{"id":"F2006B02549","name":"AD/AT/23 Amdt 1 - Aft Fuselage Structure Upper Longeron and Upper Diagonal Tube","slug":"ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41805,"registerId":"commonwealth-F2006B02549-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AT/23 Amdt 1 - Aft Fuselage Structure Upper Longeron and Upper Diagonal Tube","content":"![](image.001.png)\n\n![](image.001.png)  \nAIRWORTHINESS DIRECTIVE\n\nOn the effective date specified below, and for the reasons set out in the background section, the CASA delegate whose signature appears below revokes Airworthiness Directive (AD) AD/AT/23 and issues the following AD under subregulation 39.1 (1) of CASR 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\n![](image.001.png)\n\nAirtractor AT-300, 400 and 500 Series Aeroplanes\n\nAD/AT/23\n\nAmdt 1\n\n  \n\nAft Fuselage Structure Upper Longeron and Upper Diagonal Tube\n\n  \n10/2004\n\nApplicability: Model AT-401, AT-401B, AT-402, AT-402A, and AT-402B aircraft, serial numbers 0716 through 1144.\n\nModel AT-501, AT-502, AT-502A, AT-502B, and AT-503A aircraft, serial numbers 0037 through 0658.\n\nRequirement: 1. For Models AT-401 and AT-401B aircraft, serial numbers 0716 through 1144:\n\nInspect the upper longeron and upper diagonal tube on the left hand side of the fuselage frame, just forward of the vertical fin front spar attachment, for cracks in accordance with Snow Engineering Co. Service Letter (SL) No. 218A, dated 10 November 2003, as specified in SL No. 195, re-issued 10 November 2003.\n\n    2. For Models AT-402, AT-402A, and AT-402B aircraft, serial numbers 0716 through 1144:\n\nInspect the upper longeron and upper diagonal tube on the left hand side of the fuselage frame, just forward of the vertical fin front spar attachment, for cracks in accordance with SL No. 218A, dated 10 November 2003, as specified in SL No. 195, re-issued 10 November 2003.\n\n    3. For Models AT-501, AT-502, AT-502B, and AT-503A aircraft, serial numbers 0037 through 0658:\n\nInspect the upper longeron on the left hand side of the fuselage frame, just forward of the vertical fin front spar attachment, for cracks in accordance with SL No. 195B, dated 10 November 2003, as specified in SL No. 195, re-issued 10 November 2003.\n\n    4. For Models AT-502A aircraft, serial numbers 0037 through 0658.\n\nInspect the upper longeron on the left hand side of the fuselage frame, just forward of the vertical fin front spar attachment, for cracks in accordance with SL No. 195B, dated 10 November 2003, as specified in SL No. 195, re-issued 10 November 2003.\n\nNote: FAA AD 2004-15-15 Amdt 39-13749 refers.\n\nAirtractor AT-300, 400 and 500 Series Aeroplanes\n\nAD/AT/23 Amdt 1 (continued)\n\nCompliance: 1. Initially inspect upon the accumulation of 1,250 total hours time in service or within the next 100 hours time in service after 30 September 2004, whichever occurs later. Inspect thereafter at intervals not to exceed 100 hours time in service, until the upper and diagonal longerons are replaced and modified in accordance with SL No. 218B, dated 10 November 2003.\n\n    2. Initially inspect upon the accumulation of 1,250 total hours time in service or within the next 100 hours time in service after the last inspection required by the original issue of this Directive, whichever occurs later. Inspect thereafter at intervals not to exceed 100 hours time in service, until the upper and diagonal longerons are replaced and modified in accordance with SL No. 218B, dated 10 November 2003.\n\n    3. Initially inspect upon the accumulation of 4,800 total hours time in service or within the next 100 hours time in service after 30 September 2004, whichever occurs later. Inspect thereafter at intervals not to exceed 100 hours time in service, until the upper longeron is replaced and modified in accordance with SL No. 195A, revised 10 November 2003.\n\n    4. Initially inspect upon the accumulation of 2,800 total hours time in service or within the next 100 hours time in service after 30 September 2004, whichever occurs later. Inspect thereafter at intervals not to exceed 100 hours time in service, until the upper longeron is replaced and modified in accordance with SL No. 195A, revised 10 November 2003.\n\nThis Amendment becomes effective on 30 September 2004.\n\nBackground: The FAA received reports of three occurrences of cracks found on the left hand upper longeron and upper diagonal support tubes where they intersect on the left side of the fuselage frame just forward of the vertical fin front spar attachment point, on Model AT-602 aircraft. The cracks were found by the pilot and/or ground crew when they noticed excessive movement of the empennage due to the loss of torsional rigidity.