{"id":"F2006B04670","name":"AD/BELL 412/36 - Main Rotor Mast","slug":"ad-bell-412-36-main-rotor-mast","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":44422,"registerId":"commonwealth-F2006B04670-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/BELL 412/36 - Main Rotor Mast","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nBell Helicopter Textron 412 Series Helicopters\n\nAD/BELL 412/36 Main Rotor Mast 5/95\n\nApplicability: All model 412 series helicopters fitted with main rotor mast P/N 412-040-101-105,\n\n\\-109, \\-117, \\-121, \\-125, \\-127, \\-129 and spline plate P/N 412-010-167-101, \\-105 or\n\n412-010-177-101, \\-105, \\-109, \\-113 or \\-117.\n\nRequirement: Action in accordance with Bell Alert Service Bulletin 412-94-81 Revision A. Compliance: Within 100 hours time in service from 27 April 1995 or by 27 August 1995,\n\nwhichever occurs first.\n\nBackground: The manufacturer has revised the procedures for determining the retirement life of the main rotor mast and spline plate for the 412 and 412SP, which now has a retirement life of 10,000 hours or 80,000 maximum Retirement Index Numbers (RIN), whichever occurs first. For the 412HP and 412EP the retirement life is 10,000 hours or 60,000 RIN, whichever occurs first.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":true,"description":"The AD implements the manufacturer's revised retirement-life procedures by prescribing explicit retirement lives for the listed main rotor mast and spline plate part numbers. It therefore changes the applicable retirement-life regime for those parts by setting firm thresholds (10,000 hours or specified maximum RINs) and making compliance mandatory for affected aircraft. (Background; Requirement)"},"complexity_factors":["Multiple specific part numbers (P/Ns) listed for both main rotor mast and spline plate increasing applicability specificity (Applicability).","Different retirement-life thresholds depending on helicopter sub-model (412/412SP vs 412HP/412EP) and two competing measures (hours vs RIN) to determine retirement (Background).","Short, fixed compliance window (100 hours from a fixed date or an absolute calendar date) that requires prompt operational action (Requirement).","Mandate to follow an external technical document (Bell Alert Service Bulletin 412-94-81 Revision A) which operators must obtain and implement; operational detail is offloaded to that bulletin (Requirement).","Technical concept of Retirement Index Numbers (RIN) which operators must track alongside flight hours, adding recordkeeping and verification complexity (Background)."],"plain_english_summary":"What this directive does (mechanically):\n\n- It mandates operators to carry out the actions set out in Bell Alert Service Bulletin 412-94-81 Revision A for specified main rotor mast and spline plate part numbers. (Requirement)\n- It applies to all Bell 412 series helicopters fitted with the listed main rotor mast P/Ns (412-040-101-105, -109, -117, -121, -125, -127, -129) and the listed spline plate P/Ns (412-010-167-101, -105 or 412-010-177-101, -105, -109, -113 or -117). (Applicability)\n- Compliance must be completed within 100 hours time-in-service from 27 April 1995 or by 27 August 1995, whichever comes first. (Requirement)\n\nOfficial reason offered by the source (and how that changes the rule):\n\n- The manufacturer revised the procedures for determining retirement life of the main rotor mast and spline plate for the 412 family. The AD implements those revised procedures by setting explicit retirement lives: for model 412 and 412SP the retirement life is 10,000 hours or 80,000 maximum Retirement Index Numbers (RIN), whichever occurs first; for 412HP and 412EP the retirement life is 10,000 hours or 60,000 RIN, whichever occurs first. (Background)\n\nWho pays and who decides:\n\n- The owners/operators of the affected Bell 412 series helicopters bear the direct cost of complying (inspection, tracking, and replacing parts to the stated retirement lives) because the AD is addressed to those aircraft by part number and model. (Applicability; Requirement)\n- The Civil Aviation Safety Authority issues the directive and requires compliance with the named Bell Alert Service Bulletin, while Bell (the manufacturer) supplies the procedural content to be followed. (Title; Requirement)\n\nImmediate behavioural effects and compliance burden:\n\n- Operators must check whether their helicopter has the listed P/Ns and, if so, implement the service bulletin actions within the specified 100-hour or calendar deadline. (Applicability; Requirement)\n- The AD creates a mandatory retirement schedule for the listed parts: operators will need to track hours and RIN counts and remove/replace the mast or spline plate once the retirement threshold is reached. (Background)\n- The short compliance window (100 hours or a fixed calendar date) concentrates near-term work and planning requirements for affected operators. (Requirement)\n\nIncentives, trade-offs