{"id":"F2006B04900","name":"AD/CESSNA 120/12 - Carburettor and Cabin air heater muffs","slug":"ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":44731,"registerId":"commonwealth-F2006B04900-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"AIRWORTHINESS DIRECTIVE","sectionType":"part","heading":"AIRWORTHINESS DIRECTIVE","content":"# AIRWORTHINESS DIRECTIVE\n\n![](image.001.png)  \n\nFor the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.1 (1) of CASR 1998. The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section.\n\n![](image.001.png)  \n\nCessna 120 and 140 Series Aeroplanes","sortOrder":0},{"sectionNumber":"AD/CESSNA 120/12 Carburettor and Cabin A","sectionType":"part","heading":"AD/CESSNA 120/12 Carburettor and Cabin Air Heater Muffs 12/2003","content":"# AD/CESSNA 120/12 Carburettor and Cabin Air Heater Muffs 12/2003\n\nApplicability: All Cessna 120 and 140 aeroplanes equipped with Cessna welded exhaust muffler assemblies where the exhaust muffler assemblies are fabricated by welding the exhaust stacks to the muffler.\n\nRequirement: 1. Remove the carburettor air heater muff and cabin heater muff and inspect the muffler assemblies for evidence of cracks paying particular attention to the areas of the mufflers and stacks adjacent to where the exhaust stacks and tailpipe are welded to the muffler assembly.\n\n2. Any placard calling for an inspection of the mufflers every 100 flight hours should be revised to call for the inspection every 25 flight hours.\n\nNote: FAA AD 48-25-02 refers.\n\nCompliance: 1. Initial inspection within 25 flight hours from effective date of this Directive, or 25 flight hours from the last inspection, thereafter at intervals of 25 hours.\n\n2. At the first inspection of the muffler assemblies detailed in this Directive. This Airworthiness Directive becomes effective on 27 November 2003.\n\nBackground: Cracking of the heater muff could lead to hazardous gases entering the cockpit.\n\n![](image.002.png)  \n\nJim Coyne\n\nDelegate of the Civil Aviation Safety Authority 14 October 2003\n\n![](image.001.png)  \n\nThe above AD is notified in the Commonwealth of Australia Gazette on 5 November 2003.","sortOrder":1}],"analysis":{"flash_summary":{"complexity_score":2,"scope_assessment":{"changed":false,"description":"This Directive is narrowly focused on a single airworthiness concern: cracking in welded exhaust muffler assemblies on Cessna 120 and 140 aeroplanes that could allow exhaust gases into the cockpit. It does not broaden regulation beyond that technical safety purpose (inspection, placard change, and repair where necessary). There is no indication in the text that the measure expanded beyond that original safety-focused scope."},"complexity_factors":["Short, focused document (single-page AD with Applicability, Requirement, Compliance, Background)","Very small number of operative provisions (two main requirements and a compliance schedule)","One defined applicability group (Cessna 120/140 with welded muffler assemblies)","Simple numerical compliance interval (25 flight hours) with no nested exceptions","Minor cross-reference only (Note referring to FAA AD 48-25-02)","Clear authority cited (CASR subregulation 39.1(1)) but no complex legal definitions or multiple regulatory hooks"],"plain_english_summary":"**What this directive does (mechanically)\n\n- **Applicability:** Applies to all Cessna 120 and 140 aeroplanes that have Cessna welded exhaust muffler assemblies (that is, mufflers where the exhaust stacks are welded to the muffler). (See \"Applicability\".)\n\n- **Required actions (\"Requirement\"):**\n  - Remove the carburettor air heater muff and the cabin heater muff and inspect the underlying exhaust muffler assemblies for any cracks, with particular attention to the welded areas where the exhaust stacks and tailpipe join the muffler. (Requirement 1.)\n  - If there is a placard that currently directs inspections every 100 flight hours, that placard must be changed to require inspections every 25 flight hours. (Requirement 2.)\n\n- **When to do it (\"Compliance\"):**\n  - Do an initial inspection within 25 flight hours from the effective date of the Directive or within 25 flight hours from the last inspection if one has already been done; thereafter repeat inspections at 25‑hour intervals. (Compliance 1.)\n  - The placard revision is required at the first inspection called for in this Directive. (Compliance 2.)\n\n- **Why (background):** The Directive states that cracking of the heater muff could allow hazardous (exhaust) gases to enter the cockpit, which is the condition the Directive seeks to correct. (Background.)