{"id":"F2007L04843","name":"AD/JBK 117/26 - Tail Rotor Transmission Attachment Nuts","slug":"ad-jbk-117-26-tail-rotor-transmission-attachment-nuts","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":71584,"registerId":"commonwealth-F2007L04843-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"Kawasaki BK 117 Series Helicopters","sectionType":"division","heading":"Kawasaki BK 117 Series Helicopters","content":"### Kawasaki BK 117 Series Helicopters\n\n| AD/JBK 117/26 | Tail Rotor Transmission Attachment Nuts | 2/2008 TX |\n| ------------- | --------------------------------------- | --------- |\n\n| Applicability: | Kawasaki BK117 series helicopters.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                              |\n| -------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n| Requirement:   | 1. Inspect for cracks on the four (04) attachment nuts of the tail rotor transmission in accordance with Kawasaki Service Bulletin No.KSB-117-291 dated11 December 2007 or later Japan Civil Aviation Bureau (JCAB) approved revisions.If cracks are found, replace all attachment nuts with new nuts.2. If nuts are replaced with new nuts in accordance with Requirement 1 of this AD, inspect new nuts (after installation) for cracks.If any cracks are found, replace all attachment nuts with another set of new nuts.Note: JCAB Airworthiness Directive No. TCD-7195-2007 dated 11 December 2007 refers. |\n| Compliance:    | Before further flight, after the effective date of this AD.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                     |\n|                | This Airworthiness Directive becomes effective on 18 December 2007.                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                                             |\n| Background:    | During an inspection after installation of the tail rotor transmission, cracks have been detected on the four attachment nuts. Loss of nut self-locking torque due to cracking may lead to the failure of tail rotor transmission.The AD requirements address the loss of control of rotorcraft due to failure of attachment nuts of the tail rotor transmission.                                                                                                                                                                                                                                               |\n\n![](image.001.png)  \nCharles Lenarcic  \nDelegate of the Civil Aviation Safety Authority\n\n14 December 2007","sortOrder":0},{"sectionNumber":"AD/JBK 117/26","sectionType":"part","heading":"AD/JBK 117/26","content":"## AD/JBK 117/26","sortOrder":1},{"sectionNumber":"Tail Rotor Transmission Attachment Nuts","sectionType":"part","heading":"Tail Rotor Transmission Attachment Nuts","content":"## Tail Rotor Transmission Attachment Nuts","sortOrder":2},{"sectionNumber":"2/2008 TX","sectionType":"part","heading":"2/2008 TX","content":"## 2/2008 TX","sortOrder":3}],"analysis":{"flash_summary_failed":{"failed":true,"reason":"Unauthenticated. Configure AI_GATEWAY_API_KEY or use a provider module. Learn more: https://ai-sdk.dev/unauthenticated-ai-gateway","source":"analysis-cron"},"flash_summary":{"complexity_score":2,"scope_assessment":{"changed":false,"description":"This AD remains tightly focused on the original safety problem identified in the Background: cracked attachment nuts on the tail rotor transmission and the resulting risk of loss of control. It does not expand beyond inspection and replacement of those attachment nuts, and therefore has not grown significantly beyond its original purpose."},"complexity_factors":["Short, single-issue directive focused on one component (four attachment nuts)","Two-step requirement: initial inspection + conditional replacement and re-inspection","Cross-reference to Kawasaki Service Bulletin KSB-117-291 (external technical document)","Cross-reference to JCAB-approved revisions and JCAB AD TCD-7195-2007","Immediate compliance requirement ('before further flight')","No nested exceptions, no numerous defined terms, no complex scheduling or monitoring regime"],"plain_english_summary":"**What this instrument does (mechanics)\n\n- Requires operators of Kawasaki BK117 series helicopters to inspect the four attachment nuts that secure the tail rotor transmission for cracks, following Kawasaki Service Bulletin KSB-117-291 (11 Dec 2007) or any later revision approved by the Japan Civil Aviation Bureau (JCAB). (Requirement 1)\n- If any of those nuts are found to be cracked, all four attachment nuts must be replaced with new nuts. (Requirement 1)\n- After replacing the nuts with new ones, the new nuts must be inspected again. If any crack is found in the newly installed nuts, all four must be replaced again with another set of new nuts. (Requirement 2)\n- The inspection and any required replacements must be completed before the helicopter is flown again after the effective date. The AD became effective 18 December 2007. (Compliance)\n\n**Who is affected\n\n- Owners and operators of Kawasaki BK117 series helicopters.