AIRWORTHINESS DIRECTIVE On the commencement date specified below, and for the reasons set out in the background section, the CASA delegate whose signature appears below repeals Airworthiness Directive (AD) AD/PW100/4 and issues the following AD under subregulation 39.001 (1) of CASR 1998 and subsection 33 (3) of the Acts Interpretation Act 1901. The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct an unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section. Pratt and Whitney Canada Turbine Engines - PW100 Series AD/PW100/4 Amdt 1 Fuel Pump Input Drive Shaft 10/2017 Applicability: Pratt and Whitney Canada PW123, PW123AF, PW124B, PW125B, PW126 and PW126A engines as detailed in Pratt and Whitney Canada Service Bulletin No. 20946 as in force from time to time. Requirement: Action in accordance with the Technical Requirements of Transport Canada Airworthiness Directive CF-91-02 as in force at the commencement of this AD. Compliance: Prior to 31 October 1991. This AD commences on 26 May 2017. Note 1: Compliance with the original AD remains unchanged. Note 2: The words "as in force from time to time" require compliance with the most recent version of the referenced document. Background: The engine manufacturer has determined that fuel pump input drive shaft torsional vibration can induce oscillations of the main fuel control governor, which may result in large torque fluctuations or torque drop during flight. This Directive requires the modification of the fuel pump to incorporate an input drive shaft with increased torsional stiffness. Amendment 1 to this directive is issued to add PW126A engines to the applicability. This engine model was inadvertently omitted from the original issue of the AD. Pieter van Dijk Delegate of the Civil Aviation Safety Authority 12 May 2017