COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Bell Helicopter Textron 47 (All Variants) Series Helicopters AD/BELL 47/68 Control Tubes 9/71 Applicability: All models fitted with Control Tubes P/N 47-150-255-1. Document: Bell SB 47-146. Compliance: Within 100 hours time in service or 30 days after 29 October 1971 whichever occurs first and thereafter at intervals not exceeding 1200 hours time in service.