AD/AL 250/84 - HMU PLA Potentiometer

The Directive centres on three principal technical and regulatory concepts: the HMU PLA potentiometer inspection, the terminating redesign, and the linkage to ACEB technical instructions and prior directives.

1. HMU PLA potentiometer electrical inspection

• The operative safety check is an electrical signal inspection of the HMU Power Lever Angle (PLA) potentiometer. The Directive prescribes detailed steps "in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions" of the two specified ACEBs (Requirement 1). The Directive does not reproduce the test procedure or its pass/fail criteria; instead it delegates the technical specifics to those ACEB paragraphs.

2. Mandatory replacement upon failure

• If the inspection shows an "unacceptable" electrical signal, the HMU must be replaced "before further flight" (Requirement 2). The Directive uses the inspection outcome as a binary operational gate: an unacceptable result creates an immediate maintenance duty that prevents further flight until rectified.

3. Terminating action via redesigned HMU with dual‑element potentiometer

• The Directive introduces a mandatory terminating action: replacing the existing HMU with a redesigned HMU that contains a "dual element potentiometer" and which carries a P/N not listed in paragraph 1.A Group A of the ACEBs (Requirement 3). Once the replacement is made with such an HMU, the repetitive inspection requirement (Requirement 1) no longer applies ("Replacing the HMU with an HMU that has a P/N not specified in the referenced ACEBs terminates the Requirement 1 repetitive inspection").

4. Applicability linkage to ACEB P/N lists

• Applicability is not expressed solely by engine model in the Directive. It is conditional on the HMU having a P/N listed in paragraph 1.A Group A of the two ACEBs (Applicability). In other words, applicability requires cross‑referencing the ACEB group‑A P/N lists.

5. Temporal compliance structure

• The Directive sets a tiered timing regime (Compliance):
  • Inspect initially "within 300 time in service (TIS) after the effective date" or "within 300 hours TIS after completing the inspection in accordance with Requirement 1 of AD/AL 502, whichever occurs earlier".
  • Continue inspections "thereafter at intervals of 300 hours TIS until Requirement 3 is accomplished".
  • Replace an HMU found unacceptable "before further flight".
  • Accomplish the terminating replacement "within 600 hours TIS after the effective date ... or by 17 March 2005, whichever occurs earlier". This structure imposes both recurring operational costs (inspection at 300‑hour intervals) and a hard deadline for the terminating modification.

6. Document hierarchy and references

• The Directive relies on external technical documents: ACEBs CEB A‑73‑3103 Rev 4 and CEB A‑73‑6030 Rev 4 (both dated 2 December 2003) and references FAA AD 2004‑24‑04 (Note 2). It also supersedes AD/AL 250/82 (Background) and cross‑references AD/AL 502 for inspection timing. The Directive therefore functions as an administrative rule that delegates technical detail to manufacturer issued ACEBs and interacts with other ADs.

7. Safety objective stated by the issuer

• The Directive states its objective: to "prevent uncommanded and sudden changes in engine power" (Background). That stated objective connects the inspection and redesign obligations to a specific operational hazard: uncommanded engine power changes traced to HMU potentiometer failure modes.

Each of these concepts is expressed in the Directive text and in the cited ACEB references. The Directive itself does not provide the technical measurement thresholds or the redesign P/Ns; those are contained in the ACEBs and the P/N listings referenced in paragraph 1.A Group A of those ACEBs.

The Directive explicitly affects several, definable groups. The principal affected parties named in the instrument are:

• Operators and owners of RRC (formerly Allison) 250‑series engines of the specified models. The Applicability paragraph specifies RRC models 250‑C30R/3, ‑C30R/3M, ‑C47B and ‑C47M turboshaft engines that have an HMU with a part number listed in paragraph 1.A Group A of ACEBs CEB A‑73‑3103 Rev 4 and CEB A‑73‑6030 Rev 4. The Directive therefore applies only to engines that both match those models and contain HMUs with the listed P/Ns.

