{"id":"F2008L04173","name":"AD/HU 369/121 Amdt 2 - Vertical Stabilizer Control System Adapter Tubes","slug":"ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":76110,"registerId":"commonwealth-F2008L04173-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"McDonnell Douglas (Hughes) and Kawasaki ","sectionType":"division","heading":"McDonnell Douglas (Hughes) and Kawasaki 369 Series Helicopters","content":"### McDonnell Douglas (Hughes) and Kawasaki 369 Series Helicopters\n\n| AD/HU 369/121 Amdt 2 | Vertical Stabilizer Control SystemAdapter Tubes | 13/2008 TX |\n| -------------------- | ----------------------------------------------- | ---------- |\n\n| Applicability: | Model 500N and Model 600N helicopters, with Yaw Stability Augmentation System (YSAS) adapter tubes, part number (P/N) 500N7218-1, installed. |\n| -------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n| Requirement: | 1. Turn the YSAS switch OFF.Note 1: MDHI Service Bulletin SB500N-040R1/SB600N-047R1 dated 27 August 2008, MDHI maintenance manual CSP-HMI-3, Section 96-00-00, pertain to the subject of this AD.Note 2: The original issue of this AD required the CB to be pulled and collared. Ensure that the CB is un-collared and reset.2. Install a placard on the instrument panel as close as practicable to the airspeed indicator that states:“YSAS SYSTEM IS OFF. AIRPSEED LIMIT 100 KIAS or VNE, WHICHEVER IS LESS.”3. Make pen and ink changes or place a copy of this AD in the airworthiness limitations section of the Rotorcraft Flight Manual (RFM) to revise the airworthiness limitations as follows:“VNE is limited to 100 KIAS or less as determined by referring to the airspeed VNE placard already installed on the helicopter.”4. Replace each adaptor tube with an airworthy adaptor tube that has a production date code.Remove the placards and revisions that were made to the RFM and return the YSAS switch to its normal position.The replacement of the adaptor tube with an airworthy adaptor tube that has a production date code stamp is terminating action for the requirements of this AD.Note 3: FAA Emergency AD 2008-22-52 refers. This AD supersedes FAA Emergency AD 2008-18-52. |\n\n| Compliance: | For Requirements 1, 2 and 3 - Before further flight.For Requirement 4 - Within 45 days after the effective date of this AD. |\n| ----------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n| | This Amendment becomes effective on 27 October 2008. |\n| Background: | This Airworthiness Directive was prompted by reports that two Model MD900 helicopters experienced broken Vertical Stabilizer Control System (VSCS) adapter tubes. In one case, the helicopter experienced uncommanded yaw, resulting in loss of a window and a door. The same part-numbered adapter tube is used on the Model 500N and 600N helicopters Yaw Stability Augmentation System (YSAS). Some of the VSCS and YSAS adapter tubes were not manufactured to the required specifications and can fail. This condition, if not corrected, could result in loss of yaw control and subsequent loss of control of the helicopter.Amendment 1 was raised in response to the FAA being advised that the action of pulling the CB can result in impaired directional control which could result in loss of control of the helicopter. Therefore amendment 1 required that the YSAS is turned off instead of pulling the CB.Amendment 2 removes reference to any serial numbers in the applicability section because the unsafe condition can occur on any helicopter with the affected tubes installed. This amendment also requires that replacement adaptor tubes have a production date code stamped on them to determine that the parts are airworthy. (e.g., 08-08 indicates a production date of August 2008).The original issue of this AD became effective on 26 August 2008.Amendment 1 of this AD became effective 28 August 2008. |\n\n![](image.001.png) \nJames Coyne \nDelegate of the Civil Aviation Safety Authority\n\n24 October 2008","sortOrder":0},{"sectionNumber":"AD/HU 369/121 Amdt 2","sectionType":"part","heading":"AD/HU 369/121 Amdt 2","content":"## AD/HU 369/121 Amdt 2","sortOrder":1},{"sectionNumber":"Vertical Stabilizer Control SystemAdapte","sectionType":"part","heading":"Vertical Stabilizer Control SystemAdapter Tubes","content":"## Vertical Stabilizer Control System \nAdapter Tubes","sortOrder":2},{"sectionNumber":"13/2008 TX","sectionType":"part","heading":"13/2008 TX","content":"## 13/2008 