in Boulder, Wyoming, conducting an agricultural flight under 14 Code of Federal Regulations Part 137. Visual meteorological conditions prevailed at the time of the accident. The pilot reported minor injuries, and the helicopter sustained substantial damage.
Accident Sequence
According to the pilot, during a spray application run at approximately 15 ft above ground level and 50 mph, a sudden "bump" was felt in the cyclic control. The helicopter then began shaking violently. The pilot attempted to reduce speed for a landing, but the skid contacted bushes, causing the helicopter to roll over and come to rest on its side. The impact tore the tail boom and right skid tube from the fuselage, and the rotor head separated from the mast. An outboard section of a rotor cuff/paddle was found about 150 ft from the main wreckage. The rotor cuff's spar had separated near a bolted joint.
Examination
A materials laboratory examination of the separated rotor cuff section, conducted by the NTSB in Washington D.C., identified fatigue cracks originating at a bolt hole. These cracks propagated around the rotor cuff spar tube. The complete materials laboratory factual report is included in the docket for this accident.
Maintenance Records
The Federal Aviation Administration airworthiness directive (AD) 97-10-16, applicable to the Hiller UH-12 helicopter, mandates inspections of the control rotor blade spar tube for corrosion, cracks, or bolt hole anomalies. The AD specifies inspections during annual inspections, not to exceed 100 hours, and every 100 hours thereafter. For cuffs with part number (P/N) 36124, additional requirements include dye penetrant inspections at intervals not exceeding 50 hours time in service (TIS) for cuffs without a complete prior service history, and removal from service if cracks are found. Cuffs lacking documented service history cannot be used as replacements and must be removed before accumulating 225 total hours TIS since April 7, 1977.
Review of the accident helicopter's maintenance records revealed that a rotor cuff, P/N 36124, was installed on February 11, 2013, with a component total time of 988.7 hours and 0 hours since overhaul. A component listing dated September 28, 2015, indicated the helicopter's Hobbs meter at 1,093.3 hours. At the time of the accident, the Hobbs meter read 1,225.2 hours, representing an accumulation of 131.9 hours since the September 2015 listing. The pilot stated that the helicopter was on an annual inspection program, and the last 100-hour inspection occurred on August 5, 2016.