{"id":"F2006B04648","name":"AD/BELL 412/13 - Tail Rotor Trunnion Bearing Housing","slug":"ad-bell-412-13-tail-rotor-trunnion-bearing-housing","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":44400,"registerId":"commonwealth-F2006B04648-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/BELL 412/13 - Tail Rotor Trunnion Bearing Housing","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)  Bell Helicopter Textron 412 Series Helicopters\n\n \n\nAD/BELL 412/13 Tail Rotor Trunnion Bearing Housing 9/86\n\nDM\n\n \n\nApplicability: All model 412 S/N 33001 to 33118, 33120 and 33121.\n\nRequirement: Action in accordance with Bell Alert Service Bulletin No. 412-86-25, Revision A. Compliance:               Part I:              At the next 20 hour inspection and thereafter at each 20 hour inspection until\n\nPart II is complied with.\n\n \n\nPart II: Within 150 hours time in service or 60 days, whichever occurs first after 31 July 1986.\n\n \n\nBackground: A trunnion bearing housing end web has been found below specified dimensions.\n\nFailure of the housing caused excessive tail rotor vibration and loss of control.\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 mandating inspection and repair of the tail rotor trunnion bearing housing."},"complexity_factors":["Very short document (approximately 200 words)","Only 2 defined applicability criteria (model and serial numbers)","Simple two-part compliance structure (interim inspections + permanent fix)","Single cross-reference to external technical document (Bell Service Bulletin)","No nested exceptions or conditional logic","Clear, unambiguous deadlines (20 hours, 150 hours/60 days)","Plain language background explanation included"],"plain_english_summary":"**What this is:** This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for specific Bell 412 helicopters.\n\n**Who it affects:** Owners and operators of Bell 412 helicopters with serial numbers 33001 to 33118, 33120, and 33121.\n\n**What it requires:**\n- **Immediate action:** At the next 20-hour inspection (and every 20-hour inspection after that), operators must check the tail rotor trunnion bearing housing — a critical component that holds the tail rotor in place.\n- **Permanent fix:** Within 150 flight hours or 60 days (whichever comes first after 31 July 1986), operators must complete a more thorough repair or replacement following Bell's official service bulletin (Alert Service Bulletin No. 412-86-25, Revision A).\n\n**Why it matters:** A manufacturing defect was discovered where the \"end web\" (a structural part) of the bearing housing was too thin. If this part fails, the tail rotor vibrates excessively and the pilot can lose control of the helicopter — potentially causing a crash. This AD forces operators to inspect for the problem and fix it before it causes an accident.\n\n**In short:** If you own one of these helicopters, you must keep checking a specific tail rotor part every 20 hours until you permanently fix it within 60 days or 150 flight hours."},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The AD, as written, defines a corrective inspection and repair requirement that applies to a specific set of Bell 412 serial numbers and prescribes timing and reference to the manufacturer’s service bulletin. The text gives no indication that the AD changes scope from an earlier instrument or departs from its stated corrective purpose (detect and correct undersized trunnion bearing housing end webs to prevent tail-rotor vibration and loss of control)."},"complexity_factors":["Narrow applicability limited to specific serial-numbered airframes (Applicability)","Two-stage compliance schedule with short periodic inspection intervals (every 20 flight hours) until final action (Compliance)","Mandatory follow-up on an external manufacturer document (Bell Alert Service Bulletin No. 412-86-25, Revision A) which is not reproduced in the AD (Requirement)","Safety-critical mechanical inspection tied to a dimensional defect (trunnion bearing housing end web) requiring technical maintenance competence (Background)"],"plain_english_summary":"This AD requires owners/operators of Bell 412 helicopters S/N 33001–33118, 33120–33121 to follow Bell Alert Service Bulletin No. 412-86-25, Rev A: recurring inspections at the next and every 20‑hour check until a mandated Part II corrective action is done (within 150 flight hours or 60 days after 31 July 1986). It addresses an undersized trunnion bearing housing that can cause excessive tail‑rotor vibration and loss of control. Operators/maintenance providers must perform and pay for the inspections and repairs."}},"importantCases":[],"_links":{"self":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing","history":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing/history","analysis":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing/analysis","conflicts":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing/conflicts","importantCases":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing/important-cases","documents":"/api/acts/ad-bell-412-13-tail-rotor-trunnion-bearing-housing/documents"}}