{"id":"F2006B09785","name":"AD/F28/50 Amdt 1 - Boundary Layer Fence","slug":"ad-f28-50-amdt-1-boundary-layer-fence","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":54704,"registerId":"commonwealth-F2006B09785-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/F28/50 Amdt 1 - Boundary Layer Fence","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)  Fokker F28 Series Aeroplanes\n\n \n\nAD/F28/50\n\nAmdt 1\n\n\nBoundary Layer Fence 10/86\n\n \n\nApplicability: All model F28 with S/Nos 11092, 11093, 11111 to 11234, 11237 and 11238. Requirement:               Action in accordance with Fokker S.B. F28/57-75, Rev. 1.\n\nNote: RLD AD 85-45 Rev. 2 refers.\n\n \n\nCompliance: Prior to 31 December 1987.\n\n \n\nAir-leakage through boundary layer fences results in a deterioration of the wing performance. Modified boundary layer fences provide labyrinth-type seals to overcome this problem. Amendment 1 of this Directive references Rev 1 of Fokker SB F28/57-75 which requires additional work.\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":2,"scope_assessment":{"changed":true,"description":"Amendment 1 expands the required work by referencing Revision 1 of Fokker Service Bulletin F28/57-75, which the AD states \"requires additional work\" compared with the original directive (Requirement; Air-leakage... / Amendment 1)."},"complexity_factors":["Narrow applicability: specific serial numbers listed (Applicability).","Single prescriptive action: follow specified Fokker service bulletin (Requirement).","Fixed compliance deadline (Compliance: prior to 31 December 1987).","Technical work delegated to manufacturer's service bulletin (creates technical but not legal complexity) (Requirement).","Amendment increases scope by referencing Rev. 1 which \"requires additional work\" (Air-leakage... / Amendment 1)."],"plain_english_summary":"# What this instrument changes (mechanical effect)\n\nThis Airworthiness Directive (AD) requires specific Fokker F28 aeroplanes (those listed under \"Applicability\") to have their boundary layer fences modified by following Fokker Service Bulletin F28/57-75, Revision 1 (\"Requirement\"). The modification provides labyrinth-type seals for the boundary layer fences to stop air leakage (see \"Air-leakage...\"). The work must be completed by 31 December 1987 (\"Compliance\").\n\n# Who it affects\n\n- Operators and owners of the listed Fokker F28 aeroplanes (Applicability: all models with serial numbers 11092, 11093, 11111–11234, 11237 and 11238).  \n- Maintenance organisations that perform work to the manufacturer's service bulletin referenced by the AD (Requirement: action in accordance with Fokker S.B. F28/57-75, Rev. 1).\n\n# Why the AD is issued (stated purpose) and the direct mechanism\n\n- The AD states the problem: \"Air-leakage through boundary layer fences results in a deterioration of the wing performance.\" (Air-leakage through boundary layer fences...).  \n- The AD states the technical remedy: \"Modified boundary layer fences provide labyrinth-type seals to overcome this problem.\" (Air-leakage through boundary layer fences...).  \n- Mechanically, the regulation imposes a mandatory modification by reference to the manufacturer's service bulletin (Requirement) and sets a firm compliance deadline (Compliance).\n\n# Practical implications, incentives and trade-offs (source-grounded)\n\n- Who decides and who acts: The AD places the obligation on the operators/owners of the named aircraft to carry out the modification (Applicability; Requirement). The specific work method is determined by following Fokker S.B. F28/57-75, Rev. 1 (Requirement).  \n- Timing/compliance burden: Operators must complete the work by the stated date (Compliance: prior to 31 December 1987). That creates a scheduling requirement that will require organising maintenance and technical input consistent with the referenced service bulletin.  \n- Technical dependency: The AD relies on a manufacturer-issued service bulletin for the detailed procedure (Requirement). The AD therefore transfers technical detail-setting to the service bulletin it references.  \n- Change in required scope of work: Amendment 1 explicitly references Revision 1 of the service bulletin and states this Rev. 1 \"requires additional work\" (Air-leakage... / Amendment 1). That means the amendment increases the tasks operators must complete compared with the original AD.