# NTSB CEN16LA330: Hiller UH 12D, N97TH, Cokeville, WY, August 23, 2016

- Type: NTSB accident investigation
- Classification: Accident · Minor injuries · Limited investigation · Final report
- Date: August 23, 2016, 07:45 local (13:45 UTC)
- Place: Cokeville, WY
- Coordinates: 42.066944, -110.946945 (estimated)
- Nearby events: https://openairdocs.org/map.md?lat=42.0669&lon=-110.9469&mi=10
- Phase of flight: [Maneuvering](https://openairdocs.org/ntsb-phases/450) (OpenAirDocs' grouping)
- Defining event: [Maneuvering-low-alt flying: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338)
- Highest injury: Minor injuries
- Injuries: 1 minor
- Status: Completed, report November 14, 2017
- Report: Approved by Office Director
- NTSB office: Central Region
- Parties: FAA FSDO
- ADs cited in the narratives: [AD 97-10-16](https://openairdocs.org/ad/97-10-16) (OpenAirDocs)
- NTSB case key (mKey): 93876
- NTSB case page: https://web.ntsb.gov/investigations/?ntsbnumber=CEN16LA330
- NTSB report (PDF, downloads): https://data.ntsb.gov/carol-repgen/api/Aviation/ReportMain/GenerateNewestReport/93876/pdf
- NTSB docket: https://data.ntsb.gov/Docket?ProjectID=93876
- Page: https://openairdocs.org/ntsb/CEN16LA330

## Probable cause

Failure of the control rotor cuff due to fatigue.

## Findings

NTSB's coded findings; those marked are part of the probable cause.

- [Main rotor system: main rotor head system](https://openairdocs.org/ntsb-findings/01046220): Aircraft › Aircraft propeller/rotor · Fatigue/wear/corrosion (probable cause); cases with it: [Hiller UH-12D, 1 case](https://openairdocs.org/ntsb-findings/01046220/cases.md?model=4360118), [All aircraft, 10 cases](https://openairdocs.org/ntsb-findings/01046220/cases.md)

## Sequence of events

The events NTSB coded, in order, by phase of flight. The defining event best describes what happened.

- [Maneuvering-low-alt flying: Part(s) separation from AC](https://openairdocs.org/ntsb-events/338) (defining event); cases with it: [Hiller UH-12D, 2 cases](https://openairdocs.org/ntsb-events/338/sequence.md?model=4360118), [All aircraft, 563 cases](https://openairdocs.org/ntsb-events/338/sequence.md)
- [Maneuvering-low-alt flying: Loss of control in flight](https://openairdocs.org/ntsb-events/240); cases with it: [Hiller UH-12D, 4 cases](https://openairdocs.org/ntsb-events/240/sequence.md?model=4360118), [All aircraft, 5,002 cases](https://openairdocs.org/ntsb-events/240/sequence.md)
- [Uncontrolled descent: Collision with terr/obj (non-CFIT) ](https://openairdocs.org/ntsb-events/470); cases with it: [Hiller UH-12D, 3 cases](https://openairdocs.org/ntsb-events/470/sequence.md?model=4360118), [All aircraft, 10,189 cases](https://openairdocs.org/ntsb-events/470/sequence.md)

## Analysis

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.

## Factual narrative

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 ofthe 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.

## Aircraft

- Registration: [N97TH](https://openairdocs.org/deregistered/N97TH-2016-11-08), registration cancelled 2016-11-08, S/N 1165, built 1960 (OpenAirDocs)
- Reported make and model: HILLER UH 12D (NO SERIES)
- Registry model: [Hiller UH-12D](https://openairdocs.org/model/hiller-uh-12d) (registry code 4360118) (OpenAirDocs)
- Registry engine: [Allison 250 series](https://openairdocs.org/engine/allison-250-series) (registry code 03014, turbo-prop) (OpenAirDocs)
- Category: Helicopter
- Serial number: 1165
- Year built: 1960
- Airframe hours: 14494
- Damage: Substantial damage
- Flight: Business, Part 137
- Operator: Wyoming Helicopters Inc DBA
- Registered owner: Wyoming Helicopters Inc DBA
- Airworthiness certificate: Standard, Normal
- Last inspection: Annual, August 5, 2016
- Departed: Cokeville, WY
- Destination: Cokeville, WY

## Flight crew

As NTSB recorded them. NTSB leaves out names.

- Pilot; age 55; injuries: none; commercial; medical: class 2, with waivers/limitations; last flight review December 9, 2014; seat: center; restraint: 3-point; flight hours: 3,965.8 total, 3,007.1 in this make and model

## Weather

- Conditions: Visual (VMC), Day
- Wind: calm
- Visibility: 10 statute miles
- Lowest ceiling: none
- Temperature: 32 °F, dew point 27 °F
- Altimeter: 30.19 inHg
- Observed at: KAFO

## Docket (6 files)

The files NTSB released with the investigation on September 20, 2017, on NTSB's docket system: https://data.ntsb.gov/Docket?ProjectID=93876. 9.1 MB in all.

- Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1 (item 1, .PDF, 2.6 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456454&FileExtension=.PDF&FileName=Pilot%2FOperator%20Aircraft%20Accident%20Report,%20NTSB%20Form%206120.1-Master.PDF
- Maintenance Records (excerpts) (item 2, .PDF, 973 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456455&FileExtension=.PDF&FileName=Maintenance%20Records%20(excerpts)-Master.PDF
- Materials Laboratory Factual Report (item 3, .PDF, 17 photos, 3.2 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456456&FileExtension=.PDF&FileName=Materials%20Laboratory%20Factual%20Report-Master.PDF
- Release of Aircraft Wreckage, NTSB Form 6120.15 (item 4, .PDF, 55 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456457&FileExtension=.PDF&FileName=Release%20of%20Aircraft%20Wreckage,%20NTSB%20Form%206120.15-Master.PDF
- Hiller AD 97-10-16 (item 5, .PDF, 159 KB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456458&FileExtension=.PDF&FileName=Hiller%20AD%2097-10-16-Master.PDF
- Service Bulletin (item 6, .PDF, 2.1 MB): https://data.ntsb.gov/Docket/Document/docBLOB?ID=40456459&FileExtension=.PDF&FileName=Service%20Bulletin-Master.PDF

From the NTSB's public API. Aircraft are matched to registry aircraft by N-number and event date, checked against the reported serial number, make and model; the registry's history has gaps, so older matches can be wrong. Compare the reported make and model.

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