{"id":"F2006B02254","name":"AD/AL 250/41 Amdt 5 - Third Stage Turbine Wheel - Inspection and RPM Operational Restrictions","slug":"ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41484,"registerId":"commonwealth-F2006B02254-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AL 250/41 Amdt 5 - Third Stage Turbine Wheel - Inspection and RPM Operational Restrictions","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 106 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nAllison Turbine Engines \\- 250 Series\n\nAD/AL 250/41\n\nAmdt 5\n\n  \n\nThird Stage Turbine Wheel \\- Inspection and RPM Operational Restrictions\n\n  \n5/84\n\nApplicability: All models 250-C20, C20B, C20F, B17, B17B, B17C and B17D not fitted with solid blade shroud third stage turbine wheels.\n\nDocument: 1. Allison 250-C20, C20B, C20F, CEB-A-1174 Rev. 3; or\n\n2. Allison 250-B17, B17B, B17C B17D, CEB-A-1146 Rev. 3.\n\nNote: FAA AD 83-03-02 R1 also refers.\n\nIn addition, until all affected turbine wheels have been replaced with solid blade shroud wheels, it will be necessary to install a placard on or adjacent to the aircraft installed N2 tacho indicator, prohibiting sustained operation between 90 and 98% N2.\n\nAnd\n\nFor C20, C20B, C20F powered aircraft only install a placard in clear view of the pilot stating that:\n\n“DURING ALL GROUND OPERATION, THE ENGINE N1 SPEED MUST BE MAINTAINED AT GROUND IDLE EXCEPT DURING TRANSIENT OPERATION, WHEN PERFORMING REQUIRED OPERATIONAL CHECKS, IN HIGH OR GUSTY WIND CONDITIONS OR WHERE SAFETY WOULD BE ADVERSELY AFFECTED.”\n\nAnd\n\nFor B17, B17B, B17C B17D powered aircraft only install a placard in clear view of the pilot stating that:\n\n“DURING ALL GROUND OPERATION, THE ENGINE N2 SPEED MUST BE MAINTAINED BELOW 90%N2 RPM EXCEPT DURING TRANSIENT OPERATION, WHEN PERFORMANCE REQUIRED OPERATIONAL CHECKS IN HIGH OR GUSTY WIND CONDITIONS OR WHERE SAFETY WOULD BE ADVERSELY AFFECTED.”\n\nCompliance: In accordance with the reference document.\n\nBackground: This Directive is raised in issue to ensure compliance with Revision 3 of the manufacturers document which is a complete reissue. It now requires replacement of non-sold blade shroud wheels on a tighter phased schedule and it is no longer allowable to re-install any superseded wheel.\n\nInstallation of third stage turbine wheel with solid blade shroud deletes the engine operating speed restrictions imposed by this Directive.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 106 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES","sortOrder":0}],"analysis":{"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"},"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":true,"description":"Yes. The instrument (Revision 3 implemented by this Directive) tightens the original scope by requiring a more accelerated phased replacement programme for non-solid blade shroud third stage turbine wheels and by prohibiting the re-installation of any superseded wheel. The Background explicitly states these changes: replacement schedule is tighter and reinstating older wheels is no longer allowed. Mechanically, the Directive adds temporary operational restrictions (placards and N1/N2 limits) until replacement occurs and makes the only permanent removal of those restrictions contingent on installing solid blade shroud wheels. These additions expand the directive from a guidance/inspection focus to an enforceable operational-limitation plus mandatory replacement regime, increasing compliance costs, supply demand for replacement wheels, and enforcement needs."},"complexity_factors":["Applies to multiple engine models (C20 family and B17 family) with different, model-specific placard text","Requires compliance with external manufacturer documents (CEB-A-1174 Rev.3 and CEB-A-1146 Rev.3) rather than self-contained procedural steps","Conditional rule: temporary operating restrictions apply only 'until' the affected wheel is replaced with a solid blade shroud","Prohibition on reinstalling superseded wheels (a policy change referenced in Background) increases procedural strictness","Cross-reference to FAA AD 83-03-02 R1 and manufacturer's revision history introduces external cross-jurisdictional material","Several short exceptions to the placard-based operational limits (transient operation, required checks, high/gusty winds, safety) create operational discretion","Key operational term 'sustained operation' is used without a precise definition in the text provided, leaving room for interpretive variation"],"plain_english_summary":"**What this directive does (mechanics)\n\n- Applies to: certain Allison 250 turbine engines — models 250-C20, C20B, C20F, and B17/B17B/B17C/B17D — but only those that do not already have a \"solid blade shroud\" third stage turbine wheel (see \"Applicability\").\n\n- Inspections and replacements: The directive requires operators to follow the manufacturer documents listed (Allison CEB-A-1174 Rev.3 for C20-series or CEB-A-1146 Rev.3 for B17-series) for inspection and replacement of third stage turbine wheels. Compliance is to be carried out in accordance with those documents (see \"Compliance\").\n\n- Temporary operational limits until replacement: Until an affected turbine wheel is replaced with a solid blade shroud wheel, the directive requires a placard (a visible label) to be installed at the N2 tachometer warning pilots and maintenance staff that sustained engine operation between 90% and 98% N2 is prohibited (see paragraph beginning \"In addition, until...\").\n\n- Model-specific ground-speed placards: In addition, there are model-specific placard requirements placed in view of the pilot:\n  - For C20-series aircraft: a placard telling crews to keep N1 at ground idle during all ground operation except for short transients, required checks, high/gusty winds, or safety reasons.\n  - For B17-series aircraft: a placard telling crews to keep N2 below 90% during ground operations except for the same limited situations.