• Airframes and operators known to commonly carry those engine installations. The Directive includes Note 1 identifying aircraft types on which these engines "are known to be installed, but not limited to", namely Bell OH‑58D, Bell Helicopter Textron 407, Boeing AH/MH‑6M and MD Helicopters Inc. 600N helicopters. That note indicates practical relevance to operators of those helicopter types in Australia.

• Maintenance organisations and licensed personnel who perform engine and HMU inspections and replacements. The Directive requires electrical signal inspections "in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions" in the ACEBs and requires replacement of the HMU under specified conditions. Those technical tasks fall to maintenance certifying staff and approved maintenance facilities.

• Rolls‑Royce Corporation (RRC) as the OEM and supplier of ACEBs and replacement HMUs. The Directive creates a commercial demand for redesigned HMUs with dual element potentiometers and defines terminating action in terms of P/Ns published in RRC ACEBs.

• Regulators and inspectors. The Directive is issued by the Civil Aviation Safety Authority (CASA), signed by a delegate (James Coyne). CASA is the issuing authority and is identified as having notified the Directive in the Commonwealth of Australia Gazette. Consequently, CASA's oversight and enforcement apparatus are implicated as the administrative actors ensuring compliance.

• Operators who previously complied with AD/AL 250/82 or AD/AL 502. The Directive supersedes the earlier AD/AL 250/82 and references AD/AL 502 for timing; thus operators who have actions outstanding under those earlier ADs are affected by the interplay between compliance windows.

Excluded or unaffected parties under the Directive are implicit: engines of other models, and engines whose HMU P/Ns are not listed in paragraph 1.A Group A of the cited ACEBs, are not brought within the Directive’s stated applicability. The Directive does not, within its text, reach other engine componentry, nor does it purport to set requirements beyond the inspection, replacement and terminating‑action framework described.

The Directive also imposes operational restrictions tied to inspection outcomes. If the inspection result is unacceptable, the HMU must be replaced "before further flight", which impacts immediate flight operations for the affected aircraft until the HMU is replaced with an acceptable unit.

Finally, the Directive's legal force within Australia is signalled by the delegate’s signature date (23 December 2004), Gazette notification (26 January 2005), and the effective date (17 February 2005). Those administrative milestones define when affected parties must begin meeting the stated requirements.

The Directive creates a short list of express duties for affected operators and maintenance providers, together with a narrowly drawn procedural right that functions as a terminating action.

Express duties

1. Perform the prescribed electrical signal inspection (Requirement 1)

• Duty: operators must "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 either ACEB CEB A‑73‑3103 Rev 4 or CEB A‑73‑6030 Rev 4 (Requirement 1).
• Timing: initial inspection must occur "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, whichever occurs earlier" (Compliance). Thereafter inspections are required "at intervals of 300 hours TIS until Requirement 3 is accomplished" (Compliance).

2. Replace the HMU if inspection fails (Requirement 2)

• Duty: if the electrical signal result is "found to be unacceptable during the Requirement 1 inspection", the HMU must be replaced (Requirement 2).
• Timing: replacement is required "before further flight immediately following the Requirement 1 inspection" (Compliance).

3. Accomplish the terminating replacement (Requirement 3)

• Duty: operators must "replace the HMU with a redesigned HMU (i.e. 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" (Requirement 3).
• Timing: the replacing action must be done "within 600 hours TIS after the effective date of this Directive, or by 17 March 2005, whichever occurs earlier" (Compliance).
• Effect: "Replacing the HMU with an HMU that has a P/N not specified in the referenced ACEBs terminates the Requirement 1 repetitive inspection" (Requirement 3).

Procedural and evidentiary duties implicit in the Directive

• Use of ACEB Accomplishment Instructions: the inspection must be carried out "in accordance with" specified ACEB paragraphs, which imports the ACEB's defined procedures, records, and pass/fail criteria into the maintenance action (Requirement 1).
• Part number verification: determining whether an HMU is within scope requires checking the HMU P/N against paragraph 1.A Group A of the ACEBs (Applicability and Requirement 3). This creates a duty to verify P/N listings in the ACEBs.

