COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Bell Helicopter Textron 204 Series Helicopters AD/BELL 204/15 Fire Detection System - Audible Warning NK Applicability: All models. Requirement: Modify the engine fire detection system to include an audible warning. The warning shall be unmistakeable from any other audible warning and shall be capable of being manually silenced. Compliance: Before 1 January 1975.