Hiller UH 12E N7173S

Summary

On 1 February 2020 at about 21:00 local time, a Hiller UH 12E registered N7173S, operated by Dynamic Aviation Helicopter Service, was involved in an accident near Dinosaur, Colorado, United States. 3 people were on board and nobody was injured. The aircraft was substantially damaged. The NTSB has published a probable cause for this accident; it is quoted in full below.

Date

1 February 2020 at 21:00

Classification

Accident

Location

Dinosaur, Colorado, United States

Coordinates

40.4622, -108.9200

Aircraft

Hiller UH 12E

Registration

N7173S

Category

Helicopter

Year built

1965

Engines

1

Operator

Dynamic Aviation Helicopter Service

Operating rule

Part 135: Air Taxi & Commuter

Phase of flight

Not recorded

Route

Naples → Yampa River

Aircraft damage

Substantial

Weather

VMC

Light

Day

NTSB number

CEN20LA073

Nobody was injured.

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.

Quoted verbatim from the NTSB record. This site does not paraphrase or interpret it.

Read the full 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.

Quoted verbatim from the NTSB record.