TX","sortOrder":3}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":true,"description":"Amendment 2 broadened the applicability and tightened the acceptance criteria for replacements compared with earlier versions. Earlier drafts referenced serial numbers (limiting the affected population); Amendment 2 removes serial-number references so the AD applies to any Model 500N/600N with P/N 500N7218-1 installed, expanding the population of helicopters covered. Amendment 2 also requires that replacement adapter tubes carry a production date code stamp to demonstrate airworthiness, which narrows acceptable replacement parts to those with verifiable production-date marking. Together these changes increase the operational scope of the AD (more helicopters covered) while imposing an additional parts-verification requirement for terminating action."},"complexity_factors":["Short AD with four discrete requirements (Requirements 1–4)","Single applicable part-number (P/N 500N7218-1) limiting technical scope","Immediate compliance timing for some items (\"Before further flight\") and a single short window for replacement (45 days)","References to external documents: MDHI Service Bulletin SB500N-040R1/SB600N-047R1 and maintenance manual (cross-references increase lookup burden)","Termination condition tied to a specific marking (production date code) on replacement parts (adds verification step)","Amendment history and notes: change from pulling CB to switching YSAS off, removal of serial-number limits — creates mild conditional history to track","No large number of defined legal terms or nested exceptions (keeps logical flow simple)"],"plain_english_summary":"**What this instrument does (mechanically)**\n\n- Applies to McDonnell Douglas (Hughes) / Kawasaki Model 500N and 600N helicopters that have Yaw Stability Augmentation System (YSAS) adapter tubes, part number P/N 500N7218-1 (Applicability).\n- Immediately requires the YSAS to be turned OFF (Requirement 1) and that the circuit-breaker not be left pulled/collared (the AD replaces an earlier instruction to pull the CB) (Requirement 1, Background).\n- Requires installation of a placard on the instrument panel near the airspeed indicator that states the YSAS is off and limits airspeed to 100 KIAS or VNE, whichever is less (Requirement 2).\n- Requires a pen-and-ink change or placement of this AD in the Rotorcraft Flight Manual (RFM) airworthiness limitations to record the VNE restriction (Requirement 3).\n- Requires replacing each affected adapter tube with an airworthy adapter tube that carries a production date code stamp within 45 days; removal of the placard/RFM change and return of the YSAS switch to normal is allowed only after that replacement (Requirement 4). The replacement is the terminating action for the AD.\n- References MDHI service bulletin and maintenance manual material for related procedures and cites an FAA Emergency AD for the same hazard (Notes and Background).\n\n**Who this affects**\n\n- Owners/operators and maintenance organisations for Model 500N and 600N helicopters that have the YSAS adapter tube P/N 500N7218-1 installed. Those operators must implement the switch-off, placard and RFM changes before further flight, and must replace the adapter tube within 45 days.\n- Maintenance providers who supply replacement parts must ensure replacement adapter tubes are airworthy and carry a production date code stamp.\n\n**Why it matters (stated safety risk and the legal mechanism)**\n\n- The AD responds to reports of broken Vertical Stabilizer Control System (VSCS) adapter tubes on related helicopter models. In at least one instance a failure produced uncommanded yaw and structural damage (Background). The same adapter tube P/N is used on the YSAS of 500N/600N helicopters and some tubes were apparently not manufactured to required specification. The failure mode could lead to loss of yaw control and subsequent loss of control of the helicopter.\n- The AD imposes interim operational restrictions (YSAS off, speed limit placard, RFM note) designed to mitigate the immediate risk before affected tubes are replaced. The mandatory corrective step (terminating action) is replacement with adapter tubes that show a production date code.\n\n**Costs, incentives, trade-offs and implementation mechanics**\n\n- Who pays: owners/operators pay for the placard and RFM administration work, downtime for restricted operations, and the parts and labour to replace each adapter tube within 45 days (Requirement 4, Compliance).