Dynamic Aviation Helicopter Service Hiller UH 12E Undesignat accident
Hiller UH 12E Undesignat · Accident: loss of engine power (total) while maneuvering
From Priv · Naples, UT to Priv · Yampa River, CO
Event
- NTSB case
- CEN20LA073
- Event typeAccident: someone killed or seriously injured, or the aircraft substantially damaged (NTSB definition). Incident: an occurrence that affected or could have affected safety, short of that. Glossary
- Accident
- InvestigationHow far the investigation had got when the record was published: preliminary, ongoing, final or completed. Unknown where the source does not say. Glossary
- Completed
- API-reported fatalities
- 0 · all aircraft and ground
- Ground fatalities
- Unknown
- OperationWhat kind of flying it was, grouped by the rules it flew under: airline (US Part 121), air taxi and commuter (Part 135), general aviation (Part 91 and similar), military or government. Glossary
- Charter & commuter
- WeatherVisual meteorological conditions (VMC): good enough to fly by looking outside. Instrument conditions (IMC): cloud or low visibility, flying by instruments. Glossary
- Visual Meteorological Cond
Hiller UH 12E Undesignat
- Aircraft type
- Hiller UH-12
- Category
- Helicopter
- RegistrationThe aircraft's tail number, such as N12345 or G-ABCD. Registrations are reissued, so the same one years apart can be a different aircraft. Glossary
- N7173S
- Onboard fatalities
- Unknown
- Route
- From Priv · Naples, UTTo Priv · Yampa River, CO
- Aircraft age
- About 55 years (built 1965)
- Flight rulesThe regulations the flight operated under: in the US, Part 91 (general aviation), Part 121 (airlines), Part 135 (air taxi and commuter) and others; for flights abroad, the NTSB's coarser commercial or non-commercial code. Glossary
- Part 135: Air Taxi & Commuter
- Phase of flightThe stage of the flight when things started to go wrong: standing, taxi, takeoff, initial climb, en route, maneuvering, approach or landing. Glossary
- Maneuvering · low-altitude flying
- Defining eventThe single coded event the NTSB judges best describes what happened (records from 2008 on). Older records name the first occurrence in the sequence instead. Glossary
- Loss of engine power (total)
- DamageDestroyed: beyond practical repair. Substantial: damage that affects the structure, performance or handling and normally needs major repair. Minor: less than that. Glossary
- Substantial
Cause areas
- Aircraft › Aircraft power plant
Approximate · Coordinates as recorded by the NTSB; no uncertainty radius is established.
NTSB narrative
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.
Probable cause
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.
Verbatim NTSB analysis and probable cause from the NTSB dataset
Sources
NTSB record CEN20LA073
- Event ID
- 20200202X81855
- Case number
- CEN20LA073
- Dataset
- full-current
- Source SHA-256
- 5cf380f0061817c0331a6b2d8cc7e0ee3a79bea469a1001dc5c10e56f35f5ab3
Source notes (4)
- Unreviewed is an editorial label, not an investigation status. API-sourced is not report-checked or human-reviewed. Explicit event totals are used without summing aircraft injury tables; onboard allocation is withheld. Unknown values remain unknown. The operation category is mapped from the NTSB-reported FAR part and has not been reviewed.
- Filled from the NTSB case API where the bulk record had no value: investigation status, cause areas. Values present in the bulk record are kept.
- The date is the local date, which is the same as the UTC date the NTSB stores.
- API snapshot SHA-256: 44cd133cdaff2f9b08553ee51fca078738f24701a8b9a7a9994115ca943ce26d; retrieved 2026-09-16T11:01:59.486Z.