{"id":"F2005L00103","name":"AD/AL 250/84 - HMU PLA Potentiometer","slug":"ad-al-250-84-hmu-pla-potentiometer","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":33500,"registerId":"commonwealth-F2005L00103-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"Allison Turbine Engines - 250 Series","sectionType":"division","heading":"Allison Turbine Engines - 250 Series","content":"### Allison Turbine Engines - 250 Series\n\n| AD/AL 250/84 | HMU PLA Potentiometer | 2/2005 |\n| ------------ | --------------------- | ------ |\n\n| Applicability: | Rolls-Royce Corporation (formerly Allison Engine Company, Allison Gas Turbine Division, and Detroit Diesel Allison) (RRC) models 250-C30R/3, -C30R/3M, -C47B, and -C47M turboshaft engines that have a hydromechanical unit (HMU) with a part number (P/N) listed in paragraph 1.A. Group A of RRC Alert Commercial Engine Bulletins (ACEB) CEB A-73-3103, Revision 4, dated 2 December 2003 and CEB A-73-6030, Revision 4, dated 2 December 2003. |\n| -------------- | -------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n|                | Note 1: These engines are known to be installed on, but not limited to, Bell OH-58D, Bell Helicopter Textron 407, Boeing AH/MH-6M and MD Helicopters Inc. 600N helicopters.                                                                                                                                                                                                                                                                        |\n| Requirement:   | 1. Perform an electrical signal inspection of the HMU Power Lever Angle (PLA) potentiometer in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions of ACEB CEB A-73-3103, Revision 4, or CEB A- 73-6030, Revision 4.                                                                                                                                                                                   |\n|                | 2. If the electrical signal result is found to be unacceptable during the Requirement 1 inspection replace the HMU.                                                                                                                                                                                                                                                                                                                                |\n|                | 3. Replace the HMU with a redesigned HMU (ie one with a dual element potentiometer) that has a P/N that is not listed in paragraph 1.A. Group A of ACEB CEB A-73-3103, Revision 4 and CEB A-73-6030, Revision 4. Replacing the HMU with an HMU that has a P/N not specified in the referenced ACEBs terminates the Requirement 1 repetitive inspection.                                                                                            |\n|                | Note 2: FAA AD 2004-24-04 Amdt 39-13880 refers.                                                                                                                                                                                                                                                                                                                                                                                                    |\n| Compliance:    | For Requirements 1 - Within 300 time in service (TIS) after the effective date of this Directive or within 300 hours TIS after completing the inspection in accordance with Requirement 1 of AD/AL 502, which ever occurs earlier, and thereafter at intervals of 300 hours TIS until Requirement 3 is accomplished.                                                                                                                               |\n|                | For Requirement 2 - Before further flight immediately following the Requirement 1 inspection.                                                                                                                                                                                                                                                                                                                                                      |\n|                | For Requirement 3 - Within 600 hours TIS after the effective date of this Directive, or by 17 March 2005, whichever occurs earlier.                                                                                                                                                                                                                                                                                                                |\n\n|             | This Airworthiness Directive becomes effective on 17 February 2005.                                                                                                                                                                                                                                                                                                                                                     |\n| ----------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |\n| Background: | This Directive supersedes AD/AL 250/82, which required initial and repetitive electrical signal inspections of the HMU power lever angle (PLA) potentiometer.                                                                                                                                                                                                                                                           |\n|             | This Directive continues to require those inspections in accordance with a revised ACEB and, following the manufacturer releasing a redesigned HMU that has a dual-element potentiometer, adds replacement of the existing HMU with a new design HMU as a mandatory terminating action to the repetitive inspection requirements. These actions are intended to prevent uncommanded and sudden changes in engine power. |\n\n![](image.001.png)  \nJames Coyne  \nDelegate of the Civil Aviation Safety Authority\n\n23 December 2004\n\nThe above AD is notified in the Commonwealth of Australia Gazette on 26 January 2005.","sortOrder":0},{"sectionNumber":"AD/AL 250/84","sectionType":"part","heading":"AD/AL 250/84","content":"## AD/AL 250/84","sortOrder":1},{"sectionNumber":"HMU PLA Potentiometer","sectionType":"part","heading":"HMU PLA Potentiometer","content":"## HMU PLA Potentiometer","sortOrder":2},{"sectionNumber":"2/2005","sectionType":"part","heading":"2/2005","content":"## 2/2005","sortOrder":3}],"analysis":{"flash_summary":{"complexity_score":5,"scope_assessment":{"changed":true,"description":"This Directive supersedes AD/AL 250/82 by keeping the repetitive electrical inspections but adding a mandatory terminating action: replacement of existing HMUs with a redesigned dual‑element potentiometer HMU within specified TIS/date. That expands the earlier inspection‑only regime to require a mandatory retrofit for affected HMUs."},"complexity_factors":["Cross-references to manufacturer Alert Commercial Engine Bulletins (ACEB CEB A-73-3103 and A-73-6030) that list applicable HMU part numbers (creates dependence on external documents).","Multiple compliance triggers and deadlines: initial/repetitive inspections every 300 hours TIS, immediate replacement before further flight if unacceptable, and a terminating retrofit within 600 hours TIS or by a calendar date.","Termination condition tied to installing an HMU with a P/N not listed in the ACEBs (requires correct P/N verification and record-keeping).","Interaction with prior directives (supersedes AD/AL 250/82 and references AD/AL 502), requiring operators to reconcile earlier compliance actions.","Operational burden: electrical signal inspection procedure complexity, possible immediate grounding/parts replacement, and logistics of sourcing redesigned HMUs.","Safety-critical context increases scrutiny and potential administrative oversight by the regulator (compliance verification)."],"plain_english_summary":"Requires operators of certain Rolls‑Royce (Allison) 250‑series turboshaft engines to perform repeated electrical inspections of the HMU power‑lever‑angle (PLA) potentiometer, replace any HMU with unacceptable readings before further flight, and fit a redesigned HMU (dual‑element potentiometer) within 600 hours TIS to terminate the inspection cycle. Applies to engines with HMU part numbers listed in the referenced RRC ACEBs and affects helicopters using those engines; stated purpose is to prevent uncommanded, sudden engine power changes."},"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"},"kimi_summary":{"content_quality":"ok","complexity_score":4,"scope_assessment":{"changed":false,"description":"This AD maintains its original scope as a targeted safety directive for specific engine models. While it adds a terminating action (the redesigned HMU replacement) compared to the superseded AD/AL 250/82, this represents a natural evolution of the safety response rather than scope creep. The directive remains focused on the specific potentiometer defect in specific engine models."},"complexity_factors":["Multiple cross-references to manufacturer bulletins (ACEB CEB A-73-3103 and CEB A-73-6030, both Revision 4) without reproducing their content","Nested conditional logic: Requirement 2 only applies if Requirement 1 finds a fault; Requirement 3 terminates Requirement 1","Three different compliance timeframes with 'whichever occurs earlier' clauses","Part number tracking across two different bulletins with specific group references (paragraph 1.A. Group A)","Reference to a superseded directive (AD/AL 250/82) and a related FAA AD (2004-24-04) for context","Technical aviation terminology requiring specialized knowledge (HMU, PLA, TIS, potentiometer, turboshaft)"],"plain_english_summary":"This is an **Airworthiness Directive (AD)** — essentially a mandatory safety order for aircraft. It applies to specific Rolls-Royce helicopter engines (the 250-C30R/3, -C30R/3M, -C47B, and -C47M models) that have a particular fuel control component called a **Hydromechanical Unit (HMU)**.\n\n**What the problem is:**\nThe HMU contains a **Power Lever Angle (PLA) potentiometer** — a sensor that tells the engine how much power the pilot is requesting. Some of these sensors were giving faulty electrical signals, which could cause the engine to suddenly change power without the pilot doing anything. That's obviously dangerous in a helicopter.\n\n**What must be done:**\n- **Inspect** the sensor every 300 flight hours to check if it's working properly\n- **If it fails the test:** Replace the HMU before flying again\n- **Eventually replace** the old HMU with a new redesigned version that has a better \"dual-element\" sensor (this stops the need for repeated inspections)\n\n**Who this affects:**\nOperators of Bell OH-58D, Bell 407, Boeing AH/MH-6M, and MD Helicopters 600N helicopters that use these specific engines.\n\n**Why it matters:**\nThis prevents helicopters from experiencing sudden, uncommanded engine power changes — which could lead to loss of control or crash. It updates a previous safety order by adding a permanent fix (the redesigned part) rather than just ongoing inspections."}},"importantCases":[],"_links":{"self":"/api/acts/ad-al-250-84-hmu-pla-potentiometer","history":"/api/acts/ad-al-250-84-hmu-pla-potentiometer/history","analysis":"/api/acts/ad-al-250-84-hmu-pla-potentiometer/analysis","conflicts":"/api/acts/ad-al-250-84-hmu-pla-potentiometer/conflicts","importantCases":"/api/acts/ad-al-250-84-hmu-pla-potentiometer/important-cases","documents":"/api/acts/ad-al-250-84-hmu-pla-potentiometer/documents"}}