Inter-Island Helicopter McDonnell Douglas 369FF accident

The helicopter pilot reported that he was conducting a Part 135 air tour flight with 4 passengers on board the helicopter. The helicopter was about 1,500 feet above ground level with about 80 - 85 knots of airspeed when within a split-second he heard two loud "bangs." The nose of the helicopter pitched down and the helicopter yawed to the right. He applied left pedal but without effect. He attempted to "get more forward airspeed," but each time he did the helicopter nosed over and yawed to the right. The right yaw developed into a tight spin, and he realized that he "lost his tail rotor." The pilot executed an autorotation to a clearing located at a campground. The helicopter impacted the trees at the western edge of the clearing during the autorotation. The helicopter impacted the terrain on its right, front side. Inspection of the helicopter revealed that the tail rotor output shaft and the tail rotor blades separated from the tail rotor gearbox. The tail rotor assembly was inspected at the National Transportation Safety Board's Materials Laboratory. The inspection revealed that the tail rotor blade assembly fractured through the root fitting in the general area that coincided 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. The inspection revealed crack arrest marks typical of a fatigue cracking that emanated from multiple origins at the relief radius between the hub bore and conical hole bottom. The relief radius measured approximately 0.001 inch, much less than specified by the engineering drawing (0.12 inch and 0.15 inch). 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." According to MDHI SB, before further flight, the tail rotor should be removed from helicopter models 369 (Army YOH-6A), 369 (Army OH-6A), 369H, 369HM, 369HS, 369HE, 369D, 369E, 369F, and 369FF. The hub bore should be visually examined with a flashlight. If the relief radius does not contain a smooth transition between the bore and conical bottom hole, the root fitting should be removed from service. On April 27, 2007, the Federal Aviation Administration issued emergency Airworthiness Directive (AD) 2007-09-51 that mandated inspection of the root fittings per the MDHI SB.

Verbatim NTSB analysis and probable cause from the NTSB dataset