{"id":"F2006B10758","name":"AD/HU 369/94 Amdt 2 - Main Rotor Blade Doublers","slug":"ad-hu-369-94-amdt-2-main-rotor-blade-doublers","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":56206,"registerId":"commonwealth-F2006B10758-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/HU 369/94 Amdt 2 - Main Rotor Blade Doublers","content":"![](image.001.png)  \nMcDonnell Douglas (Hughes) and Kawasaki 369 Series Helicopters\n\nAD/HU 369/94\n\nAmdt 2\n\n  \nMain Rotor Blade Doublers 11/98\n\nApplicability: Model 369A, 369D, 369E, 369F, 369FF, 369H, 369HE, 369HM, 369HS, 500N,\n\n600N, and OH-6A helicopters with the following main rotor blades installed: part number (P/N) 369A1100-507, with serial number (S/N) D139 through D203, D209 through D223; P/N 369D21100-517 with S/N H664, H665, H667, H669, H671, H672, H674, H676, H679, H680, H683 through H724, H726 through H999, J000\n\nthrough J039, J041 through J055; or P/N 369D21102-517 with S/N 1976 through 2100, 2106 through 2115.\n\nRequirement: 1. For applicable Model 369, 500N, and OH-6 helicopters:\n\n    With each blade lifted off the droop stop, using a 10x or higher magnifying glass, visually inspect the blade for any chordwise cracking emanating from the root fitting edge on the blade lower surface doubler and skin or cracks on the doubler adjacent to the root end fitting (Figure 1), as specified in the Accomplishment Instructions in Boeing McDonnell Douglas Helicopter Systems Service Bulletin (SB) SB369H-243R3, SB369E-088R3, SB500N- 015R3, SB369D-195R3, SB369F-075R3, or SB 600N-007R2, as applicable. If any cracking is discovered, remove the blade and replace it with a serviceable blade.\n\n    b. With each blade lifted off the droop stop, inspect the lower surface for missing or cracked adhesive or paint at the root end fitting-to-doubler bond line (Figure 1 of the SB Accomplishment Instructions). If any missing or cracked adhesive or paint is discovered, remove and inspect the blade in accordance with paragraph 3.E. of Part II of the Accomplishment Instructions in McDonnell Douglas Helicopter Systems Service Information Notice HN-239, DN-188, EN- 81, FN-67, or NN-008, as applicable. If there is any disbonding in excess of the allowable margins specified in paragraph 3E of Part II of the applicable service information notice, remove the blade and replace it with a serviceable blade.\n\n    2. For the Model 600N helicopters:\n\nRemove any affected blade from service and replace it with a serviceable blade not listed in the applicability section of this Directive. Blades removed from the Model 600N are not eligible for use on any rotorcraft.\n\n    3. Affected blades are to be removed from service on or before reaching either of the applicable new life limits. The new life limits are determined by hours time in service or number of torque events (TE). A TE is defined as the transition to a hover from forward flight. For this definition of TE, forward flight is considered to be flight at any airspeed after attaining translational lift.\n\n    For blades that do not have TE logged, before further flight, log the TE in the aircraft logbook or equivalent record as follows:\n\n    Log the TE, if known.\n\nCOMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n    ii. For non-cargo hook operations, if the number of TE is unknown, log 6 TE for each hour time in service.\n\n    iii. For cargo hook (external load) operations, or for any combination of non- cargo hook operations and cargo hook (external load) operations, if the number of TE is unknown, log 20 TE for each hour time in service.\n\n    b. Make an entry into the component record or equivalent record to reflect new life limits for blade part numbers as follows:\n\n    For part number 369A1100-507, Models 369A, 369H, 369HE, 369HM, 369HS, and OH-6A, enter 1,750 hours time in service or 10,600 TE, whichever occurs first.\n\n    ii. For part number 369D21100-517, Models 369D and 369E, enter 2,500 hours time in service or 15,000 TE, whichever occurs first.\n\n    iii. For part number 369D21102-517, Model 369F, 369FF, and 500N, enter 2,500 hours time in service or 15,000 TE, whichever occurs first.\n\n    4. After compliance with Requirement 3 of this Directive, during each operation thereafter, maintain a count of TE performed and additional hours time in service accumulated, and, at the end of each day’s operations, add those counts to the accumulated number of TE and hours time in service on the aircraft logbook or equivalent record.\n\n    5. The blades are no longer retired based upon only hours time in service. This Directive revises the Airworthiness Limitations Section of the maintenance manual by establishing a new retirement life for certain blade part numbers based on hours time in service or a number of TE, whichever occurs first.\n\nNote: FAA AD 98-15-26 Amdt 39-10675 refers.