{"id":"F2006B02829","name":"AD/B737/128 - Forward Lavatory Drain - Isolation Valve","slug":"ad-b737-128-forward-lavatory-drain-isolation-valve","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":42171,"registerId":"commonwealth-F2006B02829-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/B737/128 - Forward Lavatory Drain - Isolation Valve","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nBoeing 737 Series Aeroplanes\n\nAD/B737/128 Forward Lavatory Drain \\- Isolation Valve 5/99\n\nApplicability: All Boeing model 737-100, \\-200, \\-300, \\-400, and \\-500 series aeroplanes having forward lavatories installed.\n\nRequirements: 1. Install a placard on the door beneath the forward lavatory sink, that warns the cabin crew not to put the selector valve for the forward lavatory water supply in the “DRAIN” position during flight. This installation is to be completed in accordance with the provisions of Boeing Service Bulletin 737-38-1043 dated 8 January 1998.\n\n2. Install an isolation valve in the drain line downstream of the selector valve for the forward lavatory water supply, in accordance with a method approved by the aircraft manufacturer or is approved under Regulation 35. Once this is accomplished, the placard at requirement 1 above may be removed.\n\nNote: US FAA AD 99-01-16 Amendment 39-10981 refers.\n\nCompliance: 1. Within six months of the effective date of this Directive.\n\n2. Within 36 months of the effective date of this Directive.\n\nThis Airworthiness Directive becomes effective on 20 May 1999.\n\nBackground: Reports of damage to the horizontal stabiliser and engine flame out caused by ice formed from water drained inadvertently from the forward toilet through a mispositioned selector valve. This Directive introduces measures to warn of mispositioning the valve and, eventually, modifying the system to eliminate such occurrences.","sortOrder":0}],"analysis":{"flash_summary_failed":{"failed":true,"reason":"Unauthenticated. Configure AI_GATEWAY_API_KEY or use a provider module. Learn more: https://ai-sdk.dev/unauthenticated-ai-gateway","source":"analysis-cron"},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a standard Airworthiness Directive with tightly scoped, specific safety purpose. The scope matches the original intent: addressing a known safety hazard (ice formation from lavatory drainage) on specific aircraft models."},"complexity_factors":["Very short document (under 300 words)","Only 2 substantive requirements with clear sequential logic","Single defined applicability criterion (specific Boeing 737 models with forward lavatories)","No nested exceptions or conditional logic","Simple compliance timeline (6 months and 36 months)","Single cross-reference to FAA directive for background only"],"plain_english_summary":"This safety rule requires operators of older Boeing 737s with a forward lavatory to take immediate and later actions to stop crew from dumping toilet water in flight. Within six months a placard must warn crew not to set the sink drain to “DRAIN”; within 36 months an isolation valve must be fitted to permanently block in‑flight draining. It prevents in‑flight toilet water freezing into ice that has damaged tail surfaces and caused engine flameouts."},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The Directive's measures align with its stated safety motivation and apply to the specified aircraft with forward lavatories. It does not expand or narrow the stated applicability beyond the models and installations already identified; it adds procedural and hardware requirements but does not alter the population of affected aeroplanes."},"complexity_factors":["Limited applicability to specific Boeing 737 series with forward lavatories reduces scope complexity","Two-step compliance (short-term placard, longer-term hardware modification) creates phased administrative and logistical planning","Valve installation requires method approval by the aircraft manufacturer or under Regulation 35, introducing procedural and certification complexity","Safety-critical context (ice causing stabiliser damage/engine flame-out) raises regulatory strictness but not technical ambiguity","Fixed compliance deadlines (6 months and 36 months) create scheduling and cost pressures for operators"],"plain_english_summary":"This Airworthiness Directive applies to Boeing 737‑100/200/300/400/500 aeroplanes fitted with forward lavatories. It requires operators to place a warning placard by the forward lavatory sink to prevent selecting the drain position in flight, and later to fit an isolation valve in the forward-lavatory drain line.\n\nOperators must install the placard within six months and the isolation valve within 36 months; the valve method must be approved by Boeing or under Regulation 35. Operators bear the compliance cost; approvals and maintenance scheduling are the decision and implementation points."}},"importantCases":[],"_links":{"self":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve","history":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve/history","analysis":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve/analysis","conflicts":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve/conflicts","importantCases":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve/important-cases","documents":"/api/acts/ad-b737-128-forward-lavatory-drain-isolation-valve/documents"}}