Rights and administrative effects

1. Termination right via replacement

• Right: an operator who replaces the HMU with a redesigned unit that carries a P/N not listed in paragraph 1.A Group A of the ACEBs is entitled to cease the repetitive inspections mandated by Requirement 1. This is framed as a termination of that repetitive inspection requirement (Requirement 3).

2. Choice of ACEB reference

• The Directive allows the inspection to be performed in accordance with either ACEB CEB A‑73‑3103 Revision 4 or CEB A‑73‑6030 Revision 4 (Requirement 1). That gives operators the procedural choice consistent with the ACEB that applies to their engine/installation.

3. Interaction with earlier ADs

• Timing allowance: the initial inspection deadline is linked to completion of Requirement 1 of AD/AL 502, if that earlier inspection was already undertaken. The Directive thus recognises prior compliant inspection activity and sets the earlier of two possible timing triggers as definitive for commencing the new inspection schedule (Compliance).

No explicit liability limits, exemptions, or penalty clauses are contained in the Directive text. The duties are framed as mandatory maintenance and replacement actions; the rights are procedural (terminating action and choice of ACEB instruction). The Directive defers technical measurement definitions and the list of implicated P/Ns to the ACEBs it cites.

The Directive itself does not set out statutory penalties or a penalty schedule. The text contains no express civil or criminal sanction, fine, or administrative penalty provision. It is framed as an Airworthiness Directive issued by the Civil Aviation Safety Authority (CASA) , signed by a delegate, James Coyne, and published in the Commonwealth of Australia Gazette , which signals its function as a mandatory regulatory instrument within the aviation safety framework, but the Directive text provided does not specify enforcement mechanisms or penalties.

Notwithstanding the absence of explicit penalty clauses in the Directive text, the instrument includes enforceable operational constraints that can have immediate safety consequences:

• An unacceptable inspection result gives rise to a "before further flight" replacement obligation (Requirement 2; Compliance), which practically bars continued flight operations until the HMU is replaced.
• The Directive sets fixed compliance windows for inspections and for the terminating replacement (Compliance), imposing hard deadlines on operators.

Enforcement modalities implicit in the nature of an AD and the instrument’s administrative provenance

• Issuer and notification: the Directive is issued by CASA and was "notified in the Commonwealth of Australia Gazette on 26 January 2005" with an effective date of 17 February 2005 (signature, Gazette and effective date). Publication in the Gazette is an administrative step that communicates the Directive’s effective force to regulated parties.
• Operational enforcement: because the Directive imposes airworthiness‑related maintenance actions, failure to comply is likely to have practical consequences for an aircraft’s certification as airworthy and for the operator's ability to legally conduct flights. These consequences are not enumerated in the Directive text, but are implicit in the regulatory regime that governs airworthiness directives and aircraft maintenance.

Limitations of the Directive text regarding enforcement

• No fine schedule or explicit enforcement procedure is provided.
• The Directive does not identify the CASA compliance processes, audit mechanisms, or the administrative remedies available to CASA or to operators who disagree about applicability or technical interpretation.

Where the Directive references other ADs and ACEBs, enforcement may be effected through a combination of audit of maintenance records, operational approvals, and airworthiness certification processes. The Directive’s "before further flight" requirement is an operationally enforceable condition: a maintenance action that, by its timing language, restricts air operations until remedial work is completed.

For legal certainty about penalties or enforcement practicalities, the Directive itself would need to be read alongside the broader aviation safety regulatory framework and CASA’s enforcement policies. The Directive does not, within its own text, provide those enforcement rules or penalties.

The Directive expressly interacts with a small number of related administrative documents and prior Airworthiness Directives (ADs). The text of the Directive provides the specific interconnections that affected parties must recognise.

1. Supersession of AD/AL 250/82

• The Directive expressly "supersedes AD/AL 250/82" (Background). That is a direct legal and administrative interaction: AD/AL 250/82 previously required initial and repetitive electrical signal inspections of the HMU PLA potentiometer; the present Directive continues those inspections under revised ACEBs and adds a mandatory terminating replacement. Operators must therefore treat AD/AL 250/82 as replaced by this Directive for the matters addressed.