\n\n# Costs, risk and opportunity-cost channels (mechanistic, source-grounded)\n\n- Direct financial and time costs are not itemised in the AD text. The mechanism that produces costs is the mandatory modification and the need to perform the work in accordance with the referenced service bulletin (Requirement; Compliance). Those actions imply use of labour, parts and maintenance facility time by the affected operators/maintenance organisations.  \n- Effect on aircraft availability: The compliance deadline and the requirement to perform specific work create a need to schedule downtime for the affected aeroplanes to complete the modification (Compliance; Requirement). The AD text itself does not quantify downtime or specify grounding, but the mandatory work implies aircraft will need to be taken into maintenance until the modification is completed.\n\n# Implementation and discretion\n\n- The AD prescribes a specific technical source (the Fokker service bulletin) rather than providing an alternative compliance method in the text. That narrows administrative discretion about how the modification should be performed (Requirement).  \n- The AD cross-references an earlier directive (Note: RLD AD 85-45 Rev. 2 refers), indicating continuity with prior regulatory action but not adding alternative methods or waivers in the AD text.\n\n# Net functional effect (concise)\n\nOperators of the listed F28 serial numbers must have the boundary layer fences modified to labyrinth-type seals using the procedures in Fokker S.B. F28/57-75 Rev. 1, completing that work by 31 December 1987. The stated technical goal is to stop air leakage through the fences and restore wing performance (Air-leakage...). The amendment (Amendment 1) increases the required work by referencing the revised service bulletin (Amendment 1; Requirement)."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a narrow, technical airworthiness directive focused on a specific airframe component (boundary layer fences) on a specific aircraft type (Fokker F28). The amendment simply updates the referenced technical document to a revised version requiring additional work — it does not expand the scope beyond the original safety purpose of preventing air leakage through wing fences."},"complexity_factors":["Single, specific technical requirement referencing an external manufacturer document (Fokker Service Bulletin)","Limited applicability — only 147 specific aircraft serial numbers affected (S/Nos 11092, 11093, 11111-11234, 11237, 11238)","No defined terms section, no conditional logic, no nested exceptions","Straightforward compliance deadline with no alternative methods or phased implementation","Total length is approximately 150 words of substantive content","Single amendment note explaining the change (additional work required beyond original directive)"],"plain_english_summary":"This is an **Airworthiness Directive** — a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) that requires specific repairs or modifications to aircraft.\n\n**What it does:**\nOrders owners and operators of certain **Fokker F28 series aeroplanes** to fix a design flaw in the \"boundary layer fences\" — small aerodynamic surfaces on the wings. The problem is that air leaks through these fences, which degrades wing performance and could affect flight safety.\n\n**Who it affects:**\n- Owners and operators of specific Fokker F28 aircraft with serial numbers: 11092, 11093, 11111 through 11234, 11237, and 11238\n- This covers most F28 models in this production range\n\n**What must be done:**\n- Install modified boundary layer fences that have better \"labyrinth-type seals\" (a mechanical design with a winding path that blocks air leakage, like a maze seal)\n- Follow the exact instructions in **Fokker Service Bulletin F28/57-75, Revision 1** (the manufacturer's official repair guide)\n\n**When:**\n- The work had to be completed before **31 December 1987**\n\n**Why it matters:**\nThis directive prevents potential loss of wing lift or control due to aerodynamic degradation. The \"Amendment 1\" updates the original directive to require additional work beyond what the first version specified, based on new technical information from the manufacturer."}},"importantCases":[],"_links":{"self":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence","history":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence/history","analysis":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence/analysis","conflicts":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence/conflicts","importantCases":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence/important-cases","documents":"/api/acts/ad-f28-50-amdt-1-boundary-layer-fence/documents"}}