COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Douglas DC4 Series Aeroplanes AD/DC4/38 Lower Main Plane Stringers Inspection NK Applicability: All models. Requirement: Inspect for cracking of stringers 18, 19, 20 and 21 between the front and centre lower mainplane spars from wing station 167 to wing station 281 using a radiographic technique acceptable to the Secretary.