\n\nCompliance: For Requirement 1. Unless already accomplished, before further flight after\n\n4 February 1998 for affected main rotor blades that have accumulated 600 or more hours time in service; thereafter at intervals not to exceed 25 hours time in service.\n\nFor Requirement 2: Before further flight after 4 February 1998. For Requirements 3, 4, and 5: As specified in each Requirement.\n\nThe compliance of the previous issue of this Directive remains unchanged for Requirements 1 and 2.\n\nThis Amendment becomes effective on 8 October 1998.\n\nBackground: This Directive was prompted by an accident in which a main rotor blade failed on a Model 369D helicopter due to cracks. The blade that failed had accumulated over 2,300 hours time in service. Subsequent investigation revealed cracks in two other blades on the same helicopter. The cracks had initiated in the lower inboard doubler and propagated in a chordwise direction through the blade skin and spar. These fatigue cracks may have been caused by residual stresses induced by non-conforming doublers that were used to construct the blade.\n\nAmendment 1 introduced new Requirement Documents, limited the affected blades to certain serial numbers, and reduced the inspection threshold time.\n\nAmendment 2 is issued in response to a new FAA AD which requires the same inspections required by the previous FAA AD, but deletes the Model 369, specifies recording torque events, and establishes a shorter retirement life for certain blades.\n\nAmendment 1 of this Airworthiness Directive became effective on 4 February 1998.\n\nThe original issue of this Airworthiness Directive became effective on 8 January 1998.","sortOrder":0}],"analysis":{"kimi_summary":{"content_quality":"ok","complexity_score":6,"scope_assessment":{"changed":true,"description":"The original directive (January 1998) was a straightforward inspection requirement following an accident. Amendment 1 (February 1998) added serial number limitations and reduced inspection thresholds. Amendment 2 (October 1998) significantly expanded the scope by: (1) introducing an entirely new tracking methodology (torque events) that fundamentally changes how blade life is calculated; (2) establishing a dual-parameter retirement system replacing simple hour-based limits; (3) adding permanent removal requirements for Model 600N blades; and (4) imposing ongoing operational burdens (daily TE counting and logging). The directive evolved from a one-off inspection mandate into a comprehensive lifecycle management system for affected components."},"complexity_factors":["Multiple overlapping applicability criteria: 11 helicopter models, 3 part numbers, and extensive serial number ranges requiring cross-referencing","Dual-unit tracking system: introduces new defined term 'torque events (TE)' with specific calculation rules that vary by operation type (cargo hook vs non-cargo hook)","Nested conditional logic: different life limits apply based on part number AND helicopter model, with 'whichever occurs first' triggers","Multiple reference documents: requires consultation of 6 different Service Bulletins and 5 different Service Information Notices, with 'as applicable' determinations","Phased compliance structure: 5 separate requirements with different effective dates and compliance timelines","Retroactive calculation requirements: mandates back-calculation of TE for blades without historical data using different multipliers (6 TE/hour vs 20 TE/hour)","Cross-border regulatory alignment: references and implements FAA AD 98-15-26, requiring awareness of foreign regulatory context","Amendment layering: this is Amendment 2, with background explaining changes from Amendment 1 and original issue, creating temporal complexity"],"plain_english_summary":"This CASA Airworthiness Directive mandates inspections and new retirement rules for certain McDonnell Douglas (Hughes) and Kawasaki 369-series main rotor blades after fatigue cracks in a doubler led to an in‑flight separation. Affected operators in Australia must perform regular magnified inspections, remove specified 600N blades immediately, and log both flight hours and “torque events” (stressful transitions) so blades are retired at the earlier of defined hours or TE limits. The aim is to prevent catastrophic blade failure."},"flash_summary":{"complexity_score":7,"scope_assessment":{"changed":true,"description":"The instrument’s scope has been altered since the original issue. Amendment 1 narrowed applicability by limiting affected blades to specific serial number ranges and lowered the inspection threshold time. Amendment 2 further changed scope by removing one model from applicability (the Model 369, per the amendment note), introducing mandatory logging of torque events (TE) and establishing retirement lives based on hours OR TE (shorter retirement lives for certain blades). These additions move the rule beyond solely requiring inspections toward changing maintenance policy (new life limits recorded in component history) and restricting reuse of removed blades (Model 600N rule), which has operational and asset‑value consequences not present in the original, inspection‑focused Directive."