{"id":"F2006B02148","name":"AD/AC/87 - Wing Main Spar Steel Strap Corrosion","slug":"ad-ac-87-wing-main-spar-steel-strap-corrosion","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41378,"registerId":"commonwealth-F2006B02148-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AC/87 - Wing Main Spar Steel Strap Corrosion","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/87 Wing Main Spar Steel Strap Corrosion 4/92 DM\n\nApplicability: All 500, 600 and 700 series with the original factory-fitted internal steel strap which reinforces the main spar lower cap. The steel strap was fitted in aircraft serial numbers 1731 and subsequent, but not in the 690C and 695 series which featured an internal aluminium alloy strap.\n\nNote: This excludes aircraft that have had the steel strap replaced, on the assumption that all replacement straps were 301 stainless.\n\nRequirement: 1. Determine whether the strap is made from 4130 or 301 (stainless) steel. The following simple method is suggested.\n\nRun a magnet along the wing lower surface skin, under the main spar, between the fuselage and the engine nacelle. If there is a strong magnetic attraction the strap is made from 4130 steel. Since the 4130 strap is prone to corrosion, it must be inspected in accordance with paragraph 2.\n\nIf there is no magnetic attraction the strap is made from 301 stainless steel, which is not as prone to corrosion, and does not have to be inspected. (Note that this does not obviate the need for the aluminium alloy spar caps to be inspected for corrosion under AD/AC/83).\n\n    2. Gain access to the 4130 steel strap inside the wing, removing fuel cells and lifting filler blocks as necessary. Visually inspect the exposed areas of the steel strap (in between the main spar lower cap and the next stringer aft) for corrosion, between the side of the fuselage and the engine nacelle.\n\nIf corrosion is found, determine the full extent of the corroded area on the remainder of the strap.\n\nProtect the steel strap in the area opened up for inspection by liberally applying an inhibitor conforming to MIL-C-0081309C or MIL-C-16173D.\n\n    3. Inspect the seals on all upper surface access panels through which water may reach the steel strap. They should be water tight.\n\n    4. Report defects using the normal Major Defect reporting System and in addition organisations are requested to report the steel strap material and whether or not corrosion was found in the 4130 steel straps to Mr Dick Hewitt on (06) 268 4456.\n\nNote: This aircraft has a history of wing fatigue problems. Repair of the steel strap, or the alloy spar cap, or any other primary structure, or continued flight with the defective straps on caps, must be supported by a though fatigue and residual strength evaluation.\n\nCompliance: Complete the inspections not later than 31 March 1992. If micro structure of the main spar lower cap is susceptable to stress corrosion (AD/AC/86 refers) repair or replace the defective straps before further flight.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\nBackground: Very severe corrosion has been found in the internal steel strap of an Australian- registered aircraft. The strap was made from 4130 steel, not 301 stainless as the design records indicate. Twin Commander has advised that there may be up to 200 other aircraft with 4130 steel straps, but there are unfortunately no production records. At least one corroded 4130 has be found in the USA.\n\nThe 4130 steel straps, unprotected, are very prone to corrosion. A corroded strap could seriously degrade the durability of the main spar assembly.\n\nThis Directive has been issued to protect the Australian fleet in the short term. Twin Commander are developing a more permanent solution, to be published in a forthcoming service bulletin.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive is narrowly focused on identifying and addressing corrosion in incorrectly supplied 4130 steel internal straps and was issued as a short‑term protective measure pending a manufacturer service bulletin. The instrument remains within that original, limited safety‑remedial purpose: it prescribes inspection, local protection, possible repair/replacement, panel/seal checks, and reporting. It does not expand into unrelated regulatory areas or introduce new ongoing regulatory programs, so the scope has not significantly changed beyond its original intent (see Background and Requirements paragraphs 1–4)."},"complexity_factors":["Short text focused on a single technical problem (corrosion of an internal strap)","Few conditional branches: magnet test -> either no further action (301 stainless) or mandatory internal inspection (4130)","Cross-references to two other ADs (AD/AC/83 and AD/AC/86)","Operational complexity arises from access requirements (remove fuel cells, lift filler blocks) rather than legal complexity","A small number of technical specifications (approved inhibitors MIL-C-0081309C or MIL-C-16173D) and a single compliance deadline","Reporting requirement to both Major Defect system and a named contact"],"plain_english_summary":"**What this directive does, in plain English**\n\n- **Who it applies to:** Owners/operators of Twin Commander (Gulfstream/Rockwell/Aerocommander) 500, 600 and 700 series aeroplanes that still have the original factory-fitted internal steel strap reinforcing the main spar lower cap (fitted from serial number 1731 onward). Aircraft that have had that steel strap replaced (assumed to be 301 stainless replacements) and the 690C/695 series (which used an aluminium internal strap) are excluded.