{"id":"F2006B02147","name":"AD/AC/86 Amdt 1 - Wing Main Spar Lower Cap Microstructure","slug":"ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41377,"registerId":"commonwealth-F2006B02147-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AC/86 Amdt 1 - Wing Main Spar Lower Cap Microstructure","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)  \nTwin Commander (Gulfstream/Rockwell/ Aerocommander 500, 600 and 700) Series Aeroplanes\n\nAD/AC/86\n\nAmdt 1\n\n  \nWing Main Spar Lower Cap Microstructure 4/93\n\nApplicability: All wing main spars fitted to 500, 600 and 700 series aeroplanes.\n\nRequirement: Metallographically polish, etch, and inspect a machined surface of the main spar lower cap in a manner approved by the CAA. Any scheme to be approved must recognise that this inspection requires a trained person to interpret the microstructure. The CAA may be able to provide a qualified technical officer to perform the inspection. If the metal grains are large and elongated, as is usual for an extrusion, no further action is required by this directive. If the metal grains are small and equi- axed, indicating re-crystallisation has occurred, replace the spar cap before further flight.\n\nNote: Twin Commander Service Bulletin 215 refers to a similar inspection requirement.\n\nCompliance: Inspect the microstructure before 30 April 1992, and whenever a new spar cap is installed. This amendment becomes effective 1 April 1993.\n\nBackground: The CAA has discovered that some wing main spar lower caps were made from extrusion stock which has an abnormal re-crystallised microstructure. This makes them very susceptible to stress corrosion cracking. Affected spar caps cannot be identified from production records. Amendment 1 specifies that affected spar caps must be replaced. Recent analyses have demonstrated the impracticability of controlling this stress corrosion cracking safely by inspections.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":5,"scope_assessment":{"changed":true,"description":"Amendment 1 shifts from an inspection‑based control to a mandatory replacement rule where recrystallisation is detected, requires CAA approval of inspection schemes (and offers CAA assistance), sets inspection timing and makes clear inspection alone is impracticable to control stress‑corrosion cracking (Requirement; Compliance; Background)."},"complexity_factors":["Technical metallurgy: requires metallographic polishing, etching and microstructural interpretation (Requirement).","Skill dependency: inspection must be read by a trained person; CAA may supply a qualified officer (Requirement).","Regulatory discretion: CAA approval required for inspection schemes and may assist (Requirement).","Remedial mandate: replacement is compulsory if re‑crystallisation is present, not merely increased monitoring (Requirement).","Traceability gap: affected spar caps cannot be identified from production records, creating an inspection‑wide scope (Background).","Compliance logistics: one‑off deadline plus requirement on installation of any new spar cap (Compliance)."],"plain_english_summary":"This directive requires operators of Twin Commander 500, 600 and 700 series aeroplanes to metallographically inspect the wing main spar lower cap microstructure and to replace the spar cap before further flight if re‑crystallisation is found (Applicability; Requirement; Compliance). The Civil Aviation Authority must approve inspection schemes and may provide a qualified officer; the amendment mandates replacement because analysis showed inspection cannot reliably prevent stress‑corrosion cracking where re‑crystallisation has occurred (Requirement; Background)."},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":true,"description":"The original AD/AC/86 likely permitted continued operation with periodic inspections for stress corrosion cracking. Amendment 1 significantly narrows the scope by mandating replacement of affected spar caps rather than allowing ongoing monitoring, reflecting a shift from risk management to risk elimination after safety analysis showed inspection-based controls were impracticable."},"complexity_factors":["Single technical requirement with clear conditional logic (if-then-else structure based on grain structure)","Limited cross-referencing (one reference to Twin Commander Service Bulletin 215)","No defined terms section — technical terms (e.g., 'equi-axed', 're-crystallisation') are explained in context","Short document (approximately 250 words)","Single amendment to existing directive rather than complex legislative instrument","Technical complexity is moderate (requires metallurgical expertise) but legal/structural complexity is low"],"plain_english_summary":"This CASA airworthiness directive requires owners, operators and maintenance organisations of Twin Commander 500/600/700 series aeroplanes to inspect wing main spar lower caps for metallurgical re‑crystallisation. Samples are polished and chemically treated for microscopic grain‑structure examination.\n\nIf small, rounded grains (re‑crystallised metal) are found, the spar cap must be replaced before flight because such material is highly susceptible to stress‑corrosion cracking. The rule arose from untraceable defective stock and supersedes earlier inspection-only measures."},"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"}},"importantCases":[],"_links":{"self":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure","history":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure/history","analysis":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure/analysis","conflicts":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure/conflicts","importantCases":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure/important-cases","documents":"/api/acts/ad-ac-86-amdt-1-wing-main-spar-lower-cap-microstructure/documents"}}