CHI03IA033 · Hughes OH-6

The helicopter lost tail rotor effectiveness during cruise flight, and the pilot executed a run on landing to a paved runway. The pilot reported that he was in cruise flight for approximately one hour when the helicopter "entered an estimated 20 degree uncommanded yaw to the right." Applying pressure to the anti-torque pedals had no effect. The pilot reported that there had been no advance auditory indications or unusual vibrations prior to the yaw occurring. The pilot flew to an airport about 12 nautical miles away, and executed a successful run on landing. The inspection of the helicopter revealed that the tail rotor bellcrank pivot pin was dislodged from the non-rotating swashplate and was rotated inboard towards the boom. The plain brass liner, which is a pressed liner internal to the non-rotating swashplate and surrounds the tail rotor gearbox output shaft, was broken into 15 identifiable pieces. No evidence of fatigue or abnormal wear was found on the pieces of the plain liner. The splined brass liner within the non-rotating swashplate was damaged. Eight of the seventeen splines of the swashplate splined liner had fractured areas and one of the splines was cracked. No evidence of fatigue was found on any of the fractures. The splined liner was still riveted to the rotating swashplate and was still capable of bearing a load. The tail rotor gearbox was replaced with an overhauled gearbox 29.4 flight hours prior to the time of the incident. There was no record of any mechanical discrepancies to the tail rotor assembly, including the non-rotating swashplate, when the overhauled tail rotor gearbox was installed.

Verbatim NTSB analysis and probable cause from the NTSB dataset