Summary
This document proposes the adoption of a new airworthiness directive (AD) that is applicable to all Boeing Model 737 series airplanes. This proposal would require repetitive displacement tests of the secondary slide in the dual concentric servo valve of the power control unit (PCU) for the rudder, and replacement of the valve assembly with a modified valve assembly, if necessary. This proposal is prompted by reports of cracking found in PCU secondary servo valve slides. The actions specified by the proposed AD are intended to prevent failure of the secondary slide and consequent rudder hardover and reduced controllability of the airplane.