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 28/7 Amdt 1 Applicability: All models. Main Rotor Transmission Attachment Bolts - Inspection and Replacement 4/77 Requirement: 1. Inspect the 4 bolts attaching the main rotor transmission assembly to the fuselage frame for a torque preloading of 100-110 inch pounds. 2. Modify the attachment of the main rotor transmission assembly to fuselage attachment as per Enstrom Service Information Letter No. 0057. Compliance: For Para. 1: At intervals not exceeding 50 hours time in service until modified and then once only within 100 hours time in service after modification as per SIL No. 0057. For Para. 2: Modify within 300 hours time in service after 1 June 1977.