},"complexity_factors":["Multiple affected airframe models and three distinct blade part numbers with overlapping but different serial number ranges in the Applicability section","Several discrete Requirements (1–5) with different mechanics (inspection, replacement, non‑reuse, life limits, recordkeeping)","Cross‑references to multiple manufacturer Service Bulletins (SB369H-243R3, SB369E-088R3, SB500N-015R3, SB369D-195R3, SB369F-075R3, SB600N-007R2) and Service Information Notices (HN-239, DN-188, EN-81, FN-67, NN-008) for detailed procedures and acceptance limits","Conditional life limits using a dual metric (hours OR torque events), with part‑number specific thresholds (different hours and TE values)","Defined operational term with substantive consequences: torque event (TE) is specifically defined and affects retirement timing","Default recording rules where TE history is absent (6 TE/hour for non‑cargo hook; 20 TE/hour for cargo hook), adding conditional accounting rules","Special‑case rule for Model 600N (removed blades cannot be reused on any rotorcraft) creating an inter‑model restriction","Timing and interval conditions: inspection threshold (if ≥600 hours as of 4 Feb 1998), recurring inspection interval (every 25 hours), and varied compliance timing across Requirements","Interaction with maintenance manuals (Airworthiness Limitations Section must be revised) which requires administrative changes beyond inspections","Reference to related FAA ADs and previous amendments (Amendment 1 and original issue), requiring attention to historical changes"],"plain_english_summary":"**What this instrument does (mechanically)\n\n- Applies to specific McDonnell Douglas (Hughes) and Kawasaki 369‑series, 500N, 600N and OH‑6A helicopters that have particular main rotor blades installed (see Applicability for the exact part and serial numbers).\n\n- Requires visual inspections of the blade lower surface doubler and root fitting area with a 10× (or higher) magnifying glass for: (a) chordwise cracking that starts at the root fitting edge and runs through the doubler and skin, and (b) missing or cracked adhesive/paint at the doubler-to-root fitting bond line. (Requirement 1)\n  - If cracking or excessive disbonding is found, the affected blade must be removed and replaced with a serviceable blade.\n\n- For Model 600N helicopters there is an absolute removal requirement: affected blades must be removed and replaced with blades that are not listed in the Directive’s applicability. Blades removed from Model 600N are not eligible for use on any other rotorcraft. (Requirement 2)\n\n- Establishes new retirement lives for the affected blade part numbers based on either hours-in-service or on the number of torque events (TE), whichever occurs first. A torque event (TE) is defined as the transition to a hover from forward flight; forward flight is any flight at airspeed after attaining translational lift. (Requirement 3)\n  - If previous records did not log TE, operators must estimate and record TE before further flight using the default rules (e.g. 6 TE per flight hour for non‑cargo‑hook operations; 20 TE per hour for cargo‑hook/external‑load operations).\n  - New life limits to enter into component records: \n    - P/N 369A1100‑507 (models 369A/369H/369HE/369HM/369HS and OH‑6A): 1,750 hours or 10,600 TE, whichever occurs first.\n    - P/N 369D21100‑517 (models 369D/369E): 2,500 hours or 15,000 TE, whichever occurs first.\n    - P/N 369D21102‑517 (models 369F/369FF/500N): 2,500 hours or 15,000 TE, whichever occurs first.\n\n- Requires ongoing counting and daily recording of TE and hours accumulated after compliance with the new life limits. (Requirement 4)\n\n- Changes the retirement criterion so blades are no longer retired by hours alone; the maintenance manual’s Airworthiness Limitations Section is revised to reflect the new TE‑or‑hours retirement lives. (Requirement 5)\n\n- Inspection and compliance timing: inspections must be done before further flight after 4 February 1998 for blades that already had 600 or more hours; thereafter the visual inspection interval is every 25 flight hours. Other requirements have their own timing as specified in the Directive.\n\nWhy the rule exists (source explanation and technical cause)\n\n- The Directive was prompted by an accident in which a main rotor blade on a Model 369D failed after more than ~2,300 hours. Investigation found fatigue cracks initiating at a lower inboard doubler and growing chordwise through the skin and spar. The cracks were associated with residual stresses from non‑conforming doublers used during blade construction. The Directive’s stated purpose is to detect cracking/disbonding early and to limit blade life to reduce risk of fatigue failure.