{"id":"F2006B09590","name":"AD/ENST 28/33 - Tail Rotor Drive Flexible Couplings","slug":"ad-enst-28-33-tail-rotor-drive-flexible-couplings","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":54296,"registerId":"commonwealth-F2006B09590-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/ENST 28/33 - Tail Rotor Drive Flexible Couplings","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)  \nEnstrom F-28 Series Helicopters\n\nAD/ENST 28/33 Tail Rotor Drive Flexible Couplings 11/90\n\nDM\n\nApplicability: All model F-28, F-28A, F-28C, F-28C-2, F-28F, 280, 280C, 280F and 280 FX series helicopters.\n\nRequirement: 1. Replace the 5 plate flex pack coupling P/N A005-1991 with 7 plate flex pack coupling P/N A005-1992. During pack replacement inspect flange condition in accordance with Paragraph 5.2 of Enstrom Service Directive Bulletin No 0080.\n\n2. Inspect 7 plate flex pack coupling in accordance with Paragraph 11 and 12 of Enstrom Service Directive Bulletin No 0080.\n\nNote: FAA AD 90-17-02 Amdt 39-6688 refers.\n\nCompliance: 1. Within 25 hours time in service after 7 September 1990.\n\n2. At intervals not exceeding 100 hours time in service.\n\nBackground: Due to excessive distortion of the 5 plate coupling and the results of subsequent strain gauge testing by the manufacturer, replacement with a 7 plate coupling is mandatory.","sortOrder":0}],"analysis":{"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive focused on a specific mechanical defect in specific helicopter models. It has not expanded beyond its original purpose of mandating a part replacement and inspection regime."},"complexity_factors":["Very short document (under 200 words)","Only 2 substantive requirements with clear part numbers","Single cross-reference to FAA AD 90-17-02 (informational only)","No defined terms section","Simple conditional logic: replace once, then inspect repeatedly","Clear applicability list (9 specific helicopter models)"],"plain_english_summary":"This is a **safety order for helicopter owners and operators**.\n\n**What it does:**\nThis Airworthiness Directive (AD) — essentially a mandatory safety fix — requires owners of specific Enstrom helicopter models to replace a faulty part in the tail rotor system. The old 5-plate flexible coupling (part number A005-1991) must be swapped for a stronger 7-plate version (part number A005-1992). After replacement, operators must inspect this part every 100 hours of flight time.\n\n**Who it affects:**\nAnyone who owns or operates these Enstrom helicopter models: F-28, F-28A, F-28C, F-28C-2, F-28F, 280, 280C, 280F, and 280 FX.\n\n**Why it matters:**\nThe original 5-plate coupling was distorting excessively under stress. Strain gauge testing (a way of measuring material stress) showed it could fail. Since the tail rotor keeps the helicopter from spinning uncontrollably, a coupling failure could cause a crash. This directive makes the fix legally mandatory — not optional.\n\n**Key deadlines:**\n- **Replacement:** Within 25 flight hours after 7 September 1990\n- **Ongoing inspections:** Every 100 flight hours thereafter"},"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive enforces the manufacturer-recommended replacement of the 5-plate flex coupling with the 7-plate unit and establishes a recurring inspection interval. The text indicates the measure is intended to address distortion found in the 5-plate coupling (\"Background\"). There is no indication within this instrument that the scope was broadened or narrowed from that stated purpose; it applies to the listed models and prescribes the replacement and inspection steps referenced to the manufacturer’s service bulletin (\"Applicability\", \"Requirement\", \"Compliance\")."},"complexity_factors":["Clear, narrowly targeted technical action (replace specific part number with another) reduces conceptual complexity","Multiple affected airframe models listed (several F-28/280 variants) require operators to check applicability","Direct references to manufacturer service bulletin paragraphs (5.2, 11, 12) create a dependency on external documentation for procedural detail","Short mandated compliance window for initial replacement (25 hours TIS from a specified date) increases operational scheduling pressure","Ongoing recurring inspection interval (every 100 hours TIS) creates a continuing maintenance obligation","Specific part numbers and reliance on manufacturer procedures limit substitution options absent further approval"],"plain_english_summary":"Owners/operators of Enstrom F‑28 series helicopters must replace the existing 5‑plate tail‑rotor flex coupling with a 7‑plate unit and follow the manufacturer’s flange and coupling inspection procedures. The change is mandated because testing showed excessive distortion of the 5‑plate pack; the 7‑plate is intended to reduce that safety risk.\n\nThis imposes a short‑term retrofit and recurring inspection burden (inspection intervals not to exceed 100 hours TIS), raising maintenance cost and scheduling requirements for affected operators."}},"importantCases":[],"_links":{"self":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings","history":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings/history","analysis":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings/analysis","conflicts":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings/conflicts","importantCases":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings/important-cases","documents":"/api/acts/ad-enst-28-33-tail-rotor-drive-flexible-couplings/documents"}}