COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES DHC-6 (Twin Otter) Series Aeroplanes AD/DHC-6/33 Amdt 4 Elevator Root and Nose Ribs 4/92 Applicability: Model DHC-6 series. Requirement: 1. Inspect the elevator root rib for cracks in accordance with de Havilland Service Bulletin 6/399 Revision E. If cracking is present, incorporate de Havilland modification 6/1769 before further flight. 2. Incorporate de Havilland modification 6/1769. Note: Transport Canada AD CF-82-21 R1 refers. Compliance: 1. At intervals not to exceed 600 hours time in service, until modification 6/1769 is incorporated. 2. Prior to 1 January 1996 or 15 000 hours airframe time in service, whichever occurs later. The compliance of the previous issue of this Directive was: Unless already accomplished, inspect within 200 hours time in service after 31 October 1983, with repeat inspections at intervals not exceeding 600 hours time in service until modification 6/1769 is incorporated. This Amendment is effective from 2 April 1992. Background: Cracking has occurred in the elevator root rib. Amendment 4 is issued to align this Directive with a revision of the relevant Country of Origin AD which requires terminating action in lieu of ongoing repetitive inspections.