{"id":"F2006B02071","name":"AD/AC/12 - Horizontal Stabiliser - Modification","slug":"ad-ac-12-horizontal-stabiliser-modification","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41301,"registerId":"commonwealth-F2006B02071-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AC/12 - Horizontal Stabiliser - Modification","content":"---\nmeta-content-style-type: text/css\nmeta-content-type: application/xhtml+xml; charset=utf-8\nmeta-description: CASA Airworthiness Directive\nmeta-generator: Aspose.Words for .NET 22.10.0\nmeta-keywords: Issue NK,Aircraft below 5700 kg,Twin Commander (Gulfstream/Rockwell/ Aerocommander 500, 600 and 700) Series Aeroplanes,Horizontal Stabiliser - Modification,12\ntitle: Airworthiness Directive - AD/AC/12 - Horizontal Stabiliser - Modification\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)  Twin Commander (Gulfstream/Rockwell/ Aerocommander 500, 600 and 700) Series Aeroplanes\n\n \n\nAD/AC/12 Horizontal Stabiliser - Modification NK\n\nApplicability: Model 500: S/Nos. 618 to 724 except 700 and 718. Document:  1. Aero Commander SB No. 57; or\n\n2. FAA AD 59-6-1.\n\n \n\nCompliance: Forthwith.\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 text sets a narrow, specific scope: Model 500 aircraft with serial numbers 618–724 except 700 and 718, and it mandates immediate compliance with the referenced service bulletin or FAA AD. The instrument itself gives no indication that its scope has been altered from an earlier or original intent; it simply prescribes the targeted modification and an immediate compliance timeline (Applicability; Document; Compliance)."},"complexity_factors":["References external technical documents (Aero Commander SB No.57 or FAA AD 59-6-1) rather than providing technical details in the AD itself (Document)","Immediate compliance requirement ('Forthwith'), raising operational timing and scheduling pressure (Compliance)","Narrow applicability to a discrete set of serial numbers requiring precise aircraft identification (Applicability)","Technical work involves aircraft structure (horizontal stabiliser) and thus requires qualified maintenance organisations and possible parts procurement","Regulatory enforcement and verification rely on standard airworthiness oversight processes rather than bespoke procedures in the AD (header; Compliance)"],"plain_english_summary":"This Airworthiness Directive requires an immediate modification of the horizontal stabiliser on Twin Commander Model 500 aircraft, serial numbers 618–724 (except S/N 700 and 718).  \n\nOwners/operators must comply forthwith by following Aero Commander Service Bulletin No. 57 or FAA AD 59-6-1 and are responsible for arranging and funding the required work and downtime."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive focused on a specific airframe component (horizontal stabiliser) on a narrow range of aircraft serial numbers. There is no indication of scope creep; it maintains the original purpose of addressing a specific airworthiness concern on specific aircraft."},"complexity_factors":["Extremely short document — only 4 substantive lines of regulatory content","No defined terms section — all terms are either common aviation terminology or reference external documents","No conditional logic or nested exceptions — simple applicability statement with straightforward serial number range","Single compliance requirement ('forthwith') with no alternative compliance methods or phased implementation","Only two external document references (Aero Commander SB No. 57 and FAA AD 59-6-1)","No cross-references to other legislation beyond the enabling regulation (CAR 1998 Part 39)"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA). It requires owners and operators of specific **Twin Commander aircraft** (models 500, 600, and 700 series) to modify the **horizontal stabiliser** (the small wing at the tail of the plane that helps keep the aircraft level).\n\n**Who it affects:**\n- Owners and operators of specific Twin Commander Model 500 aircraft with serial numbers 618 to 724 (excluding serial numbers 700 and 718)\n- This covers Gulfstream, Rockwell, and Aero Commander branded versions of these aircraft\n\n**What it requires:**\n- A modification to the horizontal stabiliser must be carried out **\"forthwith\"** (immediately, without delay)\n- The work must follow either:\n  - **Aero Commander Service Bulletin No. 57**, or\n  - **FAA AD 59-6-1** (the equivalent American safety order)\n\n**Why it matters:**\nAirworthiness Directives are issued when a safety defect is discovered that could affect flight safety. This directive suggests there was a structural or design issue with the horizontal stabiliser on these specific aircraft that required urgent correction to prevent potential accidents."}},"importantCases":[],"_links":{"self":"/api/acts/ad-ac-12-horizontal-stabiliser-modification","history":"/api/acts/ad-ac-12-horizontal-stabiliser-modification/history","analysis":"/api/acts/ad-ac-12-horizontal-stabiliser-modification/analysis","conflicts":"/api/acts/ad-ac-12-horizontal-stabiliser-modification/conflicts","importantCases":"/api/acts/ad-ac-12-horizontal-stabiliser-modification/important-cases","documents":"/api/acts/ad-ac-12-horizontal-stabiliser-modification/documents"}}