NTSB Factual Report

On July 24, 2005, at 1125 eastern daylight time, a Brantly Helicopter B-2B, N9018Y, was substantially damaged when it collided with terrain and rolled over while hovering at the Plymouth Municipal Airport (PYM), Plymouth, Massachusetts. The certificated commercial pilot, and the passenger were not injured. Visual meteorological conditions prevailed for the local personal flight that began about 1100. No flight plan was filed for the flight conducted under 14 CFR Part 91.

In a telephone interview, the pilot stated that he hovered the helicopter about 2 feet above the ground over the infield sod as he faced the helicopter into the wind. The helicopter pivoted sharply to the right, and the pilot applied left pedal to arrest the turn. The helicopter did not respond to the pedal input, and continued to rotate to the right.

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.

A flight instructor witnessed the accident while seated in his parked helicopter. In a telephone interview, the instructor said his attention was drawn to the accident helicopter when he noticed "a movement of the helicopter that was unusual."

The nose of the helicopter moved quickly to the right, the engine revved to "max rpm," and the helicopter "corkscrewed straight up," reaching an altitude of about 50 feet. The helicopter then descended, struck the ground, and rolled over onto its side. According to the instructor, "The engine was maxed out. The engine stayed full throttle all the way to the ground."

The helicopter was examined at the site by inspectors of the Federal Aviation Administration (FAA) and the Massachusetts Aeronautics Commission. All major components of the helicopter were accounted for at the scene.

The helicopter came to rest on its left side. The cockpit, fuselage, and empennage were intact. The main rotor blades were all fractured about mid-span. The tailboom was intact back to a fracture point about 12 inches prior to the intermediate tailrotor gearbox.

The vertical stabilizer structure was separated from the tailboom structure, but still connected by wires, cables, and the driveshaft housing. The intermediate tailrotor gearbox, vertical shaft, 90-degree gearbox, and tailrotor were all intact and still attached to the vertical stabilizer.

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 pilot held a commercial pilot certificate with ratings for airplane single engine land, rotorcraft-helicopter, and instrument airplane.

The pilot was issued an FAA third class medical certificate in May 2005. He reported approximately 4,000 hours of total flight experience, 180 hours of which were in helicopters. He reported 91 total hours of experience in make and model.

The helicopter was manufactured in 1997, and had accrued 191 total flight hours. It's most recent annual inspection was completed May 14, 2005, at 159 aircraft hours.

At 1052, the weather reported at Plymouth Municipal Airport included winds from 020 degrees at 12 knots, gusting to 17 knots. The sky was clear with 10 miles of visibility.

The intermediate tailrotor gearbox and the fractured driveshaft were examined at the Safety Board's Materials Laboratory in Washington, D.C. 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.

In a telephone interview, an FAA aviation safety engineer stated that compliance with the airworthiness directive would ensure proper alignment of the intermediate shafts, and thereby reduce the likelihood of fatigue.

NTSB Probable Cause

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