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 analysis
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.
Text from the NTSB. Open the NTSB docket for WPR14LA062 for the full record.
Event
- Date
- Dec 9, 2013
- Type
- Accident
- Place
- Chowchilla, California
- Highest injury
- None
- Weather
- Visual (VMC)
- Light
- Day
Hiller UH 12E
- N-number
- N36MV
- Category
- Helicopter
- Year built
- 1978
- Damage
- Substantial
- Operating rule
- Part 91 (general aviation)
- Purpose of flight
- Other work use
Other records under this N-number
From the NTSB aviation accident database: N-registered aircraft, events since 2008, data through Oct 28, 2026. The file is published monthly, so the latest weeks are incomplete and recent records can change. The official source is the NTSB aviation investigation search. CheckAero is independent and not affiliated with the NTSB.