{"id":"F2008L02876","name":"AD/GBK 117/21 - Pressure Altitude Limitation and Max N1 Check","slug":"ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":74757,"registerId":"commonwealth-F2008L02876-current","compilationNumber":null,"startDate":"2026-04-02","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"Eurocopter BK 117 Series Helicopters","sectionType":"division","heading":"Eurocopter BK 117 Series Helicopters","content":"### Eurocopter BK 117 Series Helicopters\n\n| AD/GBK 117/21 | Pressure Altitude Limitationand Max N1 Check | 9/2008 |\n| ------------- | -------------------------------------------- | ------ |\n\n| Applicability: | EUROCOPTER DEUTSCHLAND MBB-BK 117 C-1 and C-2 helicopters except those aircraft on which both engines have been modified in accordance with Turboméca Modification TU358. |\n| -------------- | ------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------ |\n| Requirement: | 1. Affix the selected placard and amend the Flight Manual (FLM) in accordance with the instructions of the referenced Alert Service Bulletin.2. For helicopters that are about to perform starting, landing and hovering procedures in or out of ground effect at pressure altitudes above 10,000 feet and/or flights at pressure altitudes above 13,000 feet, accomplish the “MAX N1 CHECK” to establish the OEI rating of the engines in accordance with the instructions of the referenced Alert Service Bulletin.3. Repeat the “MAX N1 CHECK” to establish the OEI rating of the engines in accordance with the instructions of the referenced Alert Service Bulletin.4. If the OEI rating is not reached, only operations at pressure altitudes up to a maximum of 10,000 feet and mission flights at pressure altitudes up to a maximum of 13,000 feet may be conducted. Amend the placard and FLM accordingly.5. Repeat the “MAX N1 CHECK” to establish the OEI rating of the engines in accordance with the instructions of the referenced Alert Service Bulletin.6. Remove the placard and FLM changes from the helicopter.Once modification TU358 is incorporated into both Fuel Control Units (FCU)’s the “MAX N1 CHECK” is no longer required.7. Do not install a spare FCU on any helicopter as a replacement part unless it has been modified in accordance with Turboméca modification TU358.Eurocopter Deutschland Alert Service Bulletin (ASB) No. ASB-MBB-BK117-60-121 Revision 4 pertains to the requirements of this AD. The use of later approved revisions of these documents is acceptable for compliance with the requirements of this AD. |\n\n| | Note: EASA AD 2008-0061 dated 27 March 2008 refers. |\n| ----------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n| Compliance: | 1. Within 30 days after the effective date of this AD, unless previously accomplished.2. Before the next such mission after the effective date of this AD.3. Within 600 Flight Hours (FH) after the effective date of this AD and thereafter at intervals not to exceed 600 FH.4. From the effective date of this AD.5. When, after failing to reach the OEI rating, and either the engine, the FCU, engine module 2 and/or engine module 3 is replaced.6. After modification of both FCUs in accordance with Turboméca modification TU358.7. After 10 April 2009. |\n| | This Airworthiness Directive becomes effective on 28 August 2008. |\n| Background: | Prior to delivery of each helicopter, Eurocopter Deutschland performs the “MAX N1 Check” at a pressure altitude (PA) of about 10,000 feet. During supplementary testing the “MAX N1 CHECK” at maximum certification altitude, a few MBB-BK 117 C-2 helicopters could not reach the specified N1 power threshold value. The cause was identified as an engine acceleration limitation due to a delivered fuel flow lower than the engine fuel flow demand to achieve the OEI rating at high altitude. The fuel flow is limited by the FCU acceleration law in those cases. This limitation could potentially occur at altitudes exceeding 10,000 feet, depending on the engine and FCU characteristics. As a conservative measure, TURBOMECA and EUROCOPTER decided that take-off, landing and HIGE (Hover In Ground Effect) and/or HOGE (Hover Out of Ground Effect) operations above 10,000 feet or level flight above 13,000 feet require a dedicated mandatory check.Turboméca has now developed a modification (TU358) that will, when accomplished on both engines installed on the helicopter, cancel the required checks and allow the removal of the placard and flight manual changes. |\n\n![](image.001.png) \nJames