NTSB Identification: IAD05LA107.
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Accident occurred Sunday, July 24, 2005 in Plymouth, MA
Probable Cause Approval Date: 11/29/2006
Aircraft: Brantly Helicopter B-2B, registration: N9018Y
Injuries: 2 Uninjured.

NTSB investigators may not have traveled in support of this investigation and used data provided by various sources to prepare this aircraft accident report.

While at a hover, the helicopter made an uncommanded sharp turn to the right, which could not be arrested with a left pedal application. When the helicopter completed approximately 3/4 of a rotation, the pilot "chopped the power" to stop the spin. The helicopter descended, struck the left skid on the ground, and then rolled over onto its left side. Examination of the intermediate tailrotor gearbox revealed that the tailrotor driveshaft was fractured at the input side of the gearbox. A portion of the fractured driveshaft remained in the flanged bushing of the gearbox. The fractured pieces of the driveshaft displayed significant mechanical damage from the driven end of the driveshaft after the fracture occurred. The driveshaft section recovered from the intermediate gearbox input bevel displayed multiple fatigue fractures with multiple origins around the attach-bolt holes. The fractures propagated from the attach-bolt holes in the direction of rotation, and opposite the direction of rotation. Fractured tail rotor vertical driveshafts from two previous events in the same make and model displayed almost identical modes of failure. As a result of those examinations, the FAA issued Airworthiness Directive 2006-08-07 on May 2, 2006, to help prevent fatigue cracking and subsequent failures of the vertical shaft.

The National Transportation Safety Board determines the probable cause(s) of this accident to be:

Fatigue failure of the intermediate tailrotor driveshaft, which resulted in a loss of directional control at a hover.

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