{"id":"F2006B09161","name":"AD/DHC-6/73 - Wing Spar Strut Lug","slug":"ad-dhc-6-73-wing-spar-strut-lug","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":52874,"registerId":"commonwealth-F2006B09161-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"AIRWORTHINESS DIRECTIVE","sectionType":"part","heading":"AIRWORTHINESS DIRECTIVE","content":"# AIRWORTHINESS DIRECTIVE\n\n![](image.001.png)\n\n![](image.001.png)  \nFor the reasons set out in the background section, the CASA delegate whose signature appears below issues the following Airworthiness Directive (AD) under subregulation 39.1 (1) of CAR 1998. The AD requires that the action set out in the requirement section (being action that the delegate considers necessary to correct the unsafe condition) be taken in relation to the aircraft or aeronautical product mentioned in the applicability section: (a) in the circumstances mentioned in the requirement section; and (b) in accordance with the instructions set out in the requirement section; and (c) at the time mentioned in the compliance section.\n\nDHC-6 (Twin Otter) Series Aeroplanes","sortOrder":0},{"sectionNumber":"AD/DHC-6/73 Wing Spar Strut Lug 12/2002","sectionType":"part","heading":"AD/DHC-6/73 Wing Spar Strut Lug 12/2002","content":"# AD/DHC-6/73 Wing Spar Strut Lug 12/2002\n\nApplicability: Model DHC-6 Series 1, 100, and 200 aircraft, with PreMod 6/1117 wings that have accumulated less than 17,400 flight hours or 34,800 flight cycles.\n\nRequirement: 1. Conduct a high frequency eddy current inspection of the wing spar strut lug fastener holes. Perform the inspection in accordance with the wing spar strut lug fastener hole inspection procedure detailed in Part 3, Inspection Requirements Manual - PSM 1-6-7.\n\n    2. Assess the results and comply with any additional action required in accordance with the acceptance/rejection criteria detailed in the wing spar strut lug fastener hole inspection of Part 3, Inspection Requirements Manual - PSM 1-6-7.\n\n    3. If corrosion, cracking or damage is found in any fastener hole, report findings to Bombardier within 14 days of the inspection.\n\nNote: Transport Canada AD CF-2002-40 refers.\n\nCompliance: Within 12 months after 28 November 2002. Thereafter at intervals not to exceed 10 years.\n\nThis Airworthiness Directive becomes effective on 28 November 2002.\n\nBackground: Non-destructive testing inspection of the wing spar strut fastener holes of several aircraft with PreMod 6/1117 wings has revealed corrosion in the critical fastener holes. While the inspection of these critical holes is addressed under AD CF-2000-14 (AD/DHC-6/31), some low-utilisation aircraft have yet to reach the fatigue based inspection threshold of 17,400 flight hours or 34,800 flight cycles. Undetected corrosion in the critical fastener holes could lead to lug cracking, compromising the structural integrity of the wing.\n\n![](image.002.jpeg)  \n\nDavid Alan Villiers\n\n![](image.001.png)Delegate of the Civil Aviation Safety Authority 21 October 2002","sortOrder":1}],"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"},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"This AD is narrowly scoped to address an identified unsafe condition (corrosion in wing spar strut lug fastener holes) on a specific subset of DHC-6 aircraft. It prescribes inspections, assessment against set criteria, and reporting. The measure remains within the original safety-correction purpose described in the Background and does not broaden the regulatory reach beyond that corrective objective."},"complexity_factors":["Targeted applicability: 3 specific DHC-6 series with PreMod 6/1117 wings and conditional on being below 17,400 flight hours or 34,800 flight cycles (Applicability)","Three straightforward substantive requirements (inspect, assess, report) (Requirement 1–3)","Single procedural cross-reference: Part 3, Inspection Requirements Manual - PSM 1-6-7 (Requirement 1–2)","Single reporting deadline (14 days) for defects (Requirement 3)","Fixed compliance timing: initial within 12 months of 28 Nov 2002, then recurring every 10 years (Compliance)","Reference to a related foreign AD (Transport Canada AD CF-2002-40) but no additional substantive foreign-law cross-obligations (Note)","Limited number of conditional branches (main condition is the flight-hour/flight-cycle threshold)"],"plain_english_summary":"**What this directive does (mechanically)**\n\n- Applies to: De Havilland Canada DHC-6 (Twin Otter) Series 1, 100 and 200 aeroplanes that have PreMod 6/1117 wings and that have accumulated *less than* 17,400 flight hours or 34,800 flight cycles (Applicability).\n\n- Requires three actions (Requirement):\n  1. Perform a high-frequency eddy-current (non‑destructive) inspection of the wing spar strut lug fastener holes, following the procedure in Part 3 of the Inspection Requirements Manual (PSM 1-6-7) (Requirement 1).\n  2. Compare inspection results with the acceptance/rejection criteria in that same Part 3 and take any remedial action required by those criteria (Requirement 2).\n  3. If corrosion, cracking or other damage is found in any fastener hole, report the findings to Bombardier within 14 days of the inspection (Requirement 3).\n\n- Timing (Compliance): The first inspection must be done within 12 months after 28 November 2002; after that the inspection must be repeated at intervals not exceeding 10 years (Compliance).\n\n- Authority and purpose (Background): The Civil Aviation Safety Authority (CASA) delegate issued this Airworthiness Directive under the CAR 1998 safety regulation noted in the AD. The stated reason is that non‑destructive testing discovered corrosion in critical fastener holes on several PreMod 6/1117 wings and that undetected corrosion could lead to lug cracking and compromise wing structural integrity (Background). The AD is intended to correct that unsafe condition by mandating inspection and reporting.