{"id":"F2006B09617","name":"AD/F100/17 - Vertical Stabiliser Torsion Box","slug":"ad-f100-17-vertical-stabiliser-torsion-box","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":54373,"registerId":"commonwealth-F2006B09617-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/F100/17 - Vertical Stabiliser Torsion Box","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)  \nFokker F100 (F28 Mk 100) Series Aeroplanes\n\nAD/F100/17 Vertical Stabiliser Torsion Box 4/99\n\nApplicability: F.28 Mk.0100 aircraft, serial numbers 11244 through 11419, and 11421. Requirement: 1. Modify the Vertical stabiliser Torsion Box in accordance with Fokker SB F100-\n\n55-009 dated 9 April 1990; and\n\nmodify the Vertical Stabiliser Torsion Box in accordance with Fokker Service Bulletin (SB) F100-55-014 dated 29 November 1990, unless Fokker SB F100-55- 018 dated 19 May 1993 was carried out prior to accumulation of 6,000 flight cycles.\n\nNote 1: The modification in accordance with Fokker SB F100-55-014 dated\n\n29 November 1990 is not required if the modification in Fokker SB F100-55-018 dated 19 May 1993 is accomplished before the aircraft accumulates 6,000 flight cycles since new.\n\nNote 2: Some aircraft had the above modifications included before delivery or as a result of compliance with other Airworthiness Directives.\n\n    2. Inspect the flanges and finger strips at Rib 5.0 of the vertical stabiliser in accordance with Fokker SB F100-55-019 Revision 1 dated 19 May 1993.\n\n    3. Modify Rib 5.0 of the vertical stabiliser in accordance with Fokker SB F100-55- 018 dated 19 May 1993.\n\nNote 3: If during any inspection as required by this AD, cracks are found, contact the manufacturer for repair instructions.\n\nNote 4: RLD BLA 93-069(A) refers.\n\nCompliance: 1. Prior to accumulating 6,000 flight cycles or within 30 days, whichever occurs last.\n\n    2. Prior to accumulating 8,500 flight cycles since new or within 30 days, whichever occurs last.\n\nThereafter reinspect every 2000 flight cycles, until modified in accordance with Requirement 3.\n\n    3. Prior to accumulating 7,300 flight cycles since new or\n\nPrior to accumulating 13,500 flight cycles since new and immediately after an inspection in accordance with Requirement 2 of this AD.\n\nNote 5: If the modifications in Requirement 3 are completed prior to 7,300 flight cycles, then the requirement to inspect immediately prior to the installation of those modifications is not required.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\nThis Airworthiness Directive becomes effective on 22 April 1999.\n\nBackground: During full-scale fatigue tests, cracks were found at bolt holes in the RH flange of the torsion box and in the finger strip on the LH flange at Rib 5.0 of the vertical stabiliser. If not corrected, these cracks may lead to a deteriorated fatigue life of the vertical stabiliser structure.","sortOrder":0}],"analysis":{"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"},"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":false,"description":"The Directive stays focused on its original, narrowly defined purpose: to address fatigue cracking found in the vertical stabiliser torsion box and associated Rib 5.0 areas of specific Fokker F100 airframes. It prescribes inspection intervals and mandated structural modifications and does not expand into unrelated regulatory domains or broader operational requirements. The requirements and timing are targeted responses consistent with the stated safety rationale (fatigue cracks found during testing)."},"complexity_factors":["Applicability limited to a specific set of serial numbers (11244–11419, 11421) — narrows scope but requires precise tracking of affected machines","References to four different Fokker Service Bulletins (F100-55-009, -014, -018, -019 Rev 1)","Multiple flight-cycle thresholds (6,000; 7,300; 8,500; 13,500) and a 30-day calendar window create conditional compliance timing","Recurring inspection requirement (every 2,000 flight cycles) until a modification is completed","Conditional exemptions and sequencing (e.g. SB -018 can replace SB -014 if done before 6,000 cycles; Note 5 exempts immediate pre-install inspection if mod done early)","Requirement to contact manufacturer for repair instructions if cracks are found (delegates repair detail outside the AD text)","Notes indicating prior compliance possible (pre-delivery or earlier ADs), requiring record checks","Technical specificity (part names: torsion box, flanges, finger strips, Rib 5.0) requires skilled maintenance actions"],"plain_english_summary":"**What this instrument does (mechanically)\n\n- Applies to Fokker F100 (F28 Mk 100) aeroplanes with serial numbers 11244–11419 and 11421. (See \"Applicability\".)\n\n- Requires three things for those aircraft:\n  - Requirement 1: Modify the vertical stabiliser torsion box following Fokker Service Bulletin (SB) F100-55-009 (9 Apr 1990), and also carry out the modification in SB F100-55-014 (29 Nov 1990) unless SB F100-55-018 (19 May 1993) was already done before the aeroplane reaches 6,000 flight cycles. (Requirement 1; Note 1.)\n  - Requirement 2: Inspect the flanges and finger strips at Rib 5.0 of the vertical stabiliser according to Fokker SB F100-55-019 Revision 1 (19 May 1993). These inspections must be done before 8,500 flight cycles or within 30 days (whichever is later), and then repeated every 2,000 flight cycles until the modification required by Requirement 3 is carried out. (Requirement 2; Compliance.)\n  - Requirement 3: Modify Rib 5.0 of the vertical stabiliser per Fokker SB F100-55-018 (19 May 1993). This must be done before 7,300 flight cycles, or before 13,500 flight cycles if timed with an inspection under Requirement 2 (there is a specific sequencing allowance). (Requirement 3; Compliance; Note 5.)\n\n- If cracks are found during inspections, operators must contact the manufacturer for repair instructions. (Note 3.)