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-6/60 Amdt 4 Wing Front Spar Attachment Adaptors 5/92 Applicability: Model DHC-6 series fitted with aluminium wing to fuselage front adaptor fittings. Requirement: 1. Inspect fittings in accordance with de Havilland Service Bulletin No 6/476 Revision C. 2. Replace all aluminium fittings with steel fittings in accordance with de Havilland Service Bulletin No 6/500 Revision C. This ends the need for the inspections of para 1 above. Note: Transport Canada AD CF-85-12R3 refers. Compliance: 1. Inspect fittings reworked in accordance with de Havilland Service Bulletin No A6/476 at intervals not exceeding 250 hours time in service. Inspect all other aluminium fittings at intervals not exceeding 500 hours time in service. 2. Replace all aluminium fittings with steel fittings before 1 September 1995. Background: Fatigue and stress corrosion cracks have been widely reported in the aluminium fittings. Earlier issues of this directive controlled the problem by means of regular inspections, and permitted rework of fittings found to be cracked. Amendment 4 requires mandatory modification - the installation of steel fittings - in line with the recommendations of Transport Canada's Aging Aircraft program.