{"id":"F2006B07324","name":"AD/TB20/20 - Horizontal Stabiliser Balance Weight","slug":"ad-tb20-20-horizontal-stabiliser-balance-weight","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":49357,"registerId":"commonwealth-F2006B07324-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/TB20/20 - Horizontal Stabiliser Balance Weight","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)  Aerospatiale (Socata) TB20 (Trinidad) Series Aeroplanes\n\n \n\nAD/TB20/20 Horizontal Stabiliser Balance Weight 4/91 DM\n\nApplicability: Model TB20 and TB21 series with serial numbers 1 through 1030. Requirement:  Action in accordance with Alert SB 57.\n\nCompliance: Within 25 hours time in service after 13 February 1991.\n\n \n\nBackground:  This Directive requires an inspection to determine whether both attaching nuts of the horizontal stabilizer balance weight are within the required safety dimension.\n\nDetachment of balance weight may lead to a dangerous flutter 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\n \n\n \n","sortOrder":0}],"analysis":{"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. It has not expanded beyond its original purpose of ensuring the horizontal stabiliser balance weight is properly secured."},"complexity_factors":["Very short document (under 200 words)","Only 4 defined elements: applicability (specific models/serials), requirement (reference to external document), compliance deadline, and background","Single conditional trigger (time in service after specific date)","No nested exceptions or cross-references to other legislation","Straightforward command structure: identify aircraft → inspect → comply by deadline"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA).\n\n**What it does:**\nIt requires owners or operators of specific Socata TB20 and TB21 aeroplanes (serial numbers 1 to 1030) to inspect the horizontal stabiliser balance weight. Specifically, they must check that both nuts holding this weight in place meet a required safety dimension.\n\n**Why it matters:**\nIf these nuts aren't properly secured, the balance weight could detach during flight. This could cause **flutter** — a dangerous vibration that can damage the aircraft or cause loss of control.\n\n**Who must act:**\nAnyone operating the affected aircraft models (TB20 and TB21 series with serial numbers up to 1030).\n\n**What they must do:**\nFollow the instructions in \"Alert SB 57\" (an alert service bulletin from the manufacturer) and complete the inspection within 25 hours of flying time after 13 February 1991.\n\n**Key point:**\nThis is a **time-critical safety fix**. The 25-hour compliance window means operators couldn't delay — they had to check their aircraft relatively quickly to prevent potentially catastrophic failure."},"flash_summary":{"complexity_score":2,"scope_assessment":{"changed":false,"description":"The AD's scope, as written, matches its stated intent: to inspect whether both attaching nuts of the horizontal stabiliser balance weight meet the required safety dimension for TB20/TB21 serial numbers 1–1030. The instrument prescribes inspection per Alert SB 57 and a 25-hour compliance window; nothing in the text expands or narrows that stated purpose beyond those procedural constraints. (Applicability; Requirement; Compliance; Background)"},"complexity_factors":["Narrow technical scope limited to horizontal stabiliser balance weight attaching nuts (Background)","Applicability limited to a defined serial-number range (Applicability)","Short compliance deadline (25 hours TIS after a specific date) which raises scheduling pressure (Compliance)","Mandate refers to an external document (Alert SB 57) for specific procedures, requiring access to and reliance on that bulletin (Requirement)","No alternative compliance routes or remedial steps are specified within the AD text (Requirement)"],"plain_english_summary":"What this instrument does (mechanically)\n\n- It orders an inspection of the horizontal stabiliser balance weight attachment on certain Socata TB20/TB21 aeroplanes. The inspection must follow the procedures set out in Alert Service Bulletin 57 (\"Alert SB 57\"). (Requirement)\n- The inspection must be completed within 25 hours time-in-service after 13 February 1991. (Compliance)\n- The inspection's purpose is to determine whether both attaching nuts of the horizontal stabiliser balance weight meet the required safety dimension. (Background)\n\nWhich aircraft are affected\n\n- All Model TB20 and TB21 series aeroplanes with serial numbers 1 through 1030 are within the AD's applicability. Operators of those specific aircraft must comply. (Applicability)\n\nWhy it matters (official rationale and its operational implication)\n\n- The AD states that if the balance weight were to detach, it could create a dangerous flutter condition. The mechanical action mandated is an inspection to verify the attaching nuts are within required dimensions; that inspection aims to reduce the risk of detachment and subsequent flutter. (Background)\n\nWho pays, who decides, and what changes in behaviour\n\n- Aircraft owners/operators of the listed TB20/TB21 serial numbers must arrange for and pay for the inspection and any follow-up work required by Alert SB 57. (Applicability; Requirement)\n- The Civil Aviation Safety Authority (via this Airworthiness Directive) sets the requirement and the compliance deadline. Operators must change behaviour by scheduling and completing the inspection within the 25-hour window. (Compliance)\n\nCompliance burden, discretion and implementation notes\n\n- The directive references an external document (Alert SB 57) for the required work. That creates a dependency on procedures outside the AD text; operators need access to and must follow that service bulletin to meet the AD. (Requirement)\n- The AD gives a short compliance window (25 hours time-in-service from the specified date), which may require rapid scheduling of maintenance and could temporarily take aircraft out of service. (Compliance)\n- The AD text specifies the inspection requirement but does not in itself set out corrective actions or alternative compliance pathways; compliance depends on following Alert SB 57. The AD does not state any discretionary relief or alternative means of compliance within its own text. (Requirement)\n\nTrade-offs and risks highlighted by the source\n\n- The stated safety risk is a possible dangerous flutter if the balance weight detaches; the AD acts by imposing an inspection to reduce that risk. (Background)\n- The immediate costs are borne by aircraft owners/operators in the form of inspection and any repair work required by Alert SB 57; the AD creates a short-term operational disruption risk because of the tight compliance window. (Applicability; Compliance)\n\nSource citations: key lines of the AD are cited in parentheses throughout: (Applicability), (Requirement), (Compliance), (Background)."}},"importantCases":[],"_links":{"self":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight","history":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight/history","analysis":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight/analysis","conflicts":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight/conflicts","importantCases":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight/important-cases","documents":"/api/acts/ad-tb20-20-horizontal-stabiliser-balance-weight/documents"}}