{"id":"F2006B09156","name":"AD/DHC-6/69 Amdt 1 - Corrosion Prevention and Control","slug":"ad-dhc-6-69-amdt-1-corrosion-prevention-and-control","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":52853,"registerId":"commonwealth-F2006B09156-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/DHC-6/69 Amdt 1 - Corrosion Prevention and Control","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)  \nDHC-6 (Twin Otter) Series Aeroplanes\n\nAD/DHC-6/69\n\nAmdt 1\n\n  \nCorrosion Prevention and Control 13/99\n\nApplicability: All Model DHC-6 aircraft.\n\nRequirement: To maintain corrosion to Level 1 or better, accomplish the following:\n\n    Develop a schedule for accomplishing the Corrosion Tasks in accordance with Requirement 2 of this Directive.\n\n    2. In accordance with the following schedule, accomplish all Corrosion Tasks specified in Part 3 of the de Havilland DHC-6 Twin Otter Corrosion Prevention and Control Manual, PSM 1-6-5, Revision 1, dated 16 December 1998 (hereafter referred to as “the Manual”), by performing the seven basic tasks defined at paragraph 3.0 of the Manual.\n\nThe final deadline and rate of initial complete aircraft inspections are specified in the following table:\n\nApplicable Aircraft Serial Numbers\n\n  \n\nAccomplishment Deadline for All Aircraft in Applicable S/N Range\n\n  \n\nMinimum Rate of Initial Inspections of Aircraft in Applicable S/N Range\n\n001 to 199 31 December 1995 All aircraft\n\n200 to 439 31 December 1996 50% or one (1) aircraft per calendar\n\nyear, whichever is greater\n\n440 to 659 31 December 1998 25% or one (1) aircraft per calendar\n\nyear, whichever is greater\n\n660 to 819 31 December 1999 20% or one (1) aircraft per calendar\n\nyear, whichever is greater\n\n820 to 844 31 December 2002 Commencing 1 January 1999, 25% or\n\none (1) aircraft per calendar year, whichever is greater\n\nOnce the first CT commences, the aircraft is not to be returned to service until all CTs, excluding the float inspections, have been completed and any resulting rework and parts replacement have been completed. Floats that were on an aircraft just prior to the aircraft being inspected in accordance with the Manual do not have to be inspected simultaneously with that same aircraft; however, the aircraft cannot be returned to service with floats that have not had their applicable CTs completed and any resulting rework and parts replacement completed.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\nWhen performing CTs with less than a five year Repeat Interval (R) after the initial inspection of the aircraft, all CTs with the same R interval are to be completed during one maintenance visit and any resulting rework and parts replacement must be performed before returning the aircraft or floats to service.\n\nCorrosion is to be addressed in accordance with the references specified in paragraph\n\n4.0 of Part 3 of the Manual.\n\nCorrosion level determinations are to be based on the definitions contained in Part 1 of the Manual.\n\nCorrosion classified as Level 1 need not be reported.\n\nLevel 2 corrosion may be consolidated with a quarterly report of corrective action, and submitted to the Authority District (Airport) Office under cover of a single MDR.\n\nLevel 3 corrosion shall be reported by MDR.\n\nNote: Transport Canada AD CF-94-12R1 refers.\n\nCompliance: 1. Prior to 31 January 1995.\n\n2. Commencing 1 February 1995.\n\nThe compliance of the original issue of this Directive remains unchanged. This Amendment becomes effective on 30 December 1999.\n\nBackground: As aircraft become older, they are more likely to exhibit indications of corrosion.\n\nTransport Canada Aviation, in conjunction with other airworthiness authorities, has committed itself to ensuring that additional maintenance programs for older aircraft are developed and implemented to minimise and control corrosive deterioration that could jeopardise airworthiness.\n\nAmendment 1 is issued in response to revision of the related Transport Canada AD which introduces Revision 1 of the CPC Manual which deletes the requirement for repetitive inspections after initial compliance. Transport Canada AD CF-99-11 (AD/DHC-6/72) now provides for the requirement to perform the inspections a second time, but on a more flexible schedule than required by the original issue of this Directive.\n\nThe original issue of this Directive became effective on 8 December 1994.","sortOrder":0}],"analysis":{"flash_summary_failed":{"failed":true,"reason":"Unauthenticated. Configure AI_GATEWAY_API_KEY or use a provider module. Learn more: https://ai-sdk.dev/unauthenticated-ai-gateway","source":"analysis-cron"},"kimi_summary":{"content_quality":"ok","complexity_score":4,"scope_assessment":{"changed":false,"description":"This Amendment 1 remains focused on its original purpose of corrosion prevention and control for DHC-6 aircraft. While it modifies the compliance structure (removing repetitive inspection requirements and shifting them to a separate directive), it does not expand beyond the original scope of ensuring corrosion is maintained at Level 1 or better. The amendment simply updates the method of compliance to align with revised Canadian standards."