2. Reference to AD/AL 502

• The compliance timing for the initial inspection is linked to completion of Requirement 1 of AD/AL 502: initial inspection must occur "within 300 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, whichever occurs earlier" (Compliance). This creates an interaction where prior compliance under AD/AL 502 can affect the initial due date under the current Directive.

3. Technical dependence on ACEBs

• The Directive delegates the technical procedures and the list of P/Ns that determine applicability and terminating action to RRC Alert Commercial Engine Bulletins (ACEBs): CEB A‑73‑3103 Revision 4 and CEB A‑73‑6030 Revision 4 (Applicability; Requirement 1; Requirement 3). Those ACEBs contain the Accomplishment Instructions and the paragraph 1.A Group A P/N lists referenced by the Directive. Legally, the Directive incorporates by reference technical material that is external to the Directive text; practitioners must therefore read the ACEBs in conjunction with the Directive to understand the full scope and the technical steps required.

4. Reference to FAA AD 2004‑24‑04

• The Directive notes that "FAA AD 2004‑24‑04 Amdt 39‑13880 refers" (Note 2). That cross‑reference indicates alignment with an FAA action addressing similar safety concerns; it does not itself create a legal binding effect within the Australian regulatory framework, but it signals that the Australian Directive addresses a condition that has also been the subject of an FAA AD.

5. Administrative publication and effective date

• The Directive was "notified in the Commonwealth of Australia Gazette on 26 January 2005" and becomes effective on 17 February 2005 (signature, Gazette and effective date). This situates the Directive within Australian administrative law practice for promulgation of regulatory instruments.

Practical implications of these interactions

• Operators must reconcile previously completed actions under AD/AL 502 and AD/AL 250/82 with the new compliance schedule. The Directive’s compliance timing explicitly uses AD/AL 502 as a trigger option, so an operator who completed the earlier inspection under AD/AL 502 may have an earlier due date for the Directive’s initial inspection.
• Because applicability and the terminating action are defined by P/N lists in the ACEBs, any legal dispute about whether a given HMU is within scope will turn on the ACEB lists and their interpretation. The Directive’s reliance on external technical documents means practitioners must have and use those ACEBs to determine legal obligations.
• The Directive does not alter other unrelated statutory obligations; it operates as an AD within the broader aviation safety regime. The text itself does not specify how it interacts with civil liability, insurance, or commercial contracts; those interactions are external to the Directive and would be governed by separate legal instruments.

In short, the Directive functions by superseding a prior AD, by cross‑referencing another AD for timing, and by incorporating manufacturer ACEBs for technical detail. Those are the only interactions with other administrative instruments the Directive text itself identifies.

The Directive contains a compact amendment history within its own text and by reference to the ACEBs and other ADs. The relevant chronological facts and text citations are:

• ACEB revisions: The Directive bases its inspection and P/N listings on RRC Alert Commercial Engine Bulletins (ACEBs) CEB A‑73‑3103, Revision 4, dated 2 December 2003, and CEB A‑73‑6030, Revision 4, dated 2 December 2003 (Applicability; Requirement 1; Requirement 3). These ACEB revisions supply the Accomplishment Instructions and the Group A P/N lists that the Directive references.

• Supersession of AD/AL 250/82: The Directive "supersedes AD/AL 250/82, which required initial and repetitive electrical signal inspections of the HMU power lever angle (PLA) potentiometer" (Background). The text therefore records that the present AD replaces AD/AL 250/82 as the operative AD on this matter.

• Link to AD/AL 502: The Directive ties initial inspection timing to the completion of an inspection required by AD/AL 502 (Compliance). That linkage indicates that some operators had prior obligations under AD/AL 502 that the current AD recognises when calculating deadlines.

• FAA reference: The Directive includes "Note 2: FAA AD 2004‑24‑04 Amdt 39‑13880 refers" (Requirement section and Note 2). This notes the existence of a comparable FAA action contemporaneous with the Directive.

