{"id":"F2006B09847","name":"AD/F50/21 - Forward Fuselage - Stringers and Frame 5775","slug":"ad-f50-21-forward-fuselage-stringers-and-frame-5775","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":54906,"registerId":"commonwealth-F2006B09847-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/F50/21 - Forward Fuselage - Stringers and Frame 5775","content":"---\nmeta-content-style-type: text/css\nmeta-content-type: application/xhtml+xml; charset=utf-8\nmeta-generator: Aspose.Words for .NET 22.10.0\n---\n\n?xml version=\"1.0\" encoding=\"utf-8\" standalone=\"no\"?>\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 - 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n \n\n![](image.001.png)  Fokker F50 (F27 Mk 50) Series Aeroplanes\n\n \n\nAD/F50/21 Forward Fuselage - Stringers and Frame 5775 8/89\n\nApplicability: Serial numbers 20103 through 20108 and 20110 through 20121.\n\n \n\nRequirement: Reinforce fuselage frame to stringer connection by installing connecting angles in accordance with Fokker Service Bulletin F50-53-004 dated 14 October 1988.\n\n \n\nNote: Netherlands AD 88-81 refers.\n\n \n\nCompliance: Prior to accumulating 10000 flight cycles.\n\n \n\nBackground:  Full scale fatigue testing has shown the possibility of cracks occurring on the subject area, due to insufficient frame to stringer connection.\n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n\n \n","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The AD's scope is limited and specific: it mandates reinforcement of the frame-to-stringer connection on the listed serial numbers because testing indicated a risk of cracking (Background, Applicability, Requirement). There is no indication in the text that the AD expands beyond that stated technical purpose; it prescribes a single corrective action and a single compliance threshold (Requirement, Compliance)."},"complexity_factors":["Limited applicability to a defined set of serial numbers reduces legal complexity (Applicability).","Technical installation must follow a specific manufacturer service bulletin, creating technical but not legal ambiguity (Requirement).","Single, clear compliance metric (10000 flight cycles) simplifies timing but requires flight-cycle tracking (Compliance).","No multiple compliance options, exemptions, or staged requirements; straightforward mandatory action (Requirement).","Potential operational complexity from parts availability and maintenance scheduling rather than from statutory wording (Requirement, Compliance)."],"plain_english_summary":"What this instrument does (mechanically)\n\n- The Civil Aviation Safety Authority requires a physical reinforcement to the forward fuselage where certain fuselage frames join the stringers. The reinforcement is the installation of \"connecting angles\" and must be done exactly as described in Fokker Service Bulletin F50-53-004 dated 14 October 1988 (Requirement).\n\nWho is affected\n\n- The rule applies only to Fokker F50 (F27 Mk 50) aeroplanes with serial numbers 20103–20108 and 20110–20121 (Applicability). The operator or owner of each affected aeroplane is the party that must have the work done.\n\nWhen it must be done\n\n- The work must be completed before the aeroplane reaches 10,000 flight cycles (Compliance).\n\nWhy this was required (stated rationale)\n\n- The AD cites full-scale fatigue testing that showed a possibility of cracks at the frame-to-stringer connection area, attributed to an insufficient connection (Background). The AD also references a Netherlands airworthiness directive on the same subject (Note).\n\nWho pays, who decides, and what changes in behaviour\n\n- Who pays: aircraft owners/operators of the listed serial numbers pay for parts, labour, downtime and any related inspections required to install the connecting angles (Requirement) — the AD makes the operator responsible for compliance.\n- Who decides: the AD is issued under the Commonwealth civil aviation framework (COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 — header) and prescribes a mandatory technical action set out by the manufacturer in the referenced service bulletin (Requirement).\n- Behavioural change: affected operators must schedule the modification before 10,000 flight cycles, diverting maintenance resources and aircraft availability toward the modification rather than other uses (Compliance, Requirement).\n\nCosts, incentives and trade-offs (mechanical and economic effects)\n\n- Direct costs: purchase or fabrication of connecting angles, approved technical labour to install them per the service bulletin, and aircraft downtime while the modification is carried out (Requirement). The AD does not state cost amounts.\n- Incentives: operators have an incentive to complete the modification to avoid the fatigue risk described and to maintain legal airworthiness. The compliance deadline (10000 flight cycles) creates time pressure that may prioritise this work ahead of other non-mandatory maintenance.\n- Opportunity costs: scheduling slots, manpower and workshop capacity will be absorbed by this mandated work; that can delay non-mandatory maintenance or operations for the affected aircraft (Compliance).\n- Implementation risks and compliance burden: successful compliance requires access to the specified service bulletin, appropriate parts and personnel competent to perform the modification to the manufacturer’s instructions (Requirement). The AD does not specify permitted maintenance organisations or approval steps beyond following the service bulletin.\n\nEffects on markets, competition and private choice\n\n- The AD imposes a technical compliance obligation on a defined set of aircraft rather than a broad industry rule. That concentrates benefits (reduced fatigue risk) on those specific aircraft and concentrates costs on their owners/operators (Applicability, Background).\n- The requirement to follow a manufacturer’s service bulletin channels the technical solution through the manufacturer’s specification (Requirement). The AD does not itself limit which approved maintenance provider may perform the work, but following the SB will affect how independent maintenance providers carry out the modification.\n\nImplementation note\n\n- The AD refers to the Fokker Service Bulletin for the exact installation method and to a Netherlands AD for related history (Requirement, Note). The AD sets a firm compliance trigger (flight cycles) rather than a calendar date (Compliance)."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive addressing a specific structural issue identified through fatigue testing. It has not expanded beyond its original purpose of mandating a particular airframe modification for a defined subset of aircraft."},"complexity_factors":["Single, specific technical requirement with clear reference to manufacturer service bulletin","Limited applicability (only 18 specific aircraft serial numbers)","Straightforward compliance deadline (prior to 10,000 flight cycles)","No nested exceptions, conditional logic, or extensive cross-referencing","Minimal defined terms (only standard aviation terminology like 'flight cycles')","Short document length (approximately 150 words of substantive content)"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA) for specific aircraft.\n\n**What it does:**\nIt requires owners or operators of certain Fokker F50 aeroplanes to reinforce a specific part of the aircraft's forward fuselage (the body of the plane near the front). The reinforcement involves installing \"connecting angles\" — metal brackets that strengthen the joint where the aircraft's frame meets its stringers (longitudinal support beams).\n\n**Who it affects:**\nOnly Fokker F50 aircraft with serial numbers 20103 through 20108 and 20110 through 20121 — a limited batch of 18 specific planes.\n\n**Why it matters:**\nFatigue testing revealed that cracks could develop in this area because the original connection between the frame and stringers wasn't strong enough. If left unaddressed, this could compromise the structural integrity of the aircraft. The directive gives operators a deadline: they must complete this reinforcement before the aircraft reaches 10,000 flight cycles (roughly, one takeoff and landing equals one cycle).\n\n**Key point:** This isn't optional — compliance is mandatory under civil aviation law."}},"importantCases":[],"_links":{"self":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775","history":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775/history","analysis":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775/analysis","conflicts":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775/conflicts","importantCases":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775/important-cases","documents":"/api/acts/ad-f50-21-forward-fuselage-stringers-and-frame-5775/documents"}}