Several occurrences have been reported of loss of tightening torque of the Shur-Lok nut, which serves as a retainer of the tail rotor (TR) drive flange of the main gearbox (MGB). Subsequent investigation determined that these events were the result of failure of the Shur-Lok nut locking function, which is normally ensured by two anti-rotation tabs engaged into two slots at the end of the MGB output shaft pinion.