and implementation risks (mechanisms, not judgments):\n\n- By fixing retirement lives and enforcing them by directive, the measure increases demand for replacement masts and spline plates and for maintenance labour at particular times. That shifts costs to operators and creates commercial opportunity for parts suppliers and maintenance providers. (Background; Requirement)\n- The operator incentive is to monitor hours and RINs closely and schedule replacements to meet the AD; failure to do so risks non-compliance with the directive. (Requirement)\n- Implementation risks include incorrect tracking of RINs or hours and logistical delays in obtaining replacement parts within the compliance window; the AD is prescriptive and leaves little operational discretion to defer actions. (Requirement)\n\nEffects on private choice and markets (mechanisms):\n\n- The AD constrains an operator's freedom to keep a listed mast or spline plate in service beyond the retirement thresholds; that restriction affects maintenance planning, asset utilisation, and the timing of capital expenditure on replacement parts. (Background)\n- Independent maintenance organisations may perform the required work provided they follow the service bulletin and regulatory procedures; the AD therefore creates demand that those businesses can supply. (Requirement)\n\nConcentrated benefits, diffuse costs and substitution effects (mechanisms):\n\n- The direct commercial benefit from replacement-parts demand will be concentrated among suppliers and maintenance providers able to supply the listed P/Ns. The cost of compliance is spread across the population of affected operators. (Applicability; Background)\n- Operators may substitute by changing maintenance schedules, pre-purchasing spares, or withdrawing aircraft from service earlier than otherwise planned to meet the retirement limits. (Requirement; Background)\n\nNet description of what changed mechanically: the AD enforces the manufacturer's revised method for determining retirement life by imposing explicit retirement thresholds (hours or RIN) for the specified mast and spline plate P/Ns and mandates compliance with the Bell Alert Service Bulletin within a short, specified timeframe. (Requirement; Background)"},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, time-bound safety directive with a single, specific purpose: mandating revised retirement life limits for particular helicopter components. It has not expanded beyond its original technical safety intent."},"complexity_factors":["Very short document (under 200 words)","Only 2 defined technical terms (RIN explained in context, part numbers listed)","Single compliance pathway with clear deadline","No cross-references to other legislation beyond the parent Civil Aviation Regulations","No conditional logic or exceptions — straightforward applicability test based on part numbers"],"plain_english_summary":"**What this is:** This is an Airworthiness Directive (AD) — a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) that requires specific maintenance action on certain Bell 412 helicopters.\n\n**Who it affects:** Owners and operators of Bell 412 series helicopters with specific part numbers for their main rotor mast and spline plate (the components that connect the main rotor blades to the helicopter's body).\n\n**What it requires:** Operators must follow the procedures in Bell Alert Service Bulletin 412-94-81 Revision A. This involves updating how they track the \"retirement life\" (the maximum safe usage limit) of two critical components:\n- The **main rotor mast** (the central shaft that spins the main rotor)\n- The **spline plate** (a component with teeth that transfers power to the rotor)\n\n**New retirement limits:**\n- For 412 and 412SP models: **10,000 flight hours** OR **80,000 Retirement Index Numbers (RIN)** — whichever comes first\n- For 412HP and 412EP models: **10,000 flight hours** OR **60,000 RIN** — whichever comes first\n\n(RIN is a fatigue-tracking system that counts stress cycles on components — more demanding flying conditions accumulate RIN faster than gentle cruising.)\n\n**When compliance is due:** Within 100 hours of flight time after 27 April 1995, or by 27 August 1995 — whichever happens first.\n\n**Why it matters:** This directive prevents catastrophic failure of the main rotor system. The manufacturer discovered the original retirement life calculations needed revision, and this AD ensures helicopters don't fly with overstressed critical components that could fail in flight."}},"importantCases":[],"_links":{"self":"/api/acts/ad-bell-412-36-main-rotor-mast","history":"/api/acts/ad-bell-412-36-main-rotor-mast/history","analysis":"/api/acts/ad-bell-412-36-main-rotor-mast/analysis","conflicts":"/api/acts/ad-bell-412-36-main-rotor-mast/conflicts","importantCases":"/api/acts/ad-bell-412-36-main-rotor-mast/important-cases","documents":"/api/acts/ad-bell-412-36-main-rotor-mast/documents"}}