• Administrative dates: The AD was signed by a CASA delegate, James Coyne, on 23 December 2004 (signature block). It was "notified in the Commonwealth of Australia Gazette on 26 January 2005" and became effective on 17 February 2005 (Gazette notice and effective date). The Directive identifies a compliance hard date for the terminating replacement (Requirement 3): "within 600 hours TIS after the effective date of this Directive, or by 17 March 2005, whichever occurs earlier" (Compliance).

• Evolution in substance: The Background paragraph explains the change in regulatory approach: the earlier AD required initial and repetitive inspections; the present Directive continues those inspections but, after a redesigned HMU is released by the manufacturer, "adds replacement of the existing HMU with a new design HMU as a mandatory terminating action to the repetitive inspection requirements" (Background). That statement records the substantive evolution from inspection‑only to inspection plus mandatory terminating modification when the part becomes available.

The Directive does not provide an extended amendment log or a history of prior ACEB revisions beyond naming Revision 4 of each ACEB. It does not recount amendment drafts, stakeholder consultations, or further revisions following the effective date in the text provided. The key historical milestones recorded in the Directive are the ACEB revision dates (2 December 2003), the delegate’s signature (23 December 2004), Gazette notification (26 January 2005), effective date (17 February 2005), and the mandatory replacement deadline (17 March 2005 or within 600 hours TIS).

The text of the Directive contains no account of litigation, judicial review, or administrative appeals. It does not name any legal cases, nor does it identify any proceedings challenging the Directive’s content, scope, or enforcement. Therefore, within the document provided, there is no litigation history to report.

Where disputes or litigation might arise, the Directive’s text suggests a small set of concrete factual or interpretive issues that are likely candidates for disagreements but the document itself does not report any such disputes. Those potential dispute areas emerge from the Directive’s reliance on external technical lists and procedures:

• Determination of applicability hinges on whether an HMU "has a part number (P/N) listed in paragraph 1.A. Group A of" the cited ACEBs (Applicability). A contested factual question about whether a given HMU P/N appears in that ACEB paragraph or whether identification of a P/N on a unit is accurate could lead to disagreement.
• The terminating action is defined by replacement with an HMU whose "P/N that is not listed in paragraph 1.A. Group A of" the ACEBs (Requirement 3). Disputes could therefore turn on the ACEB P/N lists and their interpretation, or on the correct identification of a replacement HMU’s P/N.
• The inspection is to be carried out "in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions" of the ACEBs (Requirement 1). Contention could arise over whether a particular completion of those instructions satisfies the Directive, especially where the ACEB instructions leave room for technical judgment.

None of these possible conflict areas are described as having generated litigation in the text provided. The Directive does reference an FAA AD (Note 2), which may be relevant in cross‑jurisdictional regulatory or technical debates, but again the Directive does not record any legal challenges or case law.

In sum, the Directive contains no litigation history in its text. Any litigation or administrative review that occurred after issuance would be external to this document and is not documented here.

The Directive’s short, procedural form contains several points that commonly create operational or compliance traps. The following are concrete items to watch for, with direct references to the Directive text.

1. Applicability is conditional on ACEB P/N lists, not engine model alone

• The Directive applies to certain RRC engine models "that have a hydromechanical unit (HMU) with a part number (P/N) listed in paragraph 1.A. Group A of" the cited ACEBs (Applicability). Practitioners who assume applicability solely from engine model (for example 250‑C30R/3) without verifying the HMU P/N against the ACEBs risk either over‑or under‑compliance. The controlling test is whether the HMU P/N appears in the ACEB Group A list.

2. The Directive delegates technical detail to ACEB paragraphs

• The required inspection is to be performed "in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions" of ACEB CEB A‑73‑3103 Rev 4 or CEB A‑73‑6030 Rev 4 (Requirement 1). The Directive does not reproduce the procedural steps or pass/fail thresholds. Therefore, a maintenance shop that lacks the correct ACEB revision or misreads the referenced paragraphs may perform an inspection that does not legally satisfy the Directive.

