COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Boeing 747 Series Aeroplanes AD/B747/77 APU Fire Detection 5/90 Applicability: All model Boeing 747-400 series aeroplanes as listed in Boeing SB 747-26-2135 Revision 1, or later revision. Requirement: Modify the APU electrical power circuitry in accordance with Boeing SB 747-26- 2135 Revision 1 or later revision. Compliance: Prior to 17 August 1990. Background: In the current configuration if the main battery switch is turned off whilst the APU is operating, power to the APU fire detection system is removed. The APU goes through a 60 second cool down cycle with the fire detection system inoperative. Compliance with this AD is required to correct this unacceptable condition.