\n\nAmendment 1 is issued in response to a new FAA AD, which adds certain AT-500 series aircraft to applicability, changes the compliance times, and incorporates new and revised manufacturer service information that contains a terminating action for the repetitive inspection requirement.\n\n![](image.002.png)  \n\nDavid Villiers\n\nDelegate of the Civil Aviation Safety Authority 20 August 2004\n\n![](image.001.png)  \n\nThe above AD is notified in the Commonwealth of Australia Gazette on 8 September 2004.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":true,"description":"This Amendment (Amdt 1) expanded the scope of the original AD by adding AT-500 series aircraft to the Applicability, changing the compliance (initial inspection) times for some model/serial groups, and incorporating new and revised Snow Engineering service information that provides a terminating action for the repetitive inspections. In short, the amendment increases the population of affected aircraft and replaces the previous inspection-only regime for some aircraft with a pathway to a permanent modification (see Amendment statement and Background)."},"complexity_factors":["Multiple aircraft models across two series (AT-300/400/500) with different serial number ranges (AT-401/402: 0716–1144; AT-501–503A/502 series: 0037–0658)","Four separate Requirement paragraphs with slightly different affected parts (upper longeron vs. upper longeron plus upper diagonal tube)","Four different initial inspection thresholds (1,250; 1,250; 4,800; 2,800 total hours) tied to model/serial combinations","Uniform repetitive inspection interval (every 100 hours) until terminating modification — creates conditional logic (inspect until modified)","Cross-references to multiple external documents (Snow Engineering SL Nos. 195 / 195A / 195B / 218A / 218B) that contain the detailed inspection/repair procedures","Reference to an FAA AD (FAA AD 2004-15-15) for related authority/context","Terminology tied to manufacturing service letters (terminating action resides in external documents rather than the AD text)"],"plain_english_summary":"**What this instrument does (mechanically):**\n\n- This Airworthiness Directive (AD) requires owners/operators of certain Airtractor AT-300, AT-400 and AT-500 series aeroplanes to inspect specific fuselage tubes on the left side of the tail (the upper longeron and, for some models, the upper diagonal tube) for cracks. The inspections must follow the Snow Engineering Co. Service Letters (SL Nos. 195 / 195A / 195B / 218A / 218B) that are identified in the AD (Requirement paragraphs 1–4 and Compliance paragraphs 1–4).\n- The AD sets different initial inspection thresholds (total hours in service) and then requires repeat inspections every 100 flight hours until the affected parts are replaced and modified in accordance with the specified service letters (Compliance). The AD also references an FAA Airworthiness Directive that covers the same problem.\n\n**Who this affects:**\n\n- Owners and operators of Airtractor models listed in the Applicability: AT-401/401B/402/402A/402B (serials 0716–1144) and AT-501/502/502A/502B/503A (serials 0037–0658).\n- Maintenance organisations and mechanics who must carry out the inspections and any corrective work using the manufacturer-designated service letters.\n- The regulator (CASA) enforces compliance through its AD power; the CASA delegate issued this AD (signature/date noted on the instrument).\n\n**Key compliance timings (what must happen and when):**\n\n- AT-401 & AT-401B (SN 0716–1144): initial inspection at 1,250 total hours or within 100 hours after 30 Sep 2004, then every 100 hours until parts are replaced and modified per SL No. 218B (Requirement 1; Compliance 1).\n- AT-402 / 402A / 402B (SN 0716–1144): same inspection requirement as AT-401 series (Requirement 2; Compliance 2).\n- AT-501 / 502 / 502B / 503A (SN 0037–0658): initial inspection at 4,800 total hours or within 100 hours after 30 Sep 2004, then every 100 hours until upper longeron replaced/modified per SL No. 195A (Requirement 3; Compliance 3).\n- AT-502A (SN 0037–0658): initial inspection at 2,800 total hours or within 100 hours after 30 Sep 2004, then every 100 hours until upper longeron replaced/modified per SL No. 195A (Requirement 4; Compliance 4).\n\n**Why the AD was issued (stated purpose):**\n\n- The AD cites FAA reports of cracks in the left-side upper longeron and upper diagonal support tubes found on AT-602 aircraft. These cracks produced excessive movement of the empennage (tail) by reducing torsional rigidity. The AD says Amendment 1 was issued because the FAA added AT-500 series aircraft to the affected population, changed compliance times, and added revised service information that provides a terminating action for the repetitive inspection requirement (Background and Amendment statement).