Airworthiness Directives; Boeing Model 737 Series Airplanes

This amendment supersedes seven existing airworthiness directives (ADs), applicable to certain Boeing Model 737 series airplanes that, among other things, currently require replacing the main rudder power control unit (PCU) and PCU vernier control rod bolts; testing the main rudder PCU to detect certain discrepancies and to verify proper operation of the PCU; and revising the FAA-approved Airplane Flight Manual procedures to correct a jammed or restricted flight control condition. Instead, this amendment requires installation of a new rudder control system and changes to the adjacent systems to accommodate that new rudder control system. This amendment is prompted by FAA determinations that the existing system design architecture is unsafe due to inherent failure modes, including single-jam modes and certain latent failures or jams, which, when combined with a second failure or jam, could cause an uncommanded rudder hardover event and consequent loss of control of the airplane. Additionally, the current rudder operational procedure is not effective throughout the entire flight envelope. The actions specified by the proposed AD are intended to prevent the identified unsafe condition.