COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Fokker F50 (F27 Mk 50) Series Aeroplanes AD/F50/34 Electrical Power - AC Emergency Inverter 4/90 Applicability: All F27 Mk50 aircraft S/Nos 20103 to 20161 inclusive. Requirement: Action in accordance with Fokker SB F50-24-009 or later revision. Note: RLD Airworthiness Directive BLA 90-003 also refers. Compliance: Prior to 19 January 1991. Background: When the AC Power is switched from NORMAL to EMERGENCY, the attitude and heading reference system (AHRS) does not work for approximately 45 seconds, due to a combination of tolerances built into the emergency inverter and the AHRS. This Airworthiness Directive calls for a wiring change to the circuit of the emergency inverter. The emergency inverter will now be switched ON from the standby mode. This will make sure that the AHRS does not become inoperative when the AC power is switched from normal to emergency.