},"complexity_factors":["Multiple compliance deadlines based on aircraft serial number ranges (5 different categories)","Cross-reference to external technical manual (PSM 1-6-5) not reproduced in the directive itself","Nested conditional logic for inspection rates ('whichever is greater' calculations)","Specific exceptions for float inspections and maintenance visit consolidation rules","Three-tier corrosion classification system with different reporting requirements","Amendment structure requires understanding of 'original issue' vs Amendment 1 changes","Reference to related Canadian Airworthiness Directives (CF-94-12R1, CF-99-11) for context"],"plain_english_summary":"This AD requires owners/operators of de Havilland DHC-6 Twin Otter aircraft in Australia to implement a mandated Corrosion Prevention and Control Program (per PSM 1‑6‑5 Rev 1) with a scheduled inspection regime and deadlines (initial inspections phased by serial number between Dec 1995 and Dec 2002). Operators must inspect increasing portions of their fleets until all aircraft are checked, complete required corrosion work before return to service, and report serious corrosion promptly.\n\nIt ensures aging Twin Otters are maintained to prevent structural failures and align with international safety standards."},"flash_summary":{"complexity_score":6,"scope_assessment":{"changed":true,"description":"This amendment changes the original Directive's inspection regime by aligning it with the revised Transport Canada AD and the Manual Revision 1. The change removes the original rigid requirement for repetitive inspections after initial compliance and replaces it with a more flexible scheduling approach (Transport Canada AD CF-99-11 referenced in Background). Mechanically, the Directive now: (a) mandates initial inspections according to serial-number deadlines and minimum rates, (b) requires completion of specified CTs per the updated Manual, and (c) introduces a combined-R rule and updated reporting rules. In other words, the Directive has shifted the timing and repeat-frequency structure from the original issue to a different, more flexible structure while retaining mandatory completion and reporting obligations; this is a meaningful but targeted change in scope rather than a wholesale expansion of subject matter."},"complexity_factors":["Cross-reference to an external technical manual (de Havilland DHC-6 Corrosion Prevention and Control Manual, PSM 1-6-5 Rev 1) for the substantive inspection tasks and definitions","Table of five serial-number ranges, each with its own completion deadline and differing minimum initial inspection rates","Operational exception for floats (floats may be inspected separately but cannot be left uninspected when aircraft returned to service)","Conditional rule requiring all CTs with the same Repeat Interval (<5 years) to be completed in a single maintenance visit","Differentiated reporting obligations by corrosion level: Level 1 (no report), Level 2 (quarterly consolidated MDR allowed), Level 3 (immediate MDR)","References to multiple parts and paragraphs of the Manual (Part 1 for definitions; Part 3 for tasks; paragraph 3.0 for the seven basic tasks; paragraph 4.0 for references), which requires the reader to consult the Manual to apply the Directive","Staged compliance dates (initial administrative deadline and commencement date) and an amendment that alters inspection frequency expectations relative to the original Directive","Operational return-to-service prerequisite (once CTs commence) which creates sequencing and scheduling complexity for maintenance planners"],"plain_english_summary":"**What this directive does (mechanics)\n\n- Applies to: all de Havilland DHC-6 (Twin Otter) aeroplanes (Applicability).\n\n- Core requirement: operators must keep corrosion on each aircraft at \"Level 1 or better\" by developing a schedule and carrying out the Corrosion Tasks (CTs) listed in the de Havilland DHC-6 Corrosion Prevention and Control Manual (PSM 1-6-5, Revision 1, dated 16 December 1998). The Manual defines the seven basic CTs (Requirement, Part 3; paragraph 3.0 of the Manual).\n\n- Deadlines and minimum initial inspection rates are set by serial-number ranges. For each serial-number block the Directive sets a final completion date and a minimum rate of initial inspections (Table of applicable aircraft S/N ranges and deadlines).