Accident Overview
An experimental amateur-built helicopter experienced an in-flight separation of a tail rotor blade, leading to a collision with trees and terrain. Witnesses reported observing the helicopter spinning before it impacted the ground.
Background
The helicopter had previously been involved in a hard landing in 2004. Following that incident, the main rotor blades and tail rotor assembly, including the spindle and blades, were replaced. According to the pilot's friend, the new tail rotor blades were balanced after installation but continued to run "rough." The pilot communicated these concerns to the manufacturer, who subsequently provided replacement tail rotor blades. The manufacturer and the pilot's friend reported that the helicopter operated normally after the replacement blades were installed.
Wreckage Examination
Examination of the wreckage revealed that all major components were located at the main wreckage site, except for one tail rotor blade and the outboard 2 inches of the tail rotor spindle, which were not found.
Metallurgical Analysis
The tail rotor spindle contained two integral rods extending in opposite directions, each manufactured with sections of consecutively reduced diameter. A metallurgical examination identified a fatigue failure that initiated at the fillet radius between sections where the rod stepped from 0.812-inch to 0.750-inch diameter. The relief radius in this area was rough and irregular. The manufacturer provided engineering drawings for the tail rotor spindle; Drawing #15 did not specify a relief radius for the transition area. On September 14, 2005, Canadian Home Rotors issued a revised drawing requiring a 0.0156-inch relief radius between these diameter sections. The measured relief radius at the failure site was approximately 0.0073-inch.