Wednesday, September 20, 2017

Hiller UH12D, N97TH, registered to and operated by Wyoming Helicopters Inc: Accident occurred August 23, 2016 in Cokeville, Lincoln County, Wyoming

Aviation Accident Final Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

NTSB Identification: CEN16LA330 
14 CFR Part 137: Agricultural
Accident occurred Tuesday, August 23, 2016 in Cokeville, WY
Probable Cause Approval Date: 11/14/2017
Aircraft: HILLER UH 12D, registration: N97TH
Injuries: 1 Minor.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

The commercial pilot was conducting an agricultural spray run in the helicopter when he felt a “bump” in the cyclic. The helicopter began to shake violently, and the pilot tried to conduct a forced landing; however, the skid caught on bushes and the helicopter impacted terrain.  A control rotor cuff was located about 150 ft from the main wreckage.  Examination of the component at the NTSB materials laboratory found a fatigue crack starting at the cuff’s bolt hole and progressing until the part separated from overload.  A review of maintenance records revealed that the cuff was overhauled at 988.7 hours and had accumulated 131.9 hours since overhaul.  The rotor cuff was subject to an airworthiness directive (AD) which required repetitive inspections, though the investigation was unable to determine whether the AD had been complied with. The investigation also noted ambiguity with the wording used in the AD; it was unclear whether the part was subject to replacement at 225 hours total time in service, or if the 225-hour replacement was only applicable to components without known service history.  The accident is consistent with an in-flight separation of the control rotor cuff due to fatigue failure.

The National Transportation Safety Board determines the probable cause(s) of this accident as follows:
Failure of the control rotor cuff due to fatigue.  

The National Transportation Safety Board did not travel to the scene of this accident.

Additional Participating Entity:
Federal Aviation Administration / Flight Standards District Office; Denver, Colorado

Aviation Accident Factual Report - National Transportation Safety Board: https://app.ntsb.gov/pdf

Investigation Docket - National Transportation Safety Board: https://dms.ntsb.gov/pubdms

http://registry.faa.gov/N97TH

NTSB Identification: CEN16LA330
14 CFR Part 137: Agricultural
Accident occurred Tuesday, August 23, 2016 in Cokeville, WY
Aircraft: HILLER UH 12D, registration: N97TH
Injuries: 1 Minor.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

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.

NTSB Identification: CEN16LA330
14 CFR Part 137: Agricultural
Accident occurred Tuesday, August 23, 2016 in Cokeville, WY
Aircraft: HILLER UH 12D, registration: N97TH
Injuries: 1 Minor.

This is preliminary information, subject to change, and may contain errors. Any errors in this report will be corrected when the final report has been completed. NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

On August 23, 2016, about 0745 mountain daylight time, a Hiller UH-12D helicopter, N97TH, impacted terrain near Cokeville, Wyoming. The commercial rated 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 Federal Aviation Administration (FAA) inspector reported that on the day of the accident the pilot had completed several agricultural flights. The pilot then was cleaning the spray system by it with flushing with water, on a final flight. During the flight, when the helicopter was about 10-15 ft off the ground, a control rotor cuff separated from the helicopter. The helicopter impacted terrain. The helicopter came to rest on its side/roof, and the tail boom had separated in the crash. The cuff was located about 50 yds from the helicopter wreckage. 

The helicopter was retained for further examination.

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