{"id":"F2006B10331","name":"AD/GULL/4 - Airframe Structure - Inspection","slug":"ad-gull-4-airframe-structure-inspection","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":56087,"registerId":"commonwealth-F2006B10331-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/GULL/4 - Airframe Structure - Inspection","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)  \nPercival Vega Gull Series Aeroplanes\n\nAD/GULL/4 Airframe Structure \\- Inspection 1/73\n\nApplicability: All models.\n\nRequirement: 1. Preparation: Prepare the aeroplane for inspection by removing all removable equipment, cabin linings, seats, inspection access panels, fairings, fabric patches, tape sealing and sufficient fabric and secondary structure to enable an inspection of the airframe in accordance with these requirements.\n\n    2. Inspection Procedure:\n\n    2.1 Inspect all fabricated structural components such as box spars, wooden ribs, bulkheads and braces, and all glued connections between these components for the following defects:\n\n    (a) Cracks, cuts, splits, bruises, compression shakes, warping contamination by foreign matter such as oil, attach by fungus in the wood, shrinkage of the timer at bolt holes.\n\n    (b) Separation or sponginess in the glued joints and plywood. A 0.005 in. feeler gauge should be used to determine separation of glued joints and the depth of glue failure.\n\n    2.2 Inspect all plywood skin for the following defects:\n\n    (a) Cracks, splits, cuts, bruises and shakes in the surface of the wood, deterioration and contamination by foreign matter.\n\n    (b) Separation of the glue where skin connects to the skeletal structure beneath. Particular attention should be paid to the flaps since they are exposed to moisture, and also subject to contamination by oil.\n\nNote: Load carrying wooden skin becomes inefficient and the complete structure loses strength when adhesion to the structure beneath has failed. Consequently, all structural connections and joints must be thoroughly inspected for deterioration and defects. When carrying out an inspection the following may prove helpful in detecting defects.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\nSeparation from the structure is often detectable from the exterior in several ways. It is likely to appear as a local discontinuity in the skin contour which can be pressed down without the immediate firm resistance anticipated. When the structure is flexed the skin will tend to take up waves. If these waves do not pass across the lines of gluing to the structure beneath, adhesion may be taken as sound. Oblique lighting of the surface will be found helpful in studying surface waves and contour discontinuities. Timber shakes appear as thin lines across the timber on the varnish layer. By sponging ink on the varnish and subsequently wiping off, a line where the shake exists should be apparent. Where there are signs of failure in the protective coating it should be removed to inspect the wood beneath. Special attention is to be paid to points in the structure where liquids can collect or oil contamination can occur. Casein glue deteriorates if repeatedly exposed to moisture or oil. Where possible, the inspection should be made from the interior. However, for wings, initial inspection may be made from the exterior. When inspecting the interior structure of the wings it may be necessary to cut holes in load carrying covering. In these instances reference should be made to the Secretary for an approved scheme for the incorporation of inspection holes.\n\nCompliance: 1. Within 12 months of last inspection in accordance with AD/GENERAL/29 and thereafter at intervals not exceeding 12 months.\n\n2. For aircraft which have not been inspected in accordance with AD/GENERAL/29 in the 12 months prior to 31 January 1973 - inspect within three months after 31 January 1973 and there- after at intervals not exceeding 12 months.","sortOrder":0}],"analysis":{"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":false,"description":"This directive maintains its original narrow scope — mandatory periodic inspection of wooden airframe structures for a specific vintage aircraft type. It has not expanded beyond its original safety purpose."},"complexity_factors":["Single specific aircraft type (Percival Vega Gull series) with defined applicability","Clear two-part inspection procedure with numbered subsections (2.1 and 2.2)","Specific technical measurement provided (0.005 inch feeler gauge)","Practical guidance included (ink test for timber shakes, oblique lighting technique)","Two-tier compliance schedule based on prior inspection history","Cross-reference to one other directive (AD/GENERAL/29)","No defined terms section or interpretation provisions","Straightforward conditional logic (if inspected under previous directive vs. if not)"],"plain_english_summary":"This AD requires owners/operators of Percival Vega Gull wooden-airframe aeroplanes to carry out thorough structural inspections — exposing and examining timber members and glued joints for rot, oil contamination, cracking or glue failure — because these hidden defects can cause catastrophic failure. \n\nInspections must be done within the specified initial timeframe (for aircraft not recently checked) and repeated annually, to ensure ongoing airworthiness."},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"This directive is tightly focused on annual inspection of wooden airframe structure and glued joints for Percival Vega Gull aeroplanes. It does not expand beyond that original safety purpose. The only conditional variation is a transitional compliance timing tied to a historical date for aircraft not recently inspected under AD/GENERAL/29; this does not broaden the scope of who or what must be inspected."