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 28/13 Main Rotor Hub Retention Lock Washer - Replacement 4/76 Applicability: All models F-28A with S/Nos. 4 to 275, 277, 278, 280 to 283, and models 280 with S/Nos. 1001 to 1006. Document: 1. Enstrom Service Note No. 0025A; or 2. FAA AD 75-22-10. Compliance: Within 25 hours time in service after 30 April 1976.