{"id":"F2006B09114","name":"AD/DHC-6/27 - Fuel Cell Sump Plate Flapper Valve Seals - Modification","slug":"ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":115667,"registerId":"commonwealth-F2006B09114-current","compilationNumber":null,"startDate":"2026-04-03","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/DHC-6/27 - Fuel Cell Sump Plate Flapper Valve Seals - Modification","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)  DHC-6 (Twin Otter) Series Aeroplanes\n\n \n\nAD/DHC-6/27 Fuel Cell Sump Plate Flapper Valve Seals - Modification\n\nApplicability: All models with S/Nos. 1 to 299 not incorporating Mod. 6/1406. Document:  De Havilland of Canada SB No. 6/269.\n\nCompliance: Within 100 hours time in service after 30 June 1971.\n\n\n5/71\n","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":2,"scope_assessment":{"changed":false,"description":"The instrument's scope is explicit in the text: it targets DHC-6 aircraft with serial numbers 1–299 that do not have Mod. 6/1406, requires modification per De Havilland SB No. 6/269, and sets a fixed compliance deadline (100 hours after 30 June 1971). There is no indication in the provided text that the scope differs from that expressed in the instrument itself."},"complexity_factors":["Narrow, serial-number-based applicability (S/Nos. 1 to 299) which limits the affected population.","Clear compliance window (100 hours after 30 June 1971) reduces ambiguity on timing.","Exclusion for aircraft already incorporating Mod. 6/1406 introduces a binary condition to check.","Dependence on an external service bulletin (De Havilland SB No. 6/269) for technical detail creates a secondary source-of-truth and potential implementation ambiguity.","No alternative methods or discretion described in the AD text increases procedural clarity but shifts technical detail/variability to the referenced bulletin."],"plain_english_summary":"This AD requires owners/operators of DHC‑6 (Twin Otter) aeroplanes serials 1–299 (unless they already have Mod. 6/1406) to modify the fuel‑cell sump plate flapper valve seals per De Havilland Service Bulletin No. 6/269. The work must be done within 100 hours time‑in‑service after 30 June 1971. It imposes a short scheduling and cost burden on affected operators and relies on the referenced SB for the technical procedure."},"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 a defined aircraft population. It has not expanded beyond its original technical scope."},"complexity_factors":["Single, specific technical requirement with clear applicability criteria","Only one defined term requiring external reference (Mod. 6/1406)","Simple compliance deadline structure (hours in service after fixed date)","No nested exceptions or conditional logic","Cross-reference to only one external technical document (Service Bulletin 6/269)","Extremely short length (under 100 words of substantive content)"],"plain_english_summary":"This is an **Airworthiness Directive** (AD) — a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for aircraft.\n\n**What it does:**\nOrders owners and operators of specific **De Havilland DHC-6 Twin Otter aircraft** (serial numbers 1 to 299) to modify the **fuel cell sump plate flapper valve seals**.\n\n**Why it matters:**\nThese seals are critical safety components. If they fail, fuel could leak — creating fire hazards or engine failure risks. The modification (referenced as \"Mod. 6/1406\") fixes a known design flaw.\n\n**Who must comply:**\nAnyone operating a DHC-6 Twin Otter with serial numbers 1–299 that hasn't already had Modification 6/1406 installed.\n\n**When:**\nWithin 100 hours of flight time after 30 June 1971.\n\n**Key document:**\nDe Havilland Service Bulletin No. 6/269 (the technical instructions for the fix)."}},"importantCases":[],"_links":{"self":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification","history":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification/history","analysis":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification/analysis","conflicts":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification/conflicts","importantCases":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification/important-cases","documents":"/api/acts/ad-dhc-6-27-fuel-cell-sump-plate-flapper-valve-seals-modification/documents"}}