COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 106 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Allison Turbine Engines - 250 Series AD/AL 250/8 Amdt 1 Helical Power Train Drive Gear Internal Spline Inspection 2/71 Applicability: All 250-C18 series non Blue Ribbon gearboxes and all Blue Ribbon gearboxes used with non Blue Ribbon turbines. Document: Allison 250 CSL - 35 Rev. 1. Compliance: Between 700 and 775 hours' time in service since new or last overhaul after 26 February 1961.