Incident Overview
A pilot was conducting an agricultural spraying operation. After completing a pass, the helicopter was heading westbound at an altitude of about 100 feet and a speed of 60 mph, paralleling power lines and flying 15 to 20 feet above trees. Suddenly, the pilot felt something "break," and the helicopter began to yaw. Recognizing a tail rotor failure, the pilot avoided autorotation into the trees. He reduced the throttle slightly, maintained altitude, and allowed the helicopter to spin two to three times until it cleared the trees. He then headed toward a gravel area and rolled off the throttle, but could not stop the spin. Attempting to cushion the landing, the helicopter contacted the ground hard.
Examination Findings
On-scene examination revealed that the Thomas coupling disc assembly at the aft end of the aft short shaft was twisted and bent. The tail rotor drive shaft just aft of the coupling was bent and separated about mid-length. A retaining bolt was missing from the Thomas coupling. The missing bolt and washers were found undamaged in the helicopter’s tail rotor drive shaft tunnel area. The self-locking retaining nut for the bolt was not located.
Metallurgical Review
Metallurgical review determined that the Thomas coupling at the aft end of the aft short shaft was missing one bolt and nut set that attached the coupling to the splined adapter at the forward end of the first segment of the tail rotor drive shaft. The forward face of the flange adapter exhibited rotational scoring and impact marks consistent with contact from bolt heads and discs. The damage pattern indicated that the short shaft rotated around the bolt opposite the missing one. Further examination showed that portions of the normal seating positions for the missing bolt head and washer were relatively free of dirt and grease. The bore surface of the hole for the missing bolt in the flanged adapter at the forward end of the first segment contained thread imprint marks.