ROBINSON HELICOPTER COMPANY R44 II near Destin, FL — 2015-11-26

Probable cause

The overstress fracture between the tail rotor driveshaft and intermediate flex plate for reasons that could not be determined based on the available evidence.

NTSB narrative

The commercial pilot was performing a power check before an air tour flight with three passengers onboard. While raising the collective, he heard a loud "pop." He then immediately lowered the collective and secured the helicopter. Postaccident examination of the helicopter revealed that an overstress fracture existed between the tail rotor driveshaft and intermediate flex plate. Examination revealed that all the hardware was in place except for a washer, which is required at each connection between the clutch shaft yoke and flex plate per a caution in the helicopter's maintenance manual. The maintenance manual also contained measurements to determine proper shimming required at the intermediate flex plate. A review of maintenance records revealed that, about 11 flight hours before the accident, the sprag clutch had been removed and replaced. The mechanic who performed the clutch removal and installation stated that the measurements he took at the intermediate flex plate during and after his repairs were the same. He also indicated that, when they installed new hardware, he obtained the same dimensions. Although a misalignment of the intermediate flex coupling would place significant tension along the entire length of the driveshaft between the main rotor gearbox and tail rotor gearbox and a missing washer both could have caused a fatigue-type failure of the tail rotor drive line, the parts fractured due to overstress, not fatigue; therefore, it is unlikely that the missing hardware contributed to the failure. The reason for the overstress fracture between the tail rotor driveshaft and intermediate flex plate could not be determined based on the available evidence.

Analysis

Primary failure mode

Mechanical failure

First missed decision gate

Omission of washer during maintenance could have prevented the drive shaft failure.

NTSB coding

• Sys/Comp malf/fail (non-power) · Other

Evidence available

• 9 docket documents

Docket documents9

• Pilot/Operator Aircraft Accident Report, NTSB Form 6120.1form
• Release of Aircraft Wreckage, NTSB Form 6120.15, and NTSB Evidence Control Formform
• FAA Inspector Statementinterview
• Excerpts from Maintenance Manualother
• Excerpts from Maintenance Recordsother
• Maintenance Record Entry Returning Helicopter to Service After Accidentother
• Record of NTSB Conversationsother
• NTSB Materials Laboratory Factual Reportreport
• Weather Reports and Recordsreport

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