Airworthiness Directives; Boeing Model 737 Airplanes

The FAA is superseding an existing airworthiness directive (AD) that applies to all Boeing Model 737 airplanes. The existing AD currently requires installation of a new rudder control system and changes to the adjacent systems to accommodate that new rudder control system. For certain airplanes, this new AD adds, among other actions, repetitive tests of the force fight monitor of the main rudder power control unit (PCU), repetitive tests of the standby hydraulic actuation system, and corrective action; as applicable. For those airplanes, this new AD also adds, among other actions, replacement of both input control rods of the main rudder PCU and the input control rod of the standby rudder PCU with new input control rods, as applicable, which ends the repetitive tests. For certain other airplanes, this new AD adds installation of an enhanced rudder control system in accordance with new service information. This AD results from a report of a fractured rod end of an input control rod of the main rudder PCU and a subsequent report of a fractured rod end of the input control rod of the standby rudder PCU. We are issuing this AD to prevent failure of one of the two input control rods of the main rudder PCU, which, under certain conditions, could result in reduced controllability of the airplane; and to prevent failure of any combination of two input control rods of the main rudder PCU and/or standby rudder PCU, which could cause an uncommanded rudder hardover event and result in loss of control of the airplane.