\n\n- When limits end: Installing a third stage turbine wheel that has a solid blade shroud removes (deletes) the operating speed restrictions imposed by this directive (see \"Background\").\n\n**Who is affected\n\n- Aircraft owners/operators and pilots of aircraft powered by the listed Allison 250 engine models that still have non‑solid‑shroud third stage turbine wheels.\n- Maintenance organisations and parts suppliers who inspect, repair, or replace third stage turbine wheels on these engine models.\n\n**Why it matters (stated purpose and operational effect)\n\n- The directive implements Revision 3 of the manufacturer's service documents, which requires a phased replacement programme of the older (non‑solid shroud) third stage turbine wheels and disallows reinstalling superseded wheels (see \"Background\").\n- Mechanically, it reduces the risk associated with the older wheel design by restricting engine speed bands and mandating replacement; the temporary placards and operational limits are the immediate mitigation while units are being replaced.\n\n**Costs, incentives, trade‑offs and implementation points to watch\n\n- Who pays: Aircraft owners/operators pay inspection, placard installation (minimal), downtime and the cost of replacement turbine wheels and fitting (section: \"Compliance\" and \"Background\").\n- Behaviour change: Pilots must avoid sustained operation in a specified N2 band (90–98% N2) and follow model‑specific ground speed limits; maintenance providers must inspect and replace wheels per the referenced Allison documents (\"In addition...\" and model placard paragraphs).\n- Incentives: Because installing a solid blade shroud wheel removes the speed restrictions, operators are financially incentivised to source and fit the replacement wheel to restore full engine operating flexibility (see \"Background\").\n- Implementation risk and compliance burden: The directive relies on operators actually fitting the placards and carrying out replacements per the external Allison documents. That creates enforcement and verification needs for regulators and increases workload for maintenance providers (see \"Compliance\").\n- Supply and scheduling effects: A tightened phased replacement schedule (referenced in \"Background\") can concentrate demand for replacement wheels and skilled installation slots in a short period, raising costs and downtime for operators. The prohibition on reinstalling superseded wheels further constrains options for cheaper used parts.\n- Operational trade‑offs: The engine speed restrictions may reduce operational flexibility (for example, certain ground checks, runups or taxi procedures), or lead operators to alter normal procedures to stay below the restricted bands. Exceptions for transient checks, gusty conditions or safety preserve essential operations but leave some discretion to the pilot.\n- Effects on independent businesses: Independent maintenance organisations must comply with the inspection and replacement schedule and may bear costs sourcing approved solid shroud wheels; they may gain business from retrofit demand but face procurement and certification constraints.\n\n**Concrete mechanism for change\n\n- The short‑term mitigation is procedural: install placards and observe N1/N2 restrictions during ground or sustained operation (see model‑specific placard text and the N2 tacho placard requirement).\n- The medium‑term fix is physical: replace the third stage turbine wheel with the solid blade shroud design. Once the replacement is installed, the speed limits are removed (see \"Background\").\n\n**Source attribution\n\n- The directive bases compliance on two specific Allison service documents (Allison CEB‑A‑1174 Rev.3 and CEB‑A‑1146 Rev.3) and references related FAA guidance (note: FAA AD 83‑03‑02 R1 is mentioned). The background text explicitly states the change in requirements compared with earlier revisions (see \"Background\")."},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":false,"description":"This amendment (Amdt 5) remains tightly focused on its original purpose: preventing third-stage turbine wheel failures in Allison 250 engines. While it tightens the replacement schedule and removes the option to reinstall old wheels, this represents a strengthening of existing safety controls rather than scope creep. The directive has not expanded to cover unrelated engine components or aircraft systems."},"complexity_factors":["Short, single-purpose directive focused on one mechanical component","Only 7 engine models affected, clearly listed","Two distinct operational restriction sets based on engine sub-series (C20 vs B17)","Simple conditional logic: 'until replacement completed, do X'","References external manufacturer documents (CEB-A-1174 Rev. 3 and CEB-A-1146 Rev. 3) for detailed compliance procedures","Minimal defined terms — relies on standard aviation terminology (N1, N2, RPM, placard)","No nested exceptions or complex cross-referencing within the text itself"],"plain_english_summary":"This CASA Airworthiness Directive requires operators of certain Allison 250 engines (C20 and B17 families without the solid blade shroud) to follow revised manufacturer guidance to inspect and replace older third‑stage turbine wheels on an accelerated schedule; removed wheels cannot be reinstalled.  \n\nUntil replacement, stricter ground‑running RPM limits and a cockpit placard are mandated to reduce the risk of turbine‑wheel failure that could destroy the engine and cause loss of aircraft control."}},"importantCases":[],"_links":{"self":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions","history":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions/history","analysis":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions/analysis","conflicts":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions/conflicts","importantCases":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions/important-cases","documents":"/api/acts/ad-al-250-41-amdt-5-third-stage-turbine-wheel-inspection-and-rpm-operational-restrictions/documents"}}