Accident Narrative
The helicopter pilot reported that he was conducting a Part 135 air tour flight with 4 passengers on board. The helicopter was at about 1,500 feet above ground level with an airspeed of 80 to 85 knots when the pilot heard two loud "bangs" in quick succession. The nose of the helicopter pitched down and the helicopter yawed to the right. The pilot applied left pedal but observed no effect. Attempts to increase forward airspeed resulted in the helicopter nosing over and yawing further to the right. The right yaw developed into a tight spin, and the pilot realized that the tail rotor had been lost. The pilot executed an autorotation to a clearing located at a campground. During the autorotation, the helicopter impacted trees at the western edge of the clearing and then struck the terrain on its right, front side.
Inspection Findings
Post-accident inspection of the helicopter revealed that the tail rotor output shaft and the tail rotor blades had separated from the tail rotor gearbox. The tail rotor assembly was examined at the National Transportation Safety Board's Materials Laboratory. The inspection determined that the tail rotor blade assembly had fractured through the root fitting in the area coinciding with a hole for the blade retention bolt. The root fitting contained a 1-inch nominal diameter longitudinal hub bore with a tapered (conical) hole bottom. A relief radius was located between the hub bore and the conical hole bottom.
Metallurgical examination revealed crack arrest marks characteristic of fatigue cracking that originated from multiple points at the relief radius. The relief radius measured approximately 0.001 inch, which was significantly smaller than the engineering drawing specification of 0.12 to 0.15 inch.
Subsequent Actions
On April 26, 2007, McDonnell Helicopters, Inc. (MDHI) issued mandatory Service Bulletin (SB) 369D-204, 369E-099, 369F-084, 369H-247, titled "Tail Rotor Blade Assembly, One Time Inspection." The SB required that before further flight, the tail rotor be removed on certain helicopter models (369, 369H, 369HM, 369HS, 369HE, 369D, 369E, 369F, and 369FF) and the hub bore visually examined with a flashlight. If the relief radius did not exhibit a smooth transition between the bore and the conical bottom hole, the root fitting was to be removed from service. The following day, on April 27, 2007, the Federal Aviation Administration issued emergency Airworthiness Directive (AD) 2007-09-51, mandating inspection of the root fittings per the MDHI SB.