Pilot Debrief

HILLER UH 12E near Chowchilla, CA — 2013-12-09

Final reportWPR14LA062
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Date
2013-12-09
Location
Chowchilla, CA, USA
Aircraft
HILLER UH 12E
Registration
N36MV
Category
Helicopter
Highest injury
None
Fatalities
0
Phase of flight
Climb

Probable cause

The failure of the torsional drive coupling, which resulted in a loss of drive to the main rotor system during low-altitude flight.

NTSB narrative

The pilot, who also owned the helicopter, reported that he was performing frost control operations over a citrus grove at low altitude when the helicopter's engine began to overspeed with an accompanying decrease in rotor speed. He initiated an autorotation, and the helicopter subsequently landed hard in an adjoining orchard. The pilot removed the helicopter from the accident site immediately following the accident and began disassembling it and sending components out for repair before it was examined. During the subsequent examination of the helicopter, both the elastomeric ring and lower drive lugs (engine side) of the main transmission torsional drive coupling were found sheared off, which would have resulted in the loss of drive to the main rotor system described by the pilot. The pilot did not make the torsional drive coupling available for followup examination and reported that he had misplaced the airplane's maintenance logbooks. The torsional drive coupling incorporated an internal set of lugs designed to provide positive drive to the transmission if the elastomeric portion of the coupling failed. The lugs had sheared off, which indicated that the elastomeric coupling had failed before the accident flight. The coupling had a finite service life but, because no maintenance records were provided for examination, it could not be determined if it was within its life limit at the time of the accident. Almost 4 years before the accident flight, the helicopter had been involved in another accident, which destroyed the main rotor blades. The pilot could not provide evidence that the coupling was replaced after that accident, and, without access to the airplane's maintenance logbooks, it could not be determined if a routine inspection of the coupling had been performed. It is possible that the coupling sustained damage during the original accident, which resulted in its ultimate failure.

Analysis

Primary failure mode
Mechanical failure
First missed decision gate
Failure to inspect or replace the torsional coupling as per service bulletin guidelines.

NTSB coding

Evidence available

  • Photos
  • 7 docket documents
View NTSB final reportView NTSB docket

Docket documents7

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