Summary
We are revising an earlier proposed airworthiness directive (AD) for certain Model 737 airplanes. The original NPRM would have required repetitive inspections, lubrications, and repetitive repairs/ overhauls of the ball nut and ballscrew and attachment (Gimbal) fittings for the trim actuator of the horizontal stabilizer; various installation(s); and corrective actions if necessary; as applicable. The original NPRM resulted from a report of extensive corrosion of a ballscrew used in the drive mechanism of the horizontal stabilizer trim actuator (HSTA). This action revises the original NPRM by adding airplanes to the applicability. We are proposing this supplemental NPRM to prevent an undetected failure of the primary load path for the ballscrew in the drive mechanism of the HSTA and subsequent wear and failure of the secondary load path, which could lead to loss of control of the horizontal stabilizer and consequent loss of control of the airplane.