Coyne \nDelegate of the Civil Aviation Safety Authority\n\n17 July 2008","sortOrder":0},{"sectionNumber":"AD/GBK 117/21","sectionType":"part","heading":"AD/GBK 117/21","content":"## AD/GBK 117/21","sortOrder":1},{"sectionNumber":"Pressure Altitude Limitationand Max N1 C","sectionType":"part","heading":"Pressure Altitude Limitationand Max N1 Check","content":"## Pressure Altitude Limitation \nand Max N1 Check","sortOrder":2},{"sectionNumber":"9/2008","sectionType":"part","heading":"9/2008","content":"## 9/2008","sortOrder":3}],"analysis":{"kimi_summary":{"content_quality":"ok","complexity_score":4,"scope_assessment":{"changed":false,"description":"This AD maintains its original focused scope: addressing a specific fuel control unit performance limitation in BK 117 C-1/C-2 helicopters at high altitude. The requirements (placards, repetitive checks, permanent modification) all directly serve this single safety purpose. No scope creep is evident."},"complexity_factors":["Multiple interdependent compliance deadlines (7 separate timing requirements)","Conditional logic: different requirements apply depending on whether TU358 modification is installed, whether MAX N1 CHECK passes/fails, and whether specific engine components are replaced","Cross-reference to external technical document (Eurocopter Alert Service Bulletin ASB-MBB-BK117-60-121 Revision 4) without reproducing its contents","Technical aviation terminology requiring specialized knowledge (OEI rating, FCU, N1, pressure altitude, HIGE/HOGE)","Nested conditions in Requirement 4: failure to reach OEI rating triggers permanent operational restrictions and additional placard/FLM amendments"],"plain_english_summary":"This AD requires operators of Eurocopter BK117 C‑1/C‑2 helicopters to take immediate operational safeguards and testing because some engines can’t attain required OEI (one‑engine inoperative) emergency power at high altitude due to a fuel control unit limitation. \n\nIt mandates placards and manual updates, recurring MAX N1 power checks and altitude restrictions until modified fuel control units are fitted to both engines, preventing loss of safe OEI performance."},"flash_summary":{"complexity_score":5,"scope_assessment":{"changed":false,"description":"The directive remains narrowly focused on the single technical problem identified: an FCU acceleration law that can limit fuel flow and prevent reaching OEI N1 at high pressure altitudes. The measures (placarding, FLM changes, mandatory MAX N1 CHECK, operational restrictions if the check fails, and prohibition on installing unmodified spare FCUs) are directly tied to that problem. The inclusion of the TU358 modification as a means to remove recurring checks is an expected technical solution rather than an expansion of scope. There is no clear evidence in the text that the directive grew beyond addressing the engine/FCU high‑altitude performance issue."},"complexity_factors":["Single‑aircraft type applicability with an exception (TU358 modification on both engines)","Reference to external technical document: Eurocopter Alert Service Bulletin ASB‑MBB‑BK117‑60‑121 (cross‑referenced compliance)","Multiple conditional requirements (checks required only for certain operations above defined pressure altitudes)","Several distinct compliance time rules (30 days, before next mission, within 600 FH, upon certain part replacements, after modification, after specified calendar date)","Repeated/recurring test obligation (initial, periodic every 600 FH, and after parts change)","Operational consequence branching (if OEI rating not reached → altitude restrictions and amended placard/FLM)","Prohibition on installing unmodified spare parts (spare FCU restriction)","Technical term dependence (OEI rating, MAX N1 CHECK, FCU, TU358)","Cross‑reference to an EASA AD and allowance for later approved revisions of referenced documents"],"plain_english_summary":"**What this directive does (mechanically)**\n\n- It applies to Eurocopter (MBB) BK 117 C-1 and C-2 helicopters except where both engines already have Turboméca modification TU358 to the Fuel Control Units (FCUs) (Applicability).\n- Operators must attach a visible placard and update the Flight Manual (FLM) as set out in Eurocopter Alert Service Bulletin ASB‑MBB‑BK117‑60‑121 (Requirement 1).