\n\n- Some aeroplanes may already have received these modifications before delivery or under other Airworthiness Directives; in that case the corresponding new work is not required. (Note 2.)\n\n- The directive sets specific deadlines measured in flight cycles and in calendar days (\"within 30 days\") for when the required actions must be completed.\n\nWhy the regulator says this is needed\n\n- The stated safety reason is that full-scale fatigue testing revealed cracks at bolt holes in the right-hand flange of the torsion box and in the finger strip on the left-hand flange at Rib 5.0 of the vertical stabiliser. Left uncorrected, these cracks could shorten the stabiliser's fatigue life and thereby affect structural integrity. (Background.)\n\nWho is directly affected\n\n- Owners and operators of the identified Fokker F100 serial-numbered aeroplanes. They must schedule inspections, carry out or arrange the specified structural modifications, and follow manufacturer repair instructions if cracks are found. Maintenance organisations that perform these SBs and inspections are also directly engaged.\n\nWho pays, who decides, and what changes in behaviour\n\n- Aircraft owners/operators pay for the inspections, modifications, parts and labour (Requirements 1–3 and the compliance timing). Approved maintenance organisations will decide on work sequencing and perform the SBs. Operators must track flight cycles and schedule recurring inspections until the modification of Rib 5.0 is completed. If cracks are discovered, operators must pause and seek manufacturer repair instructions, which may impose additional work or grounding.\n\nCosts, incentives and trade-offs (brief analytical test of the stated purpose)\n\n- Costs: Direct costs are concentrated on the relatively small set of affected operators (serial-numbered F100s). Costs include parts, labour, aircraft downtime (opportunity cost of out-of-service time), and administrative record-keeping. The SB references mean work is likely to be done by approved repair stations or the manufacturer, concentrating revenue to those providers.\n\n- Benefits: The intended benefit is increased structural integrity and reduced risk of fatigue-related failure in the vertical stabiliser. That benefit accrues primarily to operators (reduced failure risk and liability) and to safety regulators and the travelling public indirectly.\n\n- Implementation risk and compliance burden: The work is technical and requires following multiple SBs; scheduling around flight-cycle thresholds and repeated inspections (every 2,000 cycles until modification) adds administrative complexity. If cracks are found, operators rely on manufacturer repair instructions which may take time to obtain and implement.\n\n- Incentives and second-order effects: Operators may choose to accelerate the modification to avoid repeated inspections, or to retire or restrict use of affected airframes if retrofit costs are high relative to asset value. Maintenance providers and Fokker (as SB originator) may see concentrated demand for parts and services.\n\n- Scope and substitution: The AD targets a narrow structural deficiency. It does not change broader certification, operations, or noise/emissions rules. Its main substitution effect is operational (inspections vs. modification; accelerated retirement vs. repair).\n\nWhere the directive leaves discretion or uncertainty\n\n- Repair action if cracks are found is left to the manufacturer (Note 3), so operators must await or follow manufacturer-approved repairs rather than being given a choice of approved alternative repairs in the AD text.\n\nRelevant text references\n\n- Requirement 1 (SBs F100-55-009 and F100-55-014/018), Requirement 2 (SB F100-55-019 Rev 1), Requirement 3 (SB F100-55-018), Notes 1–5, and the Compliance timing clauses (flight-cycle thresholds and 30-day calendar windows)."},"kimi_summary":{"content_quality":"ok","complexity_score":4,"scope_assessment":{"changed":false,"description":"This Airworthiness Directive maintains its original narrow scope — addressing a specific structural fatigue issue identified during testing of Fokker F100 aircraft. It has not expanded beyond its original safety purpose of mandating inspections and modifications to the vertical stabiliser torsion box."},"complexity_factors":["Multiple interdependent requirements (Requirements 1, 2, and 3) with cross-references between them","Conditional logic with 'unless' and 'if' clauses (e.g., Requirement 1 modification not required if SB F100-55-018 done before 6,000 flight cycles)","Four separate compliance deadlines with 'whichever occurs last' conditional timing","Nested exceptions in Notes (Note 5 creates an exception to Requirement 2 based on timing of Requirement 3)","References to four different manufacturer Service Bulletins with specific dates that must be cross-referenced","Defined applicability limited to specific serial number ranges","Ongoing recurring inspection requirement (every 2,000 flight cycles) until modification condition is met"],"plain_english_summary":"This safety order covers Fokker F100 aircraft (serials 11244–11419 and 11421), requiring owners, operators and maintenance crews to inspect and modify the vertical stabiliser’s torsion box and nearby fittings, and to repair any cracks found. \n\nDeadlines depend on flight cycles and inspections repeat periodically until modifications are complete. It responds to fatigue cracks at tail bolt holes that could shorten structural life and risk failure if not fixed."}},"importantCases":[],"_links":{"self":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box","history":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box/history","analysis":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box/analysis","conflicts":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box/conflicts","importantCases":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box/important-cases","documents":"/api/acts/ad-f100-17-vertical-stabiliser-torsion-box/documents"}}