AIRWORTHINESS DIRECTIVE For the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.001(1) of CASR 1998. The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the 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 Turbine Engines - PW4000 Series AD/PW4000/21 2nd Stage High Pressure Turbine 20/2010 Air Seal Assembly Applicability: Pratt and Whitney PW4000 engines as specified by models in FAA AD 2001-20-02. Requirement: Unless previously accomplished, perform the actions in accordance with the technical requirements of FAA AD 2001-20-02. Repeat the actions in accordance with the technical requirements of FAA AD 2001-20-02. Later revisions to PW ASB PW4G-112-A72-233, Revision 3, dated 3 August 2001, approved by the United States Federal Aviation Administration (FAA) as an Alternate Method of Compliance (AMOC) to FAA AD 2001-20-02, are considered acceptable for compliance with the equivalent Requirements of this AD. Compliance: As specified in the Requirement Document with a revised effective date of 21 October 2010. This Airworthiness Directive becomes effective on 21 October 2010. Background: Reports have been received that thirteen 2nd stage High Pressure Turbine (HPT) air seal assemblies have been found cracked in the rim area. The actions specified in this AD are intended to prevent fracture of the 2nd stage HPT air seal assembly that could result in an uncontained engine failure. Charles Lenarcic Delegate of the Civil Aviation Safety Authority 22 September 2010