\n\nWho pays, who decides, and what behaviour changes\n\n- **Who decides:** The Civil Aviation Safety Authority (CASA) delegate issued the Directive under CASR subregulation 39.1(1). The Directive imposes mandatory maintenance and placard-change obligations on operators/owners of affected aircraft.\n- **Who pays:** Aircraft owners/operators pay the costs of removing the heater muffs, performing inspections, making any repairs or replacements caused by detected cracking, and labour/time for more frequent inspections. If inspections reveal cracks that require parts or shop work, additional costs follow.\n- **Behaviour changes required:** Owners/operators must increase inspection frequency from 100 flight hours (if that placard exists) to every 25 flight hours, physically remove heater muffs to inspect welded exhaust areas, and carry out repairs if cracks are found.\n\nCosts, incentives, trade-offs and implementation considerations\n\n- **Direct costs and incentives:** The Directive increases maintenance frequency and inspection time, raising ongoing operating costs for affected aircraft. Operators have an incentive to inspect promptly and repair to remain legally airworthy and to avoid risk of cockpit contamination.\n- **Opportunity costs:** Time in maintenance and possible downtime for repairs reduces aircraft availability (cost to owners/operators and potential loss of revenue if the aircraft is used commercially). There may be extra expense for parts or certified maintenance labour.\n- **Compliance burden and implementation risk:** The inspections require removal of heater muffs and focused examination of welds; this typically requires qualified maintenance personnel. Enforcement is through CASA surveillance and airworthiness oversight. Availability of suitably qualified mechanics and spare parts could affect how quickly owners can comply.\n- **Effect on private choice and businesses:** Independent maintenance organisations and mechanics will perform the work; the Directive does not prescribe where work must be done, so market choice remains. Increased inspection frequency may raise demand for maintenance services and replacement parts, with concentrated costs on owners of the specific aircraft model with welded mufflers.\n- **Safety benefit vs. cost trade-off:** The Directive is targeted (limited applicability) and aims to reduce a specific safety risk (inhalation of exhaust gases caused by muffler cracks). The practical trade-off is higher inspection/repair cost and slightly higher maintenance downtime in exchange for the reduction of that risk.\n\nOther notes\n\n- The Directive references FAA AD 48-25-02 (Note), which may provide related technical background but does not change the Directive's domestic compliance obligations.\n- Effective date was 27 November 2003; inspectors and operators should record compliance against the 25‑hour inspection intervals specified. (Compliance.)"},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a targeted safety directive addressing a specific mechanical defect in specific aircraft models. The scope remains precisely limited to exhaust muffler inspection requirements for Cessna 120/140 aircraft with welded muffler assemblies. No scope creep is evident."},"complexity_factors":["Very short document (approximately 300 words)","Only 2 defined applicability criteria (aircraft model and specific muffler type)","Simple two-part requirement with clear sequential steps","Single cross-reference to FAA AD 48-25-02 (informational only)","No nested exceptions or conditional logic beyond the initial applicability test","Plain language background and compliance sections with specific timeframes"],"plain_english_summary":"CASA’s Airworthiness Directive requires owners/operators of Cessna 120/140 aeroplanes fitted with the specified welded exhaust muffler system to remove and inspect the carburettor and cabin heater mufflers and to update inspection placards to a 25‑hour interval, with the first check within 25 hours. It shortens inspection intervals to catch welded‑joint cracks that could leak carbon monoxide into the cockpit, preventing potential poisoning."},"flash_summary_failed":{"failed":true,"reason":"A positive credit balance is required for all requests, including BYOK, so fallback providers remain available. Add credits at https://vercel.com/d?to=%2F%5Bteam%5D%2F%7E%2Fai%3Fmodal%3Dtop-up to continue.","source":"analysis-cron"}},"importantCases":[],"_links":{"self":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs","history":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs/history","analysis":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs/analysis","conflicts":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs/conflicts","importantCases":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs/important-cases","documents":"/api/acts/ad-cessna-120-12-carburettor-and-cabin-air-heater-muffs/documents"}}