{"id":"F2006B02111","name":"AD/AC/51 - Cabin Differential Pressure Limitations","slug":"ad-ac-51-cabin-differential-pressure-limitations","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41341,"registerId":"commonwealth-F2006B02111-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AC/51 - Cabin Differential Pressure Limitations","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 10/1973,Aircraft below 5700 kg,Twin Commander (Gulfstream/Rockwell/ Aerocommander 500, 600 and 700) Series Aeroplanes,Cabin Differential Pressure Limitations,51\ntitle: Airworthiness Directive - AD/AC/51 - Cabin Differential Pressure Limitations\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/51 Cabin Differential Pressure Limitations 10/73\n\nApplicability: All model 690A with S/Nos. 11102 to 11107 and 11109 to 11116 not modified in accordance with SL No. 270.\n\n \n\nDocument: Aero Commander SB No. 125. Effective:  16 October 1973.\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":false,"description":"The text provided sets out the original, narrowly circumscribed scope: it applies only to model 690A aeroplanes with the listed serial numbers unless they have already been modified under SL No. 270, and it refers implementers to Aero Commander Service Bulletin No. 125 with an effective date of 16 October 1973. There is no indication in the supplied text that the scope has been altered from that original directive."},"complexity_factors":["Very narrow applicability defined by specific serial numbers (easy to check but highly granular).","Substantive technical requirements are contained in an external document (Aero Commander SB No. 125) rather than in the AD text itself.","Applicability conditional on a separate modification status (\"not modified in accordance with SL No. 270\"), requiring verification of historical maintenance records.","Vintage directive (1973 effective date) — potential difficulty in locating referenced SB and SL documents or parts for implementation.","Regulatory basis provided (Part 39) but the AD text omits operational detail, shifting interpretive burden to operators and maintenance organisations."],"plain_english_summary":"The Civil Aviation Safety Authority AD/AC/51 makes Aero Commander Service Bulletin No.125 mandatory for a narrow set of Twin Commander model 690A aircraft (specific serial numbers 11102–11107, 11109–11116) unless already modified per SL No.270. It addresses cabin differential pressure limitations and defers technical measures to the service bulletin.\n\nOwners/operators and maintenance organisations of those airframes must verify modification status, follow SB No.125, bear compliance costs, and answer to CASA enforcement."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive from 1973 addressing a specific technical issue (cabin pressure limits) on a narrow subset of aircraft. There is no indication of scope creep; it remains precisely focused on its original purpose of preventing structural failure due to over-pressurisation in specific Twin Commander 690A serial numbers."},"complexity_factors":["Extremely short document (approximately 150 words)","Only 2 defined applicability criteria (specific serial number ranges and modification status)","Single cross-reference to external technical document (Aero Commander SB No. 125)","No conditional logic, exceptions, or nested provisions","No interpretation section or defined terms","Straightforward command structure: applies to X, must comply with Y"],"plain_english_summary":"**What this is:** This is an Airworthiness Directive (AD) — essentially a mandatory safety notice issued by Australia's Civil Aviation Safety Authority (CASA). Think of it like a compulsory recall notice for aircraft, but for safety-critical maintenance issues rather than product defects.\n\n**What it does:** This directive places strict limits on how much pressure difference (called \"cabin differential pressure\") certain Twin Commander aircraft can maintain between the inside of the cabin and the outside atmosphere.\n\n**Who it affects:** Specifically, it applies to:\n- **Twin Commander 690A model aircraft** (also known as Gulfstream/Rockwell/Aerocommander 500, 600 and 700 series)\n- **Serial numbers 11102 to 11107 and 11109 to 11116**\n- **Only those aircraft that have NOT been modified according to Service Letter No. 270**\n\n**Why it matters:** Cabin differential pressure is what keeps passengers comfortable and breathing at high altitudes — the higher the plane flies, the more the cabin needs to be \"pressurised\" compared to the thin outside air. If this pressure difference gets too high, it can stress the aircraft structure, potentially leading to cracks, failures, or even explosive decompression. This AD caps that pressure limit to prevent structural damage.\n\n**What must happen:** Operators of affected aircraft must follow the technical instructions in **Aero Commander Service Bulletin No. 125**, which became mandatory on **16 October 1973**."}},"importantCases":[],"_links":{"self":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations","history":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations/history","analysis":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations/analysis","conflicts":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations/conflicts","importantCases":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations/important-cases","documents":"/api/acts/ad-ac-51-cabin-differential-pressure-limitations/documents"}}