\n\n**Who this affects and who pays**\n\n- Primary cost-bearers: aircraft owners/operators of the listed DHC-6 models meeting the applicability criteria. They must arrange and pay for the inspections, any follow-up repairs, recordkeeping, and the 14‑day reporting to Bombardier (Requirement 1–3; Compliance).\n\n- Secondary effects: maintenance organisations and non‑destructive testing (NDT) providers may capture work to perform inspections and repairs. Transport of aircraft out of service for inspection or repair may create scheduling costs.\n\n**How it matters (trade-offs, implementation realities, incentives)**\n\n- Safety vs. operating cost: The directive imposes a safety procedure intended to reduce the risk of wing structural failure by finding corrosion early. That reduces catastrophic risk at the cost of inspection and potential repair downtime and expense (Background; Requirement).\n\n- Compliance burden and practicability: Inspecting lug fastener holes by high‑frequency eddy‑current requires qualified NDT equipment and personnel and access to the wing areas. Operators must ensure competence, maintain records of compliance, and meet the 14‑day reporting requirement if defects are found (Requirement 1–3).\n\n- Timing and frequency: The 12‑month initial window and then 10‑year recurring interval are relatively long intervals; operators with low utilisation are targeted because higher‑utilisation aircraft are addressed under a fatigue‑based inspection threshold cited in the Background (Background). The long interval lowers inspection frequency (and cost) but raises reliance on the inspection's thoroughness when performed.\n\n- Information flows and manufacturer involvement: The AD requires operators to report defects to Bombardier within 14 days, creating a channel for data collection by the manufacturer and regulators (Requirement 3). That creates a small administrative cost for operators and provides Bombardier with defect information useful for fleet-level assessment.\n\n- Potential market effects: The rule does not restrict ownership, operations, or markets directly. It imposes compliance costs on affected operators which are concentrated on those who still fall below the stated flight‑hour/flight‑cycle thresholds. Independent MROs may benefit from demand for inspections and repairs.\n\n**Cross-references and dependencies**\n\n- The inspection procedure and acceptance criteria are in Part 3 of the Inspection Requirements Manual — PSM 1-6-7. Compliance depends on access to and following that manual (Requirement 1–2).\n\n- The AD notes Transport Canada AD CF-2002-40 as related guidance (Note).\n\n**Risks, opportunity costs and unintended consequences to watch for**\n\n- Availability of qualified inspectors and equipment could delay compliance and aircraft return to service, increasing downtime costs.\n\n- If inspection thresholds or criteria are ambiguous in PSM 1-6-7, operators may face discretionary decisions about acceptable repairs or grounding, increasing administrative burden and potential disputes with regulators or manufacturers.\n\n- The 10‑year repeat interval concentrates inspection effort infrequently; any failure of inspection quality or recordkeeping could allow corrosion to progress for a long time before detection.\n\n**Who decides and enforcement**\n\n- CASA (through a delegate) mandated the AD under CAR 1998 (preamble text). Enforcement and acceptability of corrective action will be managed by the regulator and by compliance with the procedures in PSM 1-6-7.\n\n**Bottom line**\n\n- Mechanically, the AD requires periodic HF eddy‑current inspection of specified wing spar strut lug fastener holes (following PSM 1-6-7), assessment against acceptance criteria, and mandatory reporting of corrosion/cracks to Bombardier within 14 days (Requirement, Compliance, Background). The costs fall on owners/operators; the benefit is reduced risk of lug cracking and potential wing structural failure. The effect on markets is limited and concentrated (operators of the affected tail of the fleet), with incidental demand for NDT/MRO services."},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":false,"description":"This AD maintains its original narrow scope: addressing a specific structural integrity issue (corrosion in wing spar strut lug fastener holes) in specific DHC-6 aircraft variants. It does not expand beyond its original safety purpose."},"complexity_factors":["Specific applicability criteria requiring cross-checking of aircraft model, wing modification status, and flight hours/cycles","Technical inspection methodology requiring reference to external manual (PSM 1-6-7) not reproduced in the directive","Nested conditional logic: inspection → assessment → conditional reporting requirement","Reference to Canadian aviation regulation (Transport Canada AD CF-2002-40) indicating international regulatory coordination","Dual compliance timeline (initial 12-month window + recurring 10-year interval)"],"plain_english_summary":"CASA’s Airworthiness Directive applies to De Havilland DHC‑6 Twin Otter Series 1, 100 and 200 aircraft with pre‑Mod 6/1117 wings and below the hours/cycles threshold. It mandates high‑frequency eddy‑current inspection of wing‑strut attachment bolt holes per PSM 1‑6‑7, with any corrosion or cracking reported to Bombardier. First inspection was due within 12 months of 28 Nov 2002 and repeats every 10 years. It prevents undetected corrosion that could lead to wing‑strut failure."}},"importantCases":[],"_links":{"self":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug","history":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug/history","analysis":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug/analysis","conflicts":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug/conflicts","importantCases":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug/important-cases","documents":"/api/acts/ad-dhc-6-73-wing-spar-strut-lug/documents"}}