Pilot Debrief

HILLER UH 12D near Cokeville, WY — 2016-08-23

Final reportCEN16LA330
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Date
2016-08-23
Location
Cokeville, WY, USA
Aircraft
HILLER UH 12D
Registration
N97TH
Category
Helicopter
Highest injury
Minor
Fatalities
0
Phase of flight
Maneuvering

Probable cause

Failure of the control rotor cuff due to fatigue.

NTSB narrative

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.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Inspection of rotor cuff for fatigue cracks could have prevented failure.

NTSB coding

Evidence available

  • 6 docket documents
View NTSB final reportView NTSB docket

Docket documents6

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