{"id":"F2006B09897","name":"AD/F50/68 - Lower Cowling By-Pass Duct Opening","slug":"ad-f50-68-lower-cowling-by-pass-duct-opening","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":55056,"registerId":"commonwealth-F2006B09897-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/F50/68 - Lower Cowling By-Pass Duct Opening","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 F50 (F27 Mk 50) Series Aeroplanes\n\nAD/F50/68 Lower Cowling By-Pass Duct Opening 11/95\n\nApplicability: Fokker Aircraft B.V. Model F.27 Mk.050 series, serial numbers as listed in the requirement document.\n\nRequirement: Modify the air outlet opening of the engine by-pass duct in accordance with Fokker\n\nS.B. F50-71-041 dated 10 November 1993.\n\nNote: Dutch AD BLA 1995-065 (A) dated 30 June 1995 also refers.\n\nCompliance: Required within the next three (3) calendar months after the 12 October 1995, unless accomplished previously.\n\nBackground: There have been a number of in-flight power fluctuations in the service experience of the Fokker Model F.27 Mk.050 series aircraft. In certain weather conditions ice can build up in the engine air intake leading to a loss of engine power as a result of ice ingestion.\n\nThis Directive mandates the by-pass duct modification to reduce the possibility of ingestion of ice into the engine.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":4,"scope_assessment":{"changed":false,"description":"The directive, as presented, implements a narrowly defined technical modification for specified serial-number Fokker F.27 Mk.050 series aircraft to address ice ingestion risk (Applicability; Requirement; Background). The source provides no indication that the scope of affected aircraft or the mandated action has been broadened or narrowed from an earlier instrument; it simply prescribes the modification for the serial numbers listed and references the manufacturer service bulletin and a related Dutch AD (Applicability; Requirement; Note)."},"complexity_factors":["Technical maintenance action requires following a manufacturer service bulletin (Requirement).","Applicability limited to specific serial-number aircraft, requiring record checks to determine who must comply (Applicability).","Short, fixed compliance window of three calendar months creates scheduling and operational constraints (Compliance).","Interplay between national regulator's directive and a foreign (Dutch) AD requires cross-jurisdictional awareness (Note).","Directive states a safety rationale but provides no quantified risk reduction or cost data, which complicates cost–benefit assessment (Background; Requirement)."],"plain_english_summary":"This airworthiness directive requires certain Fokker F.27 Mk.050 (F50) aeroplanes to have the engine by-pass duct air outlet opening physically modified. Mechanically, the directive compels operators to carry out the modification described in Fokker Service Bulletin F50-71-041 (dated 10 November 1993) to the engine by-pass duct air outlet (Requirement). The rule applies only to the specific serial-numbered airplanes listed in the requirement document (Applicability). Operators must complete the modification within three calendar months after 12 October 1995, unless it has already been done (Compliance). The directive cites a history of in-flight engine power fluctuations caused by ice building up in the intake and being ingested by the engine in certain weather conditions; the modification is mandated to reduce the possibility of such ice ingestion (Background).\n\nWho pays and who decides\n- Aircraft owners/operators bear the direct cost of performing the modification and any related downtime, because the directive makes compliance the operator's responsibility (Applicability; Requirement; Compliance). \n- The Civil Aviation Safety Authority (CASA) issued the directive under the airworthiness regime (header: Civil Aviation Safety Authority; Part 39-105), and the technical work is specified by the manufacturer’s service bulletin (Requirement). The directive also cross-references a Dutch airworthiness directive (Note), indicating coordination with the manufacturer's state of design.\n\nPractical effects on behaviour\n- Operators must identify affected serial-number aircraft, obtain or follow Fokker Service Bulletin F50-71-041, schedule maintenance, and complete the modification within the three-month deadline (Applicability; Requirement; Compliance). \n- Where the modification is not yet done, operators will face decisions about scheduling work, incurring maintenance costs, and possible short-term operational disruption (Compliance).\n\nCosts, incentives and trade-offs\n- The directive transfers the cost of mitigating the stated ice-ingestion risk to the operators of the affected aircraft (Requirement; Compliance). The benefit claimed by the directive is reduced likelihood of ice ingestion and attendant engine power fluctuations (Background). \n- The directive concentrates costs on the subset of operators who fly the listed serial-number aircraft while distributing potential safety benefits to passengers, crew and other airspace users who benefit if the modification reduces engine-power events (Applicability; Background). \n- The source does not specify alternative mitigations or compensating measures; it mandates the specific modification in the manufacturer’s service bulletin (Requirement). That limits operator choice to following the specified modification (Requirement).\n\nImplementation, compliance burden and risks\n- Compliance requires technical maintenance work per a manufacturer’s service bulletin; operators must source parts, manpower, and possibly schedule aircraft out of service for the work (Requirement). \n- The three-month compliance window creates a firm scheduling constraint; operators face downtime/opportunity cost to meet it (Compliance). \n- The directive cites the operational problem (ice ingestion leading to power fluctuations) as the reason for the mandated change, but does not provide quantified estimates of how much the modification reduces the risk or the cost of implementation; the expected safety benefit is stated (Background; Requirement) without economic or performance metrics. \n- The directive references a Dutch AD (Note), which indicates related action by another airworthiness authority; this can affect coordination of maintenance standards and liability expectations across jurisdictions.\n\nWhat the directive does not say (source limits)\n- The text does not state the modification’s cost, detailed operational impacts, or any permitted alternative compliance means beyond following the cited Fokker service bulletin (Requirement). It does not describe enforcement mechanisms or penalties beyond the regulatory context (Compliance)."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive addressing a specific mechanical defect in a specific aircraft type. It has not expanded beyond its original purpose of mandating a single modification to prevent ice ingestion."},"complexity_factors":["Single, specific technical requirement referencing an external manufacturer document","No defined terms section — uses plain technical language","Simple conditional structure: applicability based on aircraft model and serial number, with single compliance deadline","Minimal cross-referencing (only one related Dutch airworthiness directive mentioned in a note)","Short length (approximately 150 words of substantive content)"],"plain_english_summary":"This is an **Airworthiness Directive** — a mandatory safety order issued by Australia's Civil Aviation Safety Authority (CASA). It requires owners and operators of specific Fokker F50 aircraft to make a mechanical modification to prevent engine problems.\n\n**What it does:**\n- Orders a change to the **engine by-pass duct** (a channel that routes air around the engine) on Fokker F50 planes\n- Requires enlarging or modifying the air outlet opening to prevent ice buildup from being sucked into the engine\n\n**Who it affects:**\n- Owners and operators of Fokker F.27 Mk.050 series aircraft with specific serial numbers listed in Fokker's technical bulletin\n\n**Why it matters:**\n- Ice had been building up in engine air intakes during certain weather conditions, causing **in-flight power fluctuations** (dangerous losses of engine power while flying)\n- The modification reduces the risk of ice being ingested into the engine, preventing potential engine failure or accidents\n\n**When it must be done:**\n- Within 3 months after 12 October 1995 (or earlier if already completed)\n\n**Key document to follow:**\n- Fokker Service Bulletin F50-71-041 (dated 10 November 1993) — this contains the exact technical instructions for the modification"}},"importantCases":[],"_links":{"self":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening","history":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening/history","analysis":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening/analysis","conflicts":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening/conflicts","importantCases":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening/important-cases","documents":"/api/acts/ad-f50-68-lower-cowling-by-pass-duct-opening/documents"}}