},"complexity_factors":["Short, single‑topic directive with linear steps (Preparation, Inspection, Compliance).","One external cross‑reference: AD/GENERAL/29 (for timing baseline).","A small number of technical specifics (0.005 in feeler gauge; list of defect types).","One administrative exception requiring regulator approval for cutting inspection holes.","No extensive defined term list, few nested conditions (only the special compliance rule tied to a historical date)."],"plain_english_summary":"**What this instrument requires (mechanical changes):**\n\n- Applies to: all Percival Vega Gull series aeroplanes (all models).  \n- Preparation (Requirement 1): the owner/operator must prepare the aeroplane for inspection by removing all removable equipment, cabin linings, seats, inspection access panels, fairings, fabric patches, tape sealing and sufficient fabric and secondary structure so the airframe can be inspected.  \n- Inspection (Requirement 2): inspect fabricated wooden structural components (box spars, wooden ribs, bulkheads, braces) and all glued connections for defects including cracks, splits, bruises, compression shakes, warping, contamination (e.g. oil, fungus), shrinkage at bolt holes, and separation or sponginess in glued joints. A 0.005 inch feeler gauge is specified to measure glue separation.  \n- Inspect plywood skin for cracks, splits, cuts, bruises, shakes, deterioration, contamination and for glue separation where the skin attaches to the underlying structure (with special attention to flaps).  \n- Notes and methods: the directive describes tactile and visual techniques (pressing, observing surface waves when structure is flexed, oblique lighting, sponging ink) to detect separation and timber shakes. If protective coatings show signs of failure, they must be removed to inspect the wood below. Where inspection of wing interiors requires cutting holes in load‑carrying covering, the operator must obtain an approved scheme from the Secretary (regulator).  \n- Compliance timing: inspect within 12 months of the last inspection carried out under AD/GENERAL/29 and thereafter at intervals not exceeding 12 months. Aircraft that were not inspected under AD/GENERAL/29 in the 12 months before 31 January 1973 must be inspected within three months after 31 January 1973 and then annually.\n\n**Who is affected:**\n\n- Owners and operators of Percival Vega Gull aeroplanes.  \n- Maintenance organisations and certified inspectors who perform structural inspections and repairs on these aircraft.  \n- The regulator (Secretary) when approval is needed for any permanent inspection access (cutting holes in load‑carrying coverings).\n\n**Why it matters (stated purpose and practical effect):**\n\n- Stated purpose: to detect deterioration in load‑carrying wooden structure and glued joints so that weakened structures are identified and remedied before failure. The directive notes that wooden skin that loses adhesion to the structure no longer carries load efficiently and the whole structure loses strength (Requirement 2.2 and Note).  \n- Practical safety effect: regular, systematic inspection reduces the risk of undetected deterioration in wooden primary structure which could compromise the airframe’s load‑carrying capability.  \n\n**Costs, incentives and trade‑offs (practical implications):**\n\n- Who pays: the aircraft owner/operator bears the direct costs — labour, downtime while the aircraft is prepared and inspected, possible removal of interior fittings, and any repairs or replacement of wood or glue joints.  \n- Compliance burden: annual inspections and the preparatory disassembly (removal of linings, seats, panels, etc.) increase maintenance time and complexity compared with inspections that do not require such removal (Requirement 1). The inspection may require cutting openings in load‑carrying coverings; that needs regulator approval of an incorporation scheme, adding administrative steps and potential delays.  \n- Technical/implementation risks: accurate assessment requires skilled inspectors familiar with wooden airframe pathology and the prescribed tactile/visual methods and tools (e.g. 0.005 in feeler gauge). Inadequate inspection technique or inexperienced personnel increase the risk of missed defects.  \n- Effects on private enterprise and markets: the directive creates recurring demand for qualified inspection and repair services for vintage wooden aircraft; owners may face higher maintenance costs that affect the economics of operating these types.  \n- Behavioural effects: owners are incentivised to schedule annual inspections, to remove contaminated protective coatings for inspection, and to seek approved methods if they must cut inspection holes. If inspections reveal deterioration, owners must repair or retire affected components, which can be costly.  \n\n**Bureaucratic discretion and procedural points:**\n\n- The Secretary (the regulator) has discretion to approve schemes for incorporating inspection holes in load‑carrying coverings (Note in Requirement 2). That is the explicit point of administrative discretion in the directive.  \n\n**Trade‑offs and opportunity costs:**\n\n- The directive trades operational availability and maintenance cost against structural safety assurance. Annual invasive inspections preserve airworthiness but increase operating cost and downtime for owners of these vintage wood‑structured aeroplanes.\n\n**Net mechanism summary:**\n\n- The AD mandates systematic annual inspections of the wooden airframe and glued joints, prescribes how inspections should be prepared and carried out, specifies a measurement tolerance for glue separation, and requires regulator approval for any structural access modifications. The owner pays for inspections and repairs; the regulator retains authority over approved inspection access schemes."},"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"}},"importantCases":[],"_links":{"self":"/api/acts/ad-gull-4-airframe-structure-inspection","history":"/api/acts/ad-gull-4-airframe-structure-inspection/history","analysis":"/api/acts/ad-gull-4-airframe-structure-inspection/analysis","conflicts":"/api/acts/ad-gull-4-airframe-structure-inspection/conflicts","importantCases":"/api/acts/ad-gull-4-airframe-structure-inspection/important-cases","documents":"/api/acts/ad-gull-4-airframe-structure-inspection/documents"}}