\n\nWho it affects and who pays\n\n- Primary affected parties: operators and owners of the listed helicopter models when they have the specified blade part and serial numbers installed; maintenance organisations that perform inspections and repairs; and parts suppliers/manufacturers that provide replacement blades.\n- Direct costs fall on operators: increased inspection labour (initial and recurring every 25 hours if applicable), record‑keeping for TE, and earlier replacement of blades (reduced usable life). For Model 600N operators there is added cost and inventory impact because removed blades are barred from reuse on any rotorcraft.\n\nPractical effects on behaviour, incentives and operations\n\n- Maintenance behaviour: more frequent inspections at short intervals (25 hours) and stricter removal criteria for cracking or excessive disbonding (Requirement 1).\n- Record‑keeping: operators must begin counting and daily recording of torque events (TE) and cumulative hours (Requirements 3 & 4). This increases administration and creates an operational incentive to minimise TE‑intensive manoeuvres if operators seek to prolong blade life.\n- Asset management: blades now have a two‑dimensional life metric (hours OR TE). That tends to shorten practical life for blades used in operations with many transitions to hover (e.g. utility, external‑load work), and reduces the resale value for some blades (especially due to the 600N ban on re‑use, Requirement 2).\n- Parts supply and scheduling: increased replacement demand and possible short‑term scarcity of serviceable, out‑of‑applicability blades.\n\nTrade‑offs, risks and implementation issues to note (mechanism not opinion)\n\n- Compliance burden and cost: frequent inspections and earlier retirement increase maintenance costs and downtime (Requirements 1, 3, 4).\n- Measurement and record accuracy: counting TE requires either reliable historical logs or use of the Directive’s default TE‑per‑hour assumptions; under‑ or over‑counting changes when retirement occurs and can lead to safety or economic risk.\n- Detection limits: the Directive mandates visual inspection with magnification. Visual inspection may miss subsurface or very small cracks compared with non‑destructive methods; the Directive mitigates this by short inspection intervals and mandatory removal when cracks/disbonding are visible. (Requirement 1)\n- Concentration of benefits vs diffuse costs: benefits (reduced risk of in‑flight blade failure) are concentrated on safety outcomes for each flight, while costs (inspections, replacements, recordkeeping) fall directly on operators and are spread among affected fleet operators.\n- Secondary‑market effects: the Model 600N rule (Requirement 2) explicitly prevents reuse of removed blades on other aircraft, creating an immediate loss of value for removed 600N blades.\n- Reliance on external documents: the Directive requires following detailed manufacturer Service Bulletins and Service Information Notices (it cross‑references several SB and SIN documents), which means the substantive inspection steps and allowable disbond margins are determined by the referenced manufacturer documents. That creates dependence on those documents for exact inspection technique and allowable criteria.\n\nKey sections to look up in the Directive for implementation\n\n- Applicability (exact part numbers and serial number ranges)\n- Requirement 1 (inspection steps and thresholds)\n- Requirement 2 (600N removal and non‑reuse rule)\n- Requirement 3 (new life limits, TE definition and logging defaults)\n- Requirement 4 (daily recording of TE and hours)\n- Requirement 5 (change to maintenance manual/Airworthiness Limitations)\n\nSummary sentence\n\nThis Airworthiness Directive imposes mandatory visual inspections, immediate removal for cracked or excessively disbonded main rotor blades of specified part/serial numbers, introduces a new retirement criterion based on hours or torque events (whichever comes first), requires daily TE/hours logging, and bans reuse of certain removed blades (600N), shifting costs for inspections, replacements and recordkeeping to operators while aiming to reduce the risk of fatigue failure arising from problematic doublers (root fitting repairs)."},"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"}},"importantCases":[],"_links":{"self":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers","history":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers/history","analysis":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers/analysis","conflicts":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers/conflicts","importantCases":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers/important-cases","documents":"/api/acts/ad-hu-369-94-amdt-2-main-rotor-blade-doublers/documents"}}