3. Multiple AD cross‑references can create confusing timing triggers

• Initial inspection timing is the earlier of two triggers: "within 300 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, whichever occurs earlier" (Compliance). Operators who have completed AD/AL 502 inspections must carefully calculate which deadline controls. Errors in computing "which occurs earlier" can make an operator late for a required inspection.

4. Terminating action depends on the replacement HMU P/N not being listed

• The Directive states that replacing the HMU with an HMU that has a P/N "not specified in the referenced ACEBs terminates the Requirement 1 repetitive inspection" (Requirement 3). The implication is that merely installing a redesigned unit will not terminate repetitive inspections unless its P/N is demonstrably absent from the ACEB Group A list. Operators should document the replacement part’s P/N and retain evidence that the P/N is not listed in the ACEBs.

5. Short hard deadlines for terminating replacement

• Requirement 3 must be carried out "within 600 hours TIS after the effective date of this Directive, or by 17 March 2005, whichever occurs earlier" (Compliance). That is a comparatively short calendar and hours window from the effective date (17 February 2005). Operators must be prepared to source redesigned HMUs quickly; supply chain delays could create operational pressure or non‑compliance.

6. Immediate grounding risk on failure

• If the inspection result is "unacceptable", the Directive says to "replace the HMU" and the Compliance section requires that for Requirement 2 the replacement occur "before further flight immediately following the Requirement 1 inspection". The "before further flight" language is operationally binding: an unacceptable inspection effectively grounds the aircraft until the HMU is replaced and returned to airworthy condition.

7. The Directive supersedes a prior AD but still points to other ADs

• The Directive supersedes AD/AL 250/82 (Background) but also references AD/AL 502 for initial inspection timing (Compliance). Operators who try to treat the new Directive entirely independently of prior ADs risk missing the AD/AL 502 timing link.

8. External FAA AD reference may lead to confusion about compliance pathways

• Note 2 states "FAA AD 2004‑24‑04 Amdt 39‑13880 refers". Operators and maintainers might consult the FAA AD expecting identical procedures or compliant parts lists. The Directive ties compliance to the ACEBs named in the Australian AD; reliance on the FAA document alone may not satisfy the Directive’s expressed requirements.

9. The Directive does not itself list the implicated HMU P/Ns or the redesigned HMU P/Ns

• Because the Directive refers to ACEB paragraph 1.A Group A for P/N lists and to the ACEB Accomplishment Instructions for the inspection steps, lack of access to or misinterpretation of those ACEB paragraphs is a practical "gotcha". Practitioners must ensure they use the correct revision (Revision 4) of each ACEB dated 2 December 2003 as cited.

10. Record‑keeping and proof of termination

• The text indicates that the repetitive inspection requirement is terminated by installing an HMU whose P/N is not listed in the ACEBs. The Directive does not specify what records must be retained to prove termination. Operators should therefore ensure their maintenance documentation records the replacement, the replacement HMU P/N and a contemporaneous citation to the ACEB paragraph showing that the P/N is not in Group A.

These items are drawn directly from the Directive’s text. They identify points at which a reader must consult the ACEBs, check part numbers, understand timing triggers, or respond rapidly to inspection failures. The Directive’s delegation of technical detail to ACEBs and its reliance on P/N lists are the root causes of most of the practical pitfalls.

Compliance with the Directive requires a narrow sequence of determinations and actions tied to the engine’s HMU P/N, the ACEB instructions, and the deadlines set out in the Directive. Below is a practical, source‑grounded step‑by‑step approach to satisfy the Directive’s requirements.

1. Confirm applicability

• Check engine model: confirm the engine is one of the RRC models listed in the Directive: 250‑C30R/3, ‑C30R/3M, ‑C47B, or ‑C47M (Applicability).
• Check HMU P/N: verify the installed HMU’s part number against "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" (Applicability). The Directive applies only if the HMU P/N is listed there.

