COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Aerospatiale (Socata) TB20 (Trinidad) Series Aeroplanes AD/TB20/21 Propeller Deicing Electrical Circuit Protection 10/91 Applicability: All TB20 and TB21 aircraft as specified in the requirement document. Requirement: Action in accordance with Aerospatiale TB Airplanes - SB Nr. 59. Compliance: Prior to 3 October 1992. Background: Compliance with this AD is necessary in order to ensure the integrity of the propeller de icing circuit by installing the correct rating fuse.