\n- Who decides: the regulator’s delegate issued the AD; individual operators decide scheduling for replacement and maintenance but must comply with the AD’s timelines (Compliance).\n- Behaviour changes required: pilots must operate with YSAS OFF and respect the temporary airspeed limit; maintenance staff must schedule replacement and verify production date codes on replacement parts; existing placards/RFM changes must be removed only after the replacement is done (Requirements 1–4).\n- Compliance burden: immediate compliance is required for the switch-off, placard and RFM change (Before further flight). Replacement must occur within 45 days, creating short-notice parts-sourcing and maintenance workload.\n- Implementation risk: operators relying on the YSAS for directional stability will temporarily lose that augmentation, potentially increasing pilot workload in certain conditions. The AD specifically avoids instructing operators to pull a circuit-breaker because that action had itself impaired control (Background and Amendment note).\n- Incentives and substitution effects: because replacement with a tube that carries a production date code is the terminating action, there is an incentive to obtain and install correctly stamped replacement parts quickly. Operators may, however, operate under the speed-limited configuration while sourcing parts, concentrating the immediate cost on aircraft that actually have the P/N installed.\n- Concentration of costs and benefits: costs fall directly on the subset of operators whose helicopters have P/N 500N7218-1 installed; safety benefits are for those operators and for broader airworthiness outcomes if failures are prevented (Background).\n\n**Key cross-references and documents**\n\n- The AD cross-references MDHI Service Bulletin SB500N-040R1/SB600N-047R1 and the MDHI maintenance manual CSP-HMI-3 Section 96-00-00 for related procedures and background (Note 1).\n- The AD notes an FAA Emergency AD on the same issue and supersedes an earlier FAA Emergency AD (Notes and Background).\n\n(Primary legal mechanics summarized from Requirements 1–4, Compliance and Background as provided.)"},"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"},"kimi_summary":{"content_quality":"ok","complexity_score":3,"scope_assessment":{"changed":false,"description":"This is Amendment 2 of an existing Airworthiness Directive. The amendments have remained tightly focused on the original safety issue (faulty adapter tubes in the YSAS). Amendment 1 refined the technical method (switch off vs. circuit breaker), and Amendment 2 broadened applicability to all serial numbers rather than specific ones, plus added the production date code requirement. These are refinements within the original scope, not scope creep."},"complexity_factors":["Single specific technical subject matter (helicopter yaw control system)","Four sequential requirements with clear compliance deadlines","Cross-reference to FAA Emergency AD and manufacturer service bulletins","Specific part numbers and model numbers limit applicability clearly","Simple conditional logic: if tubes replaced → restrictions can be removed","Minimal defined terms (only standard aviation abbreviations like KIAS, VNE, RFM, P/N)","No nested exceptions or complex conditional structures","Background section provides clear rationale for each amendment"],"plain_english_summary":"This emergency AD requires operators of McDonnell Douglas (Hughes) and Kawasaki Model 500N/600N helicopters with part P/N 500N7218-1 to take immediate safety action for a faulty adapter tube in the Yaw Stability Augmentation System. It mandates disabling the system and imposing temporary operating limits until the defective tubes are replaced; the measure responds to in‑flight failures that caused uncontrolled yaw and structural loss, and prevents potential catastrophic loss of control."}},"importantCases":[],"_links":{"self":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes","history":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes/history","analysis":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes/analysis","conflicts":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes/conflicts","importantCases":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes/important-cases","documents":"/api/acts/ad-hu-369-121-amdt-2-vertical-stabilizer-control-system-adapter-tubes/documents"}}