COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 106 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Pratt and Whitney Turbine Engines - JT9D Series AD/JT9D/24 HPC Turbine Drive Shafts 8/91 Applicability: JT9D-7R4 series engines. Requirement: Action in accordance with the Technical Requirements of FAA AD 91-03-03 Amdt 39-6838. Compliance: As detailed in Requirement Document with a revised effective date of 8 August 1991. Background: This Directive requires initial and repetitive inspection of certain high pressure compressor (HPC) turbine drive shafts. This requirement follows advice of circumferential cracks found in the thread area of the HPC drive shaft which mates with the number three bearing retainer nut.