2. Obtain and use the correct ACEB documents

• Acquire ACEB CEB A‑73‑3103 Revision 4 and CEB A‑73‑6030 Revision 4 (both dated 2 December 2003). The Directive mandates that the inspection be performed "in accordance with paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions" of those ACEBs (Requirement 1). Use the exact paragraphs cited for the electrical signal inspection procedure and any associated acceptance criteria or measurement technique.

3. Schedule the initial inspection within the Directive’s deadlines

• Calculate the initial inspection due date: the Directive requires that the initial inspection be performed "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, whichever occurs earlier" (Compliance). If you completed the AD/AL 502 inspection, determine whether the 300‑hour window from that inspection is earlier.
• If no AD/AL 502 inspection was performed, compute 300 TIS from the Directive effective date (17 February 2005) and schedule the inspection accordingly.

4. Conduct the inspection exactly as specified

• Perform the electrical signal inspection in strict accordance with the ACEB paragraphs referenced in Requirement 1 (paragraphs 2.B. through 2.B.(8) and 2.B.(10) of the Accomplishment Instructions). Follow the test procedure, measurement points, instrumentation guidance and pass/fail criteria in those ACEB paragraphs.

5. Act immediately on an unacceptable inspection result

• If the inspection produces an "unacceptable" electrical signal, replace the HMU as required by Requirement 2. The Directive mandates replacement "before further flight immediately following the Requirement 1 inspection" (Requirement 2; Compliance). Do not operate the aircraft until the HMU has been replaced and the aircraft returned to an airworthy state.

6. Plan and execute the terminating replacement

• Source the redesigned HMU that features a "dual element potentiometer" and that carries a P/N "that is not listed in paragraph 1.A. Group A of" the ACEBs (Requirement 3).
• Install the redesigned HMU within the time limit: "within 600 hours TIS after the effective date of this Directive, or by 17 March 2005, whichever occurs earlier" (Compliance).
• After installation, document the replacement and the replacement HMU’s P/N, and retain ACEB references showing that the installed P/N is not in Group A. This supports reliance on the Directive’s provision that such a replacement "terminates the Requirement 1 repetitive inspection".

7. Maintain inspection cadence until termination

• If the terminating replacement has not been accomplished, continue to perform the electrical signal inspection "thereafter at intervals of 300 hours TIS until Requirement 3 is accomplished" (Compliance). Keep accurate TIS records to demonstrate inspections were conducted at the specified 300‑hour intervals.

8. Record keeping and proof

• Document each inspection in maintenance records, including the ACEB paragraph used, test instrument readings or forms, the conclusion (acceptable/unacceptable), and any remedial work performed.
• For any replacement HMU, record the part number, serial number, installation date and TIS at installation. Retain the ACEB P/N lists or a contemporaneous printout showing that the installed HMU P/N is not listed in paragraph 1.A Group A.

9. Be aware of related ADs and cross‑references

• Recognise the links to AD/AL 502 for timing (Compliance) and to the superseded AD/AL 250/82 (Background). If AD/AL 502 inspections were completed, use that completion date to determine the earliest applicable initial inspection deadline under this Directive.

10. Use the ACEB choice appropriately

• The Directive permits the use of either ACEB CEB A‑73‑3103 Rev 4 or CEB A‑73‑6030 Rev 4 for the Accomplishment Instructions (Requirement 1). Use the ACEB that corresponds to your engine/installation and apply the exact paragraphs cited.

11. Consider operational planning and parts procurement

• Given the relatively short terminating replacement deadline (within 600 TIS or by 17 March 2005), plan procurement and scheduling to avoid supply delays causing missed deadlines. While the Directive text does not describe relief mechanisms, scheduling replacements early reduces exposure to the "before further flight" consequence of an unacceptable inspection.

This compliance pathway is derived directly from the Directive’s text, which prescribes the inspection method by reference to ACEB paragraphs, requires immediate replacement upon failure, sets inspection intervals and deadlines, and defines the terminating action by reference to ACEB P/N lists. The Directive does not provide alternative compliance methods, exemptions, or relief procedures in the text provided; therefore compliance must follow the steps laid down or be negotiated with CASA through applicable administrative channels outside the Directive text.