{"id":"F2006B02975","name":"AD/B737/74 - Thrust Reverser Actuator Piston Seal","slug":"ad-b737-74-thrust-reverser-actuator-piston-seal","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":42334,"registerId":"commonwealth-F2006B02975-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/B737/74 - Thrust Reverser Actuator Piston Seal","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)  Boeing 737 Series Aeroplanes\n\n \n\nAD/B737/74 Thrust Reverser Actuator Piston Seal 2/93\n\nDM\n\nApplicability: Boeing 737-300,-400 and -500 series aeroplanes as listed in Boeing Service Bulletin 737-78A1055, dated 2 April 1992.\n\n \n\nNote: FAA AD 92-24-04 refers.\n\n \n\nRequirement: Action in accordance with Boeing Service Bulletin 737-78A1055 dated 2 April 1992. Compliance:              By 1 January 1994.\n\nBackground:  Analysis by the manufacturer indicates that a potential exists for leakage across the piston seal in the thrust reverser actuator. The actions specified in this AD are intended to prevent uncommanded deployment.\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 models. It has not expanded beyond its original narrow technical scope."},"complexity_factors":["Very short document (under 200 words)","Only 1 defined applicability criterion (specific Boeing models)","Single requirement referencing an external technical document (Boeing Service Bulletin)","No nested conditions, exceptions, or cross-references to other legislation","Straightforward compliance deadline with no alternative methods or phased implementation"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for specific Boeing 737 aircraft.\n\n**What it does:**\n- Requires owners/operators of Boeing 737-300, -400, and -500 series planes to inspect and fix a problem with the **thrust reverser actuator piston seal** (a hydraulic component that helps stop the plane after landing).\n- The fix must follow instructions in a specific Boeing technical document (Service Bulletin 737-78A1055 from April 1992).\n\n**Why it matters:**\n- The manufacturer found that the piston seal could leak, which might cause the **thrust reverser to deploy unexpectedly** during flight. A thrust reverser opening mid-flight is extremely dangerous — it can cause loss of control or crash.\n- **Deadline:** All affected aircraft had to comply by **1 January 1994**.\n\n**Who it affects:**\n- Airlines and operators flying the specified Boeing 737 models in Australia.\n\nThis is a **reactive safety measure** — it addresses a known design flaw before it causes an accident."},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive maintains a narrowly targeted scope: it applies only to Boeing 737-300, -400 and -500 series aeroplanes as listed in Boeing Service Bulletin 737-78A1055 (Applicability) and mandates the actions set out in that service bulletin (Requirement). The AD’s purpose remains addressing the specific risk identified by the manufacturer (leakage across the piston seal) to prevent uncommanded thrust reverser deployment (Background). There is no indication in the AD text that the scope was expanded beyond that original technical focus."},"complexity_factors":["Narrow technical scope focused on a single component (thrust reverser actuator piston seal) in specified aircraft models (Applicability).","Dependence on an external technical document (Boeing Service Bulletin 737-78A1055) to define the exact work required (Requirement).","A fixed compliance deadline that creates scheduling and operational implications for operators (Compliance).","Cross-reference to a foreign regulator's AD (FAA AD 92-24-04) which may require operators to consult multiple documents (Note).","Regulatory authority derived from Civil Aviation Regulations Part 39, which frames the AD as mandatory (header), but the short AD text leaves procedural details (who performs the work, approval steps) to the referenced service bulletin or other maintenance approval processes."],"plain_english_summary":"This AD mandates that operators of certain Boeing 737-300/400/500 aircraft comply by 1 January 1994 with Boeing Service Bulletin 737‑78A1055 to inspect and repair the thrust‑reverser actuator piston seal. It converts a manufacturer recommendation into a legal requirement to prevent potential seal leakage and uncommanded thrust‑reverser deployment, imposing inspection/repair costs and downtime on affected operators."}},"importantCases":[],"_links":{"self":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal","history":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal/history","analysis":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal/analysis","conflicts":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal/conflicts","importantCases":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal/important-cases","documents":"/api/acts/ad-b737-74-thrust-reverser-actuator-piston-seal/documents"}}