\n- Maintenance providers and engineers who perform inspections and parts replacement on those helicopters.\n\n**Why it matters (stated safety risk)\n\n- The directive was issued after inspections detected cracks in the four nuts that attach the tail rotor transmission. The concern is that cracking could reduce the self-locking torque of the nuts and lead to failure of the tail rotor transmission, which can cause loss of rotorcraft control. (Background)\n\n**Practical effects, costs and who pays\n\n- Who pays: the helicopter owner/operator pays for the inspections, replacement nuts, and labour/downtime.\n- Behaviour change required: immediate inspection of the four attachment nuts before next flight and replacement if cracking is found; if replacements are fitted, a follow-up inspection of those new nuts with further replacement if cracks are detected.\n- Typical costs: inspection labour and aircraft downtime; cost of up to two full sets of replacement attachment nuts per helicopter in the event cracks recur on newly installed nuts; potential logistical costs if replacement nuts are in short supply.\n\n**Trade-offs, risks and implementation issues\n\n- Implementation relies on the Kawasaki service bulletin for inspection technique and on JCAB-approved revisions of that bulletin. That creates a dependency on external technical instructions (KSB-117-291) and any updates approved by JCAB. (Requirement 1)\n- Inspection reliability: the safety benefit depends on the ability of maintenance staff to detect cracks reliably. If inspections miss cracks, the risk remains.\n- Supply and downtime risk: if replacement nuts are required and not immediately available, operators may face grounding/delays.\n- Enforcement and recordkeeping: operators must ensure compliance before further flight; regulatory oversight (Civil Aviation Safety Authority) enforces the AD.\n\n**How this maps to economic and operational incentives\n\n- The directive increases immediate maintenance costs and potential downtime for BK117 operators, while reducing the risk of tail rotor transmission failure for the flying public and operators.\n- It creates demand for replacement parts (concentrated benefit to parts suppliers) and imposes an operational cost concentrated on BK117 owners/operators.\n- The mechanics are narrowly targeted (inspection and replacement) rather than imposing design changes or fleet-wide retrofits.\n\n(References in the AD: Requirement 1, Requirement 2, Compliance, Background; note also JCAB Airworthiness Directive No. TCD-7195-2007 is referenced.)"},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This AD maintains its original narrow scope: addressing a specific mechanical defect in a specific helicopter model. It has not expanded beyond its original safety purpose."},"complexity_factors":["Very short document (under 300 words)","Only 2 main requirements with a simple conditional (if cracks found → replace)","Single defined reference (Kawasaki Service Bulletin No.KSB-117-291)","No nested exceptions or cross-references to other legislation","Clear, single applicability criterion (Kawasaki BK117 series helicopters)","Straightforward compliance deadline (before further flight after effective date)"],"plain_english_summary":"This emergency AD requires owners/operators of Kawasaki BK117 helicopters to inspect four tail-rotor-transmission attachment nuts for cracking and replace them if any crack is found, following Kawasaki’s service bulletin. Replaced nuts must be reinspected and replaced again if cracked. Compliance is mandatory before next flight after 18 December 2007 to prevent loss of tail-rotor control."}},"importantCases":[],"_links":{"self":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts","history":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts/history","analysis":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts/analysis","conflicts":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts/conflicts","importantCases":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts/important-cases","documents":"/api/acts/ad-jbk-117-26-tail-rotor-transmission-attachment-nuts/documents"}}