COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Boeing 737 Series Aeroplanes AD/B737/123 Rudder Control System Component Changes 2/99 Applicability: All Boeing model 737-100, -200, -300, -400, -500 series aeroplanes. Requirements: Install new rudder pressure reducer (RPR) and yaw damper coupler (YDC) in accordance with the instructions in Boeing Alert Service Bulletin 737-27A1206 dated 19 November 1998. Note: FAA AD 97-14-03 Amdt 39-10060 refers. Compliance: On completion of AD/B737/116 but no later than 26 March 2002. This Airworthiness Directive becomes effective on 25 February 1999. Background: To lessen the effects of large rudder deflections, Boeing has provided instructions to incorporate a hydraulic rudder pressure reducer in the "A" hydraulic system supply line in the main rudder power control unit. Also, to improve significantly the reliability of the YDC, a new unit has been designed using technology that was not available at the time of the original design. These changes have been brought about following an investigation into the reliability and effectiveness of the B737 yaw control system by the FAA and NTSB. AD/B737/116 was issued in March 1998 to install the wiring changes necessary for the introduction of the hardware associated with this Directive.