Narrative:

During a run up of the aircraft; the pilot felt a buzz in the tail rotor pedals. Upon inspection; another mechanic noticed that the taper of the tail rotor output shaft was showing. Upon disassembly; the mechanic noticed that the anti-rotation key was not engaged properly into the tail rotor yoke keyway. Weeks before this discovery; I was working on the T inspection and rii'd (required inspection item) the installation of the tail rotor yoke onto the aircraft. I did not witness the yoke being installed; but verified torque wrench settings and nut torqueing. [Name] mentioned part of the taper showing but it never entered my mind that it wouldn't be in the keyway. [Name] continued the installation of the tail rotor after we discussed it- with the amount of torque applied to a properly installed yoke; there is no way it wouldn't seat properly. I performed all the ground runs with the native-1 pilot and the tail rotor was within tolerance after 3-4 runs. During maintenance flights; there was no indication given by the pilot that tail rotor authority was diminished and all flights were completed with no issues. Note- I was not able to submit with [name] added to the additional employees section. It asked for duty selected even when the block was filled out. [Name]; employee # xxxxx; duty was mechanic. During reinstallation of the yoke the old retaining washer is used; the torque is broken; the bolt and old washer removed; and new washer is installed for final torque. I witnessed the final torque but should have been there for both; along with removal of the old retaining washer and installation of the new one.

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Original NASA ASRS Text

Title: Mechanic working on AS-350 noted the anti-rotation key did not engaged properly into tail rotor yoke keyway.

Narrative: During a run up of the aircraft; the pilot felt a buzz in the tail rotor pedals. Upon inspection; another mechanic noticed that the taper of the tail rotor output shaft was showing. Upon disassembly; the mechanic noticed that the anti-rotation key was not engaged properly into the tail rotor yoke keyway. Weeks before this discovery; I was working on the T inspection and RII'd (Required Inspection Item) the installation of the tail rotor yoke onto the aircraft. I did not witness the yoke being installed; but verified torque wrench settings and nut torqueing. [Name] mentioned part of the taper showing but it never entered my mind that it wouldn't be in the keyway. [Name] continued the installation of the tail rotor after we discussed it- with the amount of torque applied to a properly installed yoke; there is no way it wouldn't seat properly. I performed all the ground runs with the Native-1 pilot and the tail rotor was within tolerance after 3-4 runs. During Maintenance flights; there was no indication given by the pilot that tail rotor authority was diminished and all flights were completed with no issues. NOTE- I was not able to submit with [name] added to the additional employees section. It asked for duty selected even when the block was filled out. [Name]; employee # XXXXX; duty was mechanic. During reinstallation of the yoke the old retaining washer is used; the torque is broken; the bolt and old washer removed; and new washer is installed for final torque. I witnessed the final torque but should have been there for both; along with removal of the old retaining washer and installation of the new one.

Data retrieved from NASA's ASRS site and automatically converted to unabbreviated mixed upper/lowercase text. This report is for informational purposes with no guarantee of accuracy. See NASA's ASRS site for official report.