Hiller UH 12D accident near Cokeville, Wyoming, August 23, 2016
On August 23, 2016 at about 1:45 pm local time, a 1960 Hiller UH 12D (helicopter), registered N97TH, was substantially damaged in an accident during maneuvering (low-alt flying) near Cokeville, Wyoming. It was a business flight under agricultural flying rules (Part 137). 1 person had minor injuries. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
Failure of the control rotor cuff due to fatigue.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- August 23, 2016 · about 1:45 pm local time
- Place
- Cokeville, Wyoming · map
- Type
- Accident
- Injuries
- 1 person had minor injuries.
- Weather
- visual conditions (good weather)
- Aircraft
- Hiller UH 12D NO SERIES, built 1960 · all UH 12Ds on the register
- Registration
- N97TH · no longer on the register · serial 1165
- Damage
- Substantial damage
- Flight
- Business flight · agricultural flying rules (Part 137)
The NTSB's narrative final · quoted from the NTSB record
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 complete narrative as the NTSB published it. The NTSB's docket holds the report as a PDF and any photographs, statements and other documents from the investigation.
The factual record from the NTSB's investigation tables, in plain English
What happened, in order
- Part(s) separation from AC during maneuvering (low-alt flying) defining event
- Loss of control in flight during maneuvering (low-alt flying)
- Collision with terrain or object (not controlled flight into terrain) during uncontrolled descent
The NTSB's findings
- cause Aircraft › Aircraft propeller/rotor › Main rotor system › Main rotor head system › Fatigue/wear/corrosion
Pilot
- Certificate: commercial pilot
- Ratings: rotorcraft: helicopter
- Flight time: 3,965.8 hours in all; 3,007.1 in this make and model
- Last flight review: December 9, 2014
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: ctr
- Injury: no injuries
The aircraft
- Airframe total time: 14,494 hours
- Last inspection: annual inspection, August 5, 2016
- Landing gear: fixed
- Operator: Wyoming Helicopters INC DBA
The flight
- Departed from: Cokeville WY
- Destination: Cokeville WY
- Flight plan: none
Weather at the time
- Light: daylight
- Visibility: 10 statute miles
- Sky: clear
- Temperature: 32°F (0°C), dew point 27°F (-3°C)
- Altimeter: 30.19 inHg
- Observation at 1:35 pm from KAFO
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
6 documents, released by the NTSB on September 20, 2017. View them here, or download them; the NTSB redacts some personal details before release, and this site shows the NTSB's own titles rather than its file names.
| # | Document | What it is | |
|---|---|---|---|
| 1 | Pilot/operator Aircraft Accident Report, NTSB Form 6120.1 | PDF, 12 pages | View Download |
| 2 | Maintenance Records (Excerpts) | PDF, 4 pages | View Download |
| 3 | Materials Laboratory Factual Report | PDF, 13 pages | View Download |
| 4 | Release of Aircraft Wreckage, NTSB Form 6120.15 | PDF, 1 page | View Download |
| 5 | Hiller Ad 97-10-16 | PDF, 6 pages | View Download |
| 6 | Service Bulletin | PDF, 4 pages | View Download |
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
Everything on this page comes from the NTSB's public records. The narrative, probable cause and findings are the NTSB's own words; the coded tables behind the report are written out in plain English, with pilots' ages, home towns and medical details left out. The documents and photographs are the NTSB's docket, shown as the NTSB released them. This site's own text never names anyone involved. A preliminary report can change; the final report usually follows one to two years later, and the page is refreshed when it does.