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-6/67 Lower Skin and L/E Rivets at Front Spar 1/94 Applicability: Model DHC-6 aircraft that do not have Modifications 6/1645 and 6/1893 incorporated. Requirement: 1. Carry out a detailed visual inspection in accordance with Part A of de Havilland Service Bulletin 6/517. If any unserviceable rivets are found, proceed in accordance with Part C of Service Bulletin 6/517. In addition, if four or more rivets in the joint are missing or defective, proceed in accordance with Part B of Service Bulletin 6/517. 2. Carry out the following modifications in accordance with Part C of Service Bulletin 6/517: 1. On aircraft up to and including serial number 580, incorporate Modification 6/1645. b. On all aircraft, incorporate Modification 6/1893. Note: Transport Canada AD CF-93-25 refers. Compliance: 1. Within 500 hours time in service after 6 January 1994, or prior to 6 April 1994, whichever occurs first. 2. Prior to 6 July 1994. Background: Corrosion has been found on the heads of the rivets attaching the wing lower and leading edge skins to the front spar flanges in the area between the landing light and the engine nacelle. Loss of these rivets will destroy the structural integrity of the joint.