COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Enstrom F-28 Series Helicopters AD/ENST 28/33 Tail Rotor Drive Flexible Couplings 11/90 DM Applicability: All model F-28, F-28A, F-28C, F-28C-2, F-28F, 280, 280C, 280F and 280 FX series helicopters. Requirement: 1. Replace the 5 plate flex pack coupling P/N A005-1991 with 7 plate flex pack coupling P/N A005-1992. During pack replacement inspect flange condition in accordance with Paragraph 5.2 of Enstrom Service Directive Bulletin No 0080. 2. Inspect 7 plate flex pack coupling in accordance with Paragraph 11 and 12 of Enstrom Service Directive Bulletin No 0080. Note: FAA AD 90-17-02 Amdt 39-6688 refers. Compliance: 1. Within 25 hours time in service after 7 September 1990. 2. At intervals not exceeding 100 hours time in service. Background: Due to excessive distortion of the 5 plate coupling and the results of subsequent strain gauge testing by the manufacturer, replacement with a 7 plate coupling is mandatory.