COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 106 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Rolls Royce Turbine Engines - Dart Series AD/DART/25 High Pressure Impeller Replacement 13/94 Applicability: All Rolls Royce Dart Series engines that incorporate a pre-Mod.1475 standard High Pressure Impeller. Requirement: Replace the high pressure impeller in accordance with Rolls Royce Service Bulletin Da 72-307, Revision 5 dated Sept.94 (RR Mod.1475). Note: CAA (UK) AD 004-09-94 also refers. Compliance: Effective 8 December 1994, at the first engine shop visit at which the HP compressor area is exposed but no later than 30 June 1995. Background: Pre-Mod 1475 H.P. Impellers have experienced vane tip failures caused by a particular mode of vibration being excited within the engine operating R.P.M. range.