{"id":"F2006B02950","name":"AD/B737/50 - APU - EGT Overtemperature Condition","slug":"ad-b737-50-apu-egt-overtemperature-condition","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":42292,"registerId":"commonwealth-F2006B02950-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/B737/50 - APU - EGT Overtemperature Condition","content":"---\nmeta-content-style-type: text/css\nmeta-content-type: application/xhtml+xml; charset=utf-8\nmeta-generator: Aspose.Words for .NET 22.10.0\n---\n\n?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"no\"?>\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n \n\n![](image.001.png)  Boeing 737 Series Aeroplanes\n\n \n\nAD/B737/50 APU - EGT Overtemperature Condition 2/91\n\nApplicability: Boeing 737-300 and 737-400 series.\n\n \n\nRequirement: Inspect and modify subject airplanes in accordance with FAA AD 90-23-12.\n\n \n\nNote: FAA AD 90-23-12 refers to Boeing Service Bulletin 737-49-1071, dated May 10, 1990.\n\n \n\nCompliance: As per Requirement Document.\n\n \n\nBackground: 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.\n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"This directive implements the FAA AD 90-23-12 requirements for Boeing 737-300/-400 APU EGT indication faults and refers to Boeing Service Bulletin 737-49-1071; it does not broaden applicability beyond those models or add requirements beyond the referenced FAA/Boeing documents."},"complexity_factors":["Narrow applicability: limited to Boeing 737-300 and 737-400 series (straightforward population but requires fleet identification).","Cross-reference dependency: compliance is defined by an external FAA AD (90-23-12) and a Boeing Service Bulletin (737-49-1071), requiring access to and interpretation of those documents.","Technical maintenance action: inspections and physical modifications to the APU/EGT system require certified maintenance personnel and potential parts/workshop time.","Safety-critical monitoring: the defect concerns EGT indications that can mask overtemperature, increasing operational risk and the need for reliable corrective work.","Compliance scheduling: operators must plan maintenance windows and record compliance 'as per Requirement Document', introducing operational and administrative coordination."],"plain_english_summary":"This airworthiness directive requires operators of Boeing 737-300 and 737-400 aeroplanes to inspect and modify their auxiliary power unit (APU) installation as specified in FAA AD 90-23-12 (which refers to Boeing Service Bulletin 737-49-1071). It addresses a fault where the APU exhaust gas temperature indication can read “zero” after shutdown, risking undetected overtemperature damage and possible rotor failure; operators must follow the FAA/Boeing instructions to detect and correct the condition."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive addressing a specific mechanical defect in specific aircraft models. It has not expanded beyond its original purpose of mandating inspection and modification for the APU overtemperature issue."},"complexity_factors":["Very short document (approximately 200 words)","Only 2 defined applicability criteria (Boeing 737-300 and 737-400 series)","Single requirement referencing external document (FAA AD 90-23-12)","No nested conditional logic or exceptions","No cross-references to other Australian legislation","Straightforward compliance mechanism ('As per Requirement Document')","Simple factual background with clear causal chain"],"plain_english_summary":"**What this is:** This is an Airworthiness Directive (AD) — essentially a mandatory safety notice issued by Australia's Civil Aviation Safety Authority (CASA) for specific Boeing 737 aircraft.\n\n**What it does:** It requires airlines and aircraft operators to inspect and modify the Auxiliary Power Unit (APU) on certain Boeing 737-300 and 737-400 planes. The APU is a small engine at the back of the aircraft that provides power when the main engines are off.\n\n**The safety problem:** The APU's exhaust gas temperature (EGT) gauge was incorrectly showing \"zero\" after shutdown. This meant pilots and mechanics couldn't tell if the APU had overheated. Undetected overheating could damage the APU's spinning parts (rotor), potentially causing them to break apart and damage the aircraft.\n\n**Who must comply:** Anyone operating Boeing 737-300 or 737-400 aircraft in Australia.\n\n**What they must do:** Follow the specific inspection and modification instructions in an American Federal Aviation Administration (FAA) directive (AD 90-23-12), which itself references a Boeing technical bulletin.\n\n**Why it matters:** This prevents a hidden mechanical failure that could cause serious damage to the aircraft and potentially endanger passengers and crew."}},"importantCases":[],"_links":{"self":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition","history":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition/history","analysis":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition/analysis","conflicts":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition/conflicts","importantCases":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition/important-cases","documents":"/api/acts/ad-b737-50-apu-egt-overtemperature-condition/documents"}}