COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Cessna 170, 172, F172, FR172 and 175 Series Aeroplanes AD/CESSNA 170/52 Muffler 3/86 Applicability: Cessna aircraft S/N 172RG 1135 to 172RG 1187. Requirement: Inspect muffler core below each end plate attachment screw for damage in accordance with Cessna SB No SEB85-17. Compliance: Prior to issue of Certificate of Airworthiness. Background: A possibility exists that holes may have been drilled through the muffler allowing exhaust gases into the cabin during cabin heat operation.