Airworthiness Directives; Boeing Model 737 Series Airplanes

This document corrects and clarifies information in an existing airworthiness directive (AD) that applies to all Boeing Model 737 series airplanes. That AD currently requires 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 document corrects certain PCU part and serial numbers and clarifies that PCU's with certain other serial numbers are not required to comply with the requirements of this AD. This correction is necessary to prevent failure of the secondary slide and consequent rudder hardover and reduced controllability of the airplane.