\n\n**Practical effects, incentives and costs (testing the stated purpose against trade-offs):**\n\n- Who pays: aircraft owners/operators pay for inspections, any required repairs, and for the terminating modification that removes the need for repetitive inspections.\n- Behaviour change: operators must add periodic structural inspections to maintenance cycles (every 100 flight hours until modification). Operators face either ongoing recurring inspection cost and downtime or a one-time cost for the modification that terminates the inspections.\n- Administrative/implementation dependencies: the AD requires compliance with specific Snow Engineering service letters. That creates dependence on availability of those documents and on maintenance organisations being able to carry out the modification in accordance with them (Requirement, Compliance).\n- Safety trade-off: repetitive inspections are a short/medium-term mitigation; the manufacturer service letters set out a replacement/modification that the AD treats as a terminating action — that shifts the long-run cost from repeated inspection to an upgrade/repair cost.\n- Enforcement and discretion: CASA enforces the AD; the AD specifies actions to be taken and the service letters that describe how to do them. The AD does not itself set detailed repair steps — it delegates the technical method to the specified service documents.\n\n**Cross-references and dependencies:**\n\n- The AD cross-references multiple Snow Engineering Co. Service Letters (SL Nos. 195, 195A, 195B, 218A, 218B) and an FAA AD (FAA AD 2004-15-15). Compliance therefore requires consulting those documents.\n\n**Immediate practical takeaway:**\n\n- If you own or operate an affected Airtractor in the listed serial ranges you must inspect the left upper longeron (and the upper diagonal tube for certain models) on the schedule set out in the AD and either continue inspections every 100 hours or carry out the replacement/modification in the applicable Snow Engineering service letter to stop the repetitive checks.\n\n(References: Applicability; Requirement paragraphs 1–4; Compliance paragraphs 1–4; Background; Amendment description.)"},"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":true,"description":"The amendment significantly expanded the original directive's scope. It added AT-500 series aircraft (AT-501, AT-502, AT-502A, AT-502B, AT-503A) that were not in the original AD, introduced different compliance times for different model groups, and incorporated new manufacturer service information that provides terminating actions. The original directive appears to have covered only the AT-400 series, while Amendment 1 creates a more complex, multi-tiered regime covering both 400 and 500 series with distinct requirements for each."},"complexity_factors":["Multiple applicability groups with different serial number ranges (4 distinct groups across 10 aircraft models)","Four separate requirement sections with different inspection procedures referencing different manufacturer service letters","Four separate compliance schedules with different flight hour thresholds (1,250; 2,800; 4,800 hours)","Cross-references to external documents (FAA AD 2004-15-15 and Snow Engineering Service Letters 195, 195A, 195B, 218A, 218B) that are not reproduced in the text","Conditional timing logic ('whichever occurs later') for initial inspections","Nested conditions for terminating actions (inspections continue until replacement AND modification per specific service letter)"],"plain_english_summary":"This Airworthiness Directive (Amendment 1) replaces AD/AT/23 and mandates inspections of the rear fuselage on certain Air Tractor crop-dusters (AT‑300/400/500-series, specific AT‑401/402/501–503 variants within defined serial ranges) for cracks in the left upper longeron/upper diagonal tube just forward of the tail fin. Owners/operators must carry out initial and repetitive 100‑hour inspections until the affected parts are replaced per manufacturer service letters. It addresses a known crack risk that can cause tail instability and potential loss of control."}},"importantCases":[],"_links":{"self":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube","history":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube/history","analysis":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube/analysis","conflicts":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube/conflicts","importantCases":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube/important-cases","documents":"/api/acts/ad-at-23-amdt-1-aft-fuselage-structure-upper-longeron-and-upper-diagonal-tube/documents"}}