Hiller UH 12E accident near Dinosaur, Colorado, February 1, 2020
On February 1, 2020 at about 9:00 pm local time, a 1965 Hiller UH 12E (helicopter), registered N7173S, was substantially damaged in an accident during maneuvering (low-alt flying) near Dinosaur, Colorado. It was flown under charter and air-taxi rules (Part 135). No one was hurt; 3 people were on board or involved. The weather was visual conditions (good weather).
The NTSB's probable cause their words, unchanged
Failure of a S4 compressor blade due to a fatigue crack that likely initiated at a corrosion pit. Contributing to the accident was maintenance personnel’s failure to detect the corrosion pits during maintenance.
Source: NTSB aviation accident database, copy made October 5, 2026. Docket and reports at the NTSB.
What the record shows
- Date
- February 1, 2020 · about 9:00 pm local time
- Place
- Dinosaur, Colorado · map
- Type
- Accident
- Injuries
- No one was hurt; 3 people were on board or involved.
- Weather
- visual conditions (good weather)
- Aircraft
- Hiller UH 12E Undesignat, built 1965 · all UH 12Es on the register
- Registration
- N7173S · registry record · serial 2351
- Damage
- Substantial damage
- Flight
- Flight · charter and air-taxi rules (Part 135)
The NTSB's narrative final · quoted from the NTSB record
The commercial pilot and two passengers were searching for lost cattle during an air taxi flight. While at low altitude, the pilot heard a grinding noise and noticed a loss of engine power. The pilot made a forced landing, which resulted in substantial damage to the tail rotor. The engine examination revealed compressor damage consistent with a S4 blade separation during operation. The debris released from the S4 failure caused severe downstream damage, resulting in a loss of compressor function and a total engine power loss. Metallurgical examination of the S4 wheel found corrosion on the forward rim face and at both the suction and pressure sides of the blade stubs. Scanning electron microscopy examination of areas of S4 blade stub fracture surfaces not obliterated by secondary damage found cracks initiating in corrosion-damaged areas on the suction side of the blade crown and evidence that the cracks progressed in fatigue across the blade thickness until failure in overload. Review of the operator’s aircraft service records found that the compressor was installed in 2012 with unknown time since new and zero time since overhaul. The records indicated that the compressor module was overhauled in 2006. Exposure to a corrosive environment is required to initiate corrosion. The operator reported that the compressor was not operated in a corrosive atmosphere since the 2006 overhaul. No compressor module service records before 2006 were available. The Rolls-Royce 250-C20B Overhaul Manual requires that the S4 compressor wheel satisfy overhaul inspection criteria during overhaul. The Rolls-Royce 250-C20B Operation and Maintenance Manual requires a periodic detailed compressor inspection whenever engines are operated in corrosive environments. The manual includes a CAUTION regarding this requirement.
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
- Loss of engine power (total) during maneuvering (low-alt flying) defining event
- Autorotation Loss of engine power (total)
The NTSB's findings
- Aircraft › Aircraft power plant › Engine (turbine/turboprop) › Compressor section › Fatigue/wear/corrosion
- Aircraft › Aircraft power plant › Engine (turbine/turboprop) › Compressor section › Inadequate inspection
Pilot
- Certificate: commercial pilot
- Ratings: single-engine land; rotorcraft: helicopter
- Flight time: 15,600 hours in all; 15,300 in this make and model; 14 in the last 90 days; 1 in the last 30 days; 15,600 as pilot in command; 0 on instruments
- Last flight review: October 10, 2019
- Medical certificate: Class 2 (with waivers/limitations)
- Seat: left
- Injury: no injuries
The aircraft
- Airframe total time: 13,795 hours
- Last inspection: 100-hour inspection, March 12, 2019; 59 hours since
- Maximum gross weight: 3,100 lb
- Seats: 3
- Landing gear: fixed
- Engine: Allison Rolls-Royce 250 (turboshaft); 0 hours total
- Operator: Dynamic Aviation Helicopter Service
The flight
- Departed from: Priv Naples UT at 8:15 pm
- Destination: Priv Yampa River CO
Weather at the time
- Light: daylight
- Visibility: 7 statute miles
- Sky: clear
- Temperature: 28°F (-2°C), dew point 21°F (-6°C)
- Altimeter: 30.44 inHg
- Observation at 8:53 pm from KVEL, 26 miles away
Injuries
| Fatal | Serious | Minor | None | |
|---|---|---|---|---|
| Flight crew | 1 | |||
| Passengers | 2 |
Documents from the investigation the NTSB's docket: the evidence folder behind the report
5 documents, released by the NTSB on February 9, 2022. 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, 9 pages | View Download |
| 2 | Maintenance Records | PDF, 12 pages | View Download |
| 3 | Compressor Airworthiness Approval Tag | PDF, 1 page | View Download |
| 4 | Engine Examination Report | PDF, 8 pages | View Download |
| 5 | Compressor Examination Report | PDF, 13 pages | View Download |
The same docket at the NTSB · documents without a copy here are fetched from the NTSB when you open them.
Other NTSB records under N7173S the same tail number, which may have belonged to a different aircraft at the time
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.