{"id":"F2006B03255","name":"AD/B747/77 - APU Fire Detection","slug":"ad-b747-77-apu-fire-detection","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":42679,"registerId":"commonwealth-F2006B03255-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/B747/77 - APU Fire Detection","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 747 Series Aeroplanes\n\n \n\nAD/B747/77 APU Fire Detection 5/90\n\nApplicability: All model Boeing 747-400 series aeroplanes as listed in Boeing SB 747-26-2135 Revision 1, or later revision.\n\n \n\nRequirement: Modify the APU electrical power circuitry in accordance with Boeing SB 747-26- 2135 Revision 1 or later revision.\n\n \n\nCompliance: Prior to 17 August 1990.\n\n \n\nBackground: 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.\n\nCompliance with this AD is required to correct this unacceptable condition.\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\n \n\n \n","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":false,"description":"The directive’s scope—modifying APU electrical power circuitry on the Boeing 747-400 series aeroplanes listed in Boeing SB 747-26-2135 Revision 1—is set out explicitly and aligns with the stated objective of correcting the identified condition (Applicability; Requirement; Background). The instrument does not indicate any expansion or narrowing of scope beyond that explicit applicability."},"complexity_factors":["Reference to an external technical document (Boeing SB 747-26-2135 Revision 1) for the detailed modification steps (Requirement).","Safety-critical nature of APU fire-detection and aircraft electrical systems (Background).","Mandatory compliance by a fixed deadline creates operational scheduling and downtime impacts (Compliance).","Need for certified maintenance personnel and correct electrical work increases implementation risk and compliance burden (Requirement; Background).","Applicability limited to specific aircraft models listed in the referenced SB, requiring operators to check the SB to confirm whether a particular airframe is covered (Applicability)."],"plain_english_summary":"This directive mandates a modification to the APU electrical power circuitry on specified Boeing 747‑400s, to be carried out per Boeing Service Bulletin 747‑26‑2135 (or later), to prevent loss of APU fire‑detection power if the main battery switch is turned off. It affects owners/operators and maintenance organisations, who must fund and schedule the work by the compliance date (17 August 1990) to remove the identified safety vulnerability."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, time-bound safety directive with a single purpose: correcting a specific wiring defect. It has not expanded beyond its original technical scope."},"complexity_factors":["Single, specific technical requirement referencing an external Boeing service bulletin","No defined terms section — uses plain technical language","No conditional logic or nested exceptions","Single compliance deadline with no alternative methods or phased implementation","Extremely short document (approximately 150 words of substantive content)","Narrow applicability (one aircraft model series)"],"plain_english_summary":"**What this is:** This is an Airworthiness Directive (AD) — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for aircraft.\n\n**What it does:** It requires airlines and aircraft operators to fix a dangerous electrical wiring problem in Boeing 747-400 aircraft. Specifically, they must modify the circuitry for the Auxiliary Power Unit (APU) — a small engine at the back of the plane that provides power when the main engines are off.\n\n**The safety problem:** If a pilot turned off the main battery switch while the APU was running, the fire detection system for that unit would lose power. The APU would keep running for 60 seconds to cool down, but during that time, there would be **no fire warning system** if something went wrong. This created a dangerous blind spot where a fire could start undetected.\n\n**Who it affects:** Operators of Boeing 747-400 series aircraft (the \"jumbo jet\" variant with the extended upper deck).\n\n**What had to happen:** Operators had to rewire the system according to Boeing's official service bulletin (SB 747-26-2135) before 17 August 1990.\n\n**Why it matters:** Fire on an aircraft is catastrophic. This directive closed a safety gap where a routine action (turning off a battery switch) could silently disable a critical fire protection system."}},"importantCases":[],"_links":{"self":"/api/acts/ad-b747-77-apu-fire-detection","history":"/api/acts/ad-b747-77-apu-fire-detection/history","analysis":"/api/acts/ad-b747-77-apu-fire-detection/analysis","conflicts":"/api/acts/ad-b747-77-apu-fire-detection/conflicts","importantCases":"/api/acts/ad-b747-77-apu-fire-detection/important-cases","documents":"/api/acts/ad-b747-77-apu-fire-detection/documents"}}