COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY SCHEDULE OF AIRWORTHINESS DIRECTIVES Boeing 737 Series Aeroplanes AD/B737/50 APU - EGT Overtemperature Condition 2/91 Applicability: Boeing 737-300 and 737-400 series. Requirement: Inspect and modify subject airplanes in accordance with FAA AD 90-23-12. Note: FAA AD 90-23-12 refers to Boeing Service Bulletin 737-49-1071, dated May 10, 1990. Compliance: As per Requirement Document. Background: FAA AD action prompted by reports that the APU exhaust gas temperature (EGT) indication incorrectly read "zero" following an APU shut down. This condition, if not corrected, could result in undetected overtemperature damage to the APU rotor structure, which could then result in rotor failure and possible structural damage to the aeroplane.