\n- For any helicopter that is going to perform take‑off, landing or hovering in or out of ground effect (HIGE/HOGE) above 10,000 feet pressure altitude, or any flight at pressure altitudes above 13,000 feet, the operator must perform a “MAX N1 CHECK” to establish the One‑Engine Inoperative (OEI) engine rating per the Alert Service Bulletin (Requirement 2).\n- The MAX N1 CHECK must be repeated on a scheduled basis (initially within 600 flight hours and then at intervals not exceeding 600 flight hours) and also whenever specified parts are replaced (Requirements 3, 5; Compliance 3, 5).\n- If a helicopter cannot reach the required OEI rating on the check, operations are restricted to pressure altitudes up to 10,000 feet for take‑offs/landings/hover and up to 13,000 feet for mission flights; the placard and FLM must be amended to reflect this limitation (Requirement 4).\n- Once both FCUs on a helicopter have been modified in accordance with Turboméca modification TU358, the MAX N1 CHECK and placard/FLM changes are no longer required (Requirement 6; Compliance 6).\n- Spare FCUs must not be installed unless they have the TU358 modification (Requirement 7).\n\n**Who this affects**\n\n- Aircraft operators and flight crews of BK 117 C‑1 and C‑2 helicopters (they must perform the checks, comply with placard/FLM limits, and may need to alter operations when a check fails).\n- Maintenance organisations and owners (they must carry out checks, update FLMs/placards, install TU358 modifications if chosen, and ensure spare FCUs are modified before installation).\n- Turboméca and Eurocopter as suppliers (Turboméca supplies the TU358 modification that, when installed on both engines, removes the need for recurring checks).\n\n**Why it matters (safety mechanism and rationale)**\n\n- The directive responds to findings that some BK 117 C‑2 helicopters could not reach the required N1 (engine gas generator speed) OEI power threshold at high pressure altitudes because the FCU limited fuel flow during acceleration. The MAX N1 CHECK is a safety precaution to verify engine performance at altitude and to restrict operations if required (Background).\n\n**Who pays and what behaviour changes**\n\n- Operators bear the direct costs: time to perform the MAX N1 CHECKs, updates to flight manuals and placarding, potential limitations on missions (lost revenue or rerouting), and the cost of installing TU358 on one or both FCUs if they wish to remove the recurring check requirement.\n- Maintenance organisations bear labour and documentation costs to carry out the checks, perform the TU358 modification, and ensure spare FCUs comply before installation.\n- Behavioural changes include conducting regular engine performance checks, amending operations when checks fail (restricting high‑altitude flights), and a possible commercial decision to purchase and fit TU358 modifications to avoid repeated checks and operational limits.\n\n**Trade‑offs, risks and implementation notes**\n\n- Compliance burden: recurring checks (every 600 flight hours) create an ongoing maintenance/operational cost and record‑keeping requirement (Compliance 3).\n- Concentrated benefit: operators who invest in TU358 for both engines remove recurring checks and operational placards; the modification cost is concentrated on those operators who choose to buy it (Requirement 6).\n- Inventory impact: prohibition on installing unmodified spare FCUs forces owners to keep modified spares or face downtime while modifying spares (Requirement 7).\n- Measurement and variability risk: OEI attainment is an engine/FCU performance test; environmental and measurement variability could affect pass/fail outcomes, producing operational uncertainty for individual flights.\n- Implementation risk: the directive requires amendments to FLM and placarding per the ASB — effectiveness depends on correct application of the ASB instructions and accurate documentation.\n\n(Primary sources: Requirements 1–7, Compliance timings 1–7, and Background text; see Eurocopter ASB referenced.)"},"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-gbk-117-21-pressure-altitude-limitation-and-max-n1-check","history":"/api/acts/ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check/history","analysis":"/api/acts/ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check/analysis","conflicts":"/api/acts/ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check/conflicts","importantCases":"/api/acts/ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check/important-cases","documents":"/api/acts/ad-gbk-117-21-pressure-altitude-limitation-and-max-n1-check/documents"}}