\n\n- Operational restriction: once the first CT on an aircraft begins, the aircraft must not be returned to service until all CTs (except float inspections) and any resulting rework or parts replacement are completed. Floats (if fitted) can be inspected separately, but an aircraft may not be returned to service with floats that still require CTs or repairs (Operational restriction paragraph).\n\n- When CTs with a Repeat Interval (R) of less than five years fall due after the initial inspection, all CTs with that same R must be completed in the same maintenance visit and any resulting rework and parts replacement completed before return to service (combined-R rule).\n\n- Corrosion levels and reporting: the Manual's Part 1 defines corrosion Levels 1–3. Level 1 need not be reported. Level 2 may be consolidated into a quarterly corrective-action report and submitted to the Authority District (Airport) Office under a single Maintenance Defect Report (MDR). Level 3 must be reported by MDR (Reporting rules).\n\n- Compliance timing: initial administrative step before 31 January 1995 and substantive tasks commencing 1 February 1995; the Amendment became effective on 30 December 1999 (Compliance).\n\n**Who this affects and who pays\n\n- Aircraft owners/operators of DHC-6 aircraft must develop the inspection/maintenance schedule, perform the CTs, fund any rework and parts replacement, and manage the required reporting (Requirement; Operational restriction; Reporting rules).\n\n- Maintenance organisations and parts suppliers will perform the work and receive the business (costs to operators, revenue to maintainers).\n\n- The civil aviation authority (the regulator) receives reports (MDRs) and enforces compliance (Reporting rules).\n\n**Why it matters (stated purpose and practical implications)\n\n- Stated purpose: to maintain corrosion at Level 1 or better so that corrosion does not jeopardise airworthiness; the Background connects the Directive to Transport Canada’s related AD and a general concern that older aircraft show more corrosion.\n\n- Practical effects and trade‑offs: operators of older DHC-6s face upfront inspection costs, possible aircraft downtime while CTs and rework are completed, and ongoing scheduling and reporting obligations. Maintenance workload will be concentrated around the required initial inspections and to the extent Repeat Intervals cluster. The Directive changes inspection timing and repeat requirements relative to the original issue (see Background), which shifts some scheduling flexibility but keeps the obligation to complete CTs before return to service.\n\n**Tests of the stated purpose against costs, incentives and risks\n\n- Who decides: operators decide the detailed schedule to meet the Directive (they must \"develop a schedule\"), but the Manual prescribes the CTs to be performed (Requirement 1–2; Manual references).\n\n- Who pays: operators pay for labour, parts and downtime. Maintenance providers gain work; costs concentrate on operators of older serial-number aircraft because of the staged deadlines.\n\n- Incentives and substitution: operators may choose between increased maintenance expenditure, earlier retirement of heavily corroded airframes, outsourcing inspections/repairs, or redistribution of flying tasks among less-affected aircraft.\n\n- Compliance burden and enforcement: record‑keeping and MDR reporting are required (Level 2 and 3 reporting rules). The restriction on returning aircraft to service until CTs/rework are finished creates a clear operational compliance lever; it also creates potential opportunity costs from aircraft downtime.\n\n- Implementation risks: concentrated inspection demand (per the table) could create maintenance bottlenecks and parts shortages; the combined-R rule (complete all CTs with same R in one visit) can increase a single-visit scope and downtime; there is some administrative complexity about floats being inspected separately but still subject to the 'no return to service' rule (Operational restriction).\n\n**References within the Directive**\n\n- Applicability; Requirement (develop schedule, perform CTs per Manual PSM 1-6-5 Rev 1, paragraph 3.0).\n- Serial-number table with deadlines and minimum inspection rates.\n- Operational restriction on return to service and float inspection rule.\n- Combined-R rule for CTs with Repeat Intervals < 5 years.\n- Corrosion-level definitions (Manual Part 1) and reporting rules for Levels 1–3.\n- Compliance dates and Background connecting this AD to Transport Canada ADs."}},"importantCases":[],"_links":{"self":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control","history":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control/history","analysis":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control/analysis","conflicts":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control/conflicts","importantCases":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control/important-cases","documents":"/api/acts/ad-dhc-6-69-amdt-1-corrosion-prevention-and-control/documents"}}