\n\n- **What the rule requires (mechanical steps):**\n  1. Identify the strap material. A simple suggested test is to run a magnet along the wing lower skin under the main spar, between the fuselage and engine nacelle. If the magnet is strongly attracted, the strap is 4130 steel (magnetic); if there is no attraction, it is 301 stainless (non‑magnetic). (See paragraph 1.)\n  2. If the strap is 4130 steel, gain access to the strap (remove fuel cells and filler blocks if needed) and visually inspect the exposed strap area between the main spar lower cap and the next stringer aft, between the fuselage and engine nacelle. If corrosion is found, determine the full extent of corrosion and protect the exposed area with an approved inhibitor (MIL-C-0081309C or MIL-C-16173D). (See paragraph 2.)\n  3. Check seals on all upper surface access panels that could let water reach the strap; they must be watertight. (See paragraph 3.)\n  4. Report defects using the Major Defect reporting system and additionally provide the strap material and corrosion findings to the named contact (see paragraph 4).\n\n- **When compliance is required:** Complete the inspections by 31 March 1992. If the spar lower cap microstructure is susceptible to stress corrosion (cross-reference AD/AC/86), repair or replace defective straps before further flight.\n\n- **Why this matters (official rationale and the directive's stated purpose):** The directive responds to discovery of very severe corrosion in an internal steel strap of an Australian‑registered aircraft where the strap was 4130 steel instead of the design’s 301 stainless. 4130 steel straps are prone to corrosion; a corroded strap can weaken the main spar assembly and reduce structural durability. The directive is a short‑term protective measure while the manufacturer develops a permanent fix (to be issued as a service bulletin). (See Background.)\n\n- **Who pays and who decides:** Operators/owners pay for the inspection access work (removing fuel cells, opening panels), any corrosion remediation, inhibitors, and any repairs or replacements. The Civil Aviation Safety Authority (CASA) mandates the inspection and reporting; Twin Commander is developing a longer‑term technical solution (service bulletin).\n\n- **Practical effects and trade-offs:**\n  - Direct costs to operators: labour and downtime to open wings, remove/replace fuel cells and filler blocks, carry out inspections and any repairs or replacements, and to apply corrosion inhibitor where required (paragraphs 2–3).  \n  - Incentives: Operators with older aircraft face immediate inspection and possible repair costs; replacing suspect straps with 301 stainless removes future inspection burden but requires up‑front expense.  \n  - Implementation risks: Accessing internal straps requires qualified maintenance work and safe handling of fuel system components; incomplete access could miss corrosion.  \n  - Compliance burden: The rule prescribes a simple identification test (magnet) but then requires intrusive access if the strap is 4130. Reporting requirements increase administrative load.  \n  - Market effects: The directive imposes costs on aircraft owners/operators and on maintenance providers (workload), but does not alter ownership, operating rules beyond the inspection mandate, or competitive permissions. It may increase demand for parts and qualified maintenance labour.\n\n- **Other notes and cross‑references:** The directive cross‑references AD/AC/83 (inspection of aluminium alloy spar caps for corrosion) and AD/AC/86 (stress corrosion susceptibility of main spar lower cap microstructure) for related inspection/repair obligations. The directive explicitly treats this measure as short‑term while the manufacturer develops a permanent repair (Background)."},"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":3,"scope_assessment":{"changed":false,"description":"This Airworthiness Directive maintains its original narrow scope: addressing a specific safety defect in a specific aircraft type. The background section explains the immediate safety imperative and notes that a permanent solution is being developed separately, but this does not expand the directive's operational scope beyond the urgent inspection requirement."},"complexity_factors":["Single, specific technical subject matter (one aircraft part on specific models)","Simple conditional logic (magnet test determines inspection requirement)","Clear, step-by-step procedural requirements (numbered paragraphs 1-4)","Limited cross-references (only references to related ADs AD/AC/83 and AD/AC/86)","Straightforward applicability section with one exclusion (690C/695 series)","Plain language explanations of technical concepts (magnet test described simply)","No defined terms section or complex interpretive provisions"],"plain_english_summary":"Owners/operators of Twin Commander 500/600/700 series (factory-fitted steel strap, s/n 1731+) must urgently inspect the wing main-spar steel strap because some straps were made from corrosion-prone 4130 steel instead of stainless 301. A strong magnet on the wing indicates 4130 and triggers inspection.\n\nIf 4130 is present, open access, check for corrosion, apply anti‑corrosion treatment and ensure watertight seals, and report steel type and any defects to CASA. This emergency order prevents wing‑structure failure while a permanent fix is developed."}},"importantCases":[],"_links":{"self":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion","history":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion/history","analysis":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion/analysis","conflicts":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion/conflicts","importantCases":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion/important-cases","documents":"/api/acts/ad-ac-87-wing-main-spar-steel-strap-corrosion/documents"}}