Summary
This document proposes the supersedure of two existing airworthiness directives (AD), applicable to certain Boeing Model 737 series airplanes, that currently require tests of the main rudder power control unit (PCU) to detect excessive internal leakage of hydraulic fluid, stalling, or reversal, and to verify proper operation of the PCU; and replacement of the PCU with a unit having a different part number, if necessary. This action would add requirements for replacement of the PCU and the vernier control rod bolt with newly designed units. This action also would add a requirement for leak tests of the PCU, and replacement of the PCU with a serviceable or newly designed unit, if necessary. This proposal is prompted by reports of fracturing of the vernier control rod bolts as a result of the shank of the bolt running into the threads on the nutplate during installation of the rod. The actions specified by the proposed AD are intended to prevent such fracturing, which could result in uncommanded movements of the rudder, and consequent reduced controllability of the airplane.