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 170/55 Exhaust Heater/Muffler 6/91 Applicability: All Cessna 172 Series (Serial Numbers 36216 through 36769) that have not been modified with Cessna Service kit SK-172-10 or SK-172-10A. Requirement: Inspect and/or rectify subject aircraft in accordance with FAA AD90-06-03 R1. Note: FAA AD 90-06-03 R1 refers to Cessna Service Letter (SL) 170/172-11 dated 3 February 1958 and Cessna SL 170/172-13, dated 7 April 1958 contain information that pertains to this subject AD. Compliance: Within 100 hours time-in-service from 13 June 1991. Background: FAA AD action is intended to prevent cracking of the exhaust heater/muffler that could result in dangerous carbon monoxide levels.