AD/CFM56/32 - Engine Hydromechanical Unit

The AD revolves around three technical and regulatory concepts: the Hydromechanical Unit (HMU) as an engine control component, specific ECAM warnings that indicate HMU-related faults, and service-bulletin-driven corrective actions.

Hydromechanical Unit (HMU)

• The AD targets CFM56-5B engines "equipped with Hydromechanical units (HMU) VIN 8061-532 (P/N 1348M79P10), VIN 8061-535 (P/N 1348M79P13), VIN 8061-536 (P/N 1348M79P14), Serial Numbers (S/N) as identified within paragraph 1.A of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Applicability). The AD does not provide an internal technical definition of HMU, but treats it as the component implicated in VSV/VBV scored-vanes malfunctions (Background).

ECAM warnings as triggers

• The AD identifies two ECAM warning text strings as the operational indicators that trigger replacement: "ENG X START FAULT- ENG X STALL" and "ENG X COMPRESSOR VANE" (Requirement 1). The AD clarifies that the character "X" is a placeholder for engine number 1 or 2 (Note 2). Operators must search engine records "since entry into service of the HMUs" for any occurrence of those warnings (Requirement 1). The AD therefore uses recorded ECAM events as the observable manifestation of the underlying HMU failure mode.

Service bulletin compliance as the means of corrective action

• Replacement actions are not freeform. The AD requires replacement "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Requirement 2; Replacement instruction). The AD also provides an Optional Terminating Action: modification of both HMUs per CFM SB 73-0208, 73-0239 or 73-0180, and allows later approved revisions of those SBs to be used for compliance (Optional Terminating Action). The AD therefore delegates technical detail and work-scope to manufacturer service information and accepts subsequent approved revisions.

Risk framing provided by the Background

• The Background identifies in-flight compressor stall events on the CFM56-5B series, and explicitly connects the events to HMU VSV/VBV scored vanes malfunction, the risk of in-flight engine shutdown, and the greater hazard if more than one engine is affected (Background). That sequence is the expressed causal logic for the AD’s prescribed inspections and replacements.

Regulatory cross-reference

• The AD notes that "EASA AD 2009-0154-E dated 17 July 2009 refers" (Note 3). This indicates contemporaneous parallel regulatory action by the European authority and positions the CASA AD in an international regulatory context. The AD otherwise relies on CASA issuance and on CFM service bulletins for the technical compliance path.

Operational mechanics and timing

• The AD sets strict compliance windows: the initial records check within 24 hours of the AD becoming effective, and various "before further flight" or 30‑day deadlines depending on the situation (Compliance). These timings convert the safety rationale into immediate maintenance and operational tasks.

Collectively, the main concepts tie an observed cockpit warning signature to a manufacturer-prescribed replacement or modification action, with the regulator enforcing rapid inspection and remedial work through an AD that delegates technical detail to specific service bulletin paragraphs.

The AD explicitly limits applicability to a defined set of engines and HMU serial numbers, but that set maps to a broader group of operators, maintenance organisations and parts suppliers.

Directly affected aircraft and operators

• The AD applies to "CFM International SA CFM56-5B engine models equipped with Hydromechanical units (HMU) VIN 8061-532 (P/N 1348M79P10), VIN 8061-535 (P/N 1348M79P13), VIN 8061-536 (P/N 1348M79P14), Serial Numbers (S/N) as identified within paragraph 1.A of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1 and which have not been modified in accordance with CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180" (Applicability). Operators must compare their engines’ HMU VINs and S/Ns to the list in paragraph 1.A of the service bulletin to determine applicability.

Aircraft types where these engines are known to be installed

• The AD notes that these engines "are known to be installed on, but not limited to Airbus A318, A319, A320 and A321 series aeroplanes" (Applicability Note 1). That means fleet operators of those Airbus types should prioritise checks, but the AD does not confine applicability to those models alone.

Maintenance organisations and shops

• Replacement and modification work must be carried out "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" or, for the terminating action, per the listed SBs (Requirement text; Optional Terminating Action). This places maintenance providers, both in-house and contracted, squarely within the affected cohort. They must be able to obtain and execute the relevant SB procedures.

Parts suppliers and inventory managers

• The AD requires replacement of affected HMUs with a "serviceable unit" (Requirement 2; Requirement 3.a/b). Operators and their supply chains therefore must source serviceable HMUs or organise the modifications described in the terminating action. Procurement and spare inventory management are thus affected.

Regulatory and oversight bodies

• The AD is issued by the Civil Aviation Safety Authority via a delegate (signed by James Coyne) and becomes effective on a specific date (23 July 2009 signature; effective 27 July 2009). National aviation authorities, continuing airworthiness inspectors and safety managers will be the ones to verify compliance against the AD’s timing and records requirements (signature and effective date lines; Compliance).

Who pays, who decides, who acts

• The document itself places the operational burden on operators and maintenance agents: operators must check records, perform replacements, and arrange modifications. The AD sets the decisions about what must be done (the regulator’s mandate) and delegates the technical implementation to maintenance in accordance with the specified SB paragraphs. The AD does not allocate funding or state a cost-recovery mechanism; the practical outcome is that operators incur the direct costs of inspections, component replacements and any associated aircraft downtime.

Secondary effects on scheduling and operations

• Because some actions are "before further flight" and others are within short windows (24 hours, 30 days), operational planners, flight dispatch and maintenance schedulers are affected. If a replacement is required before further flight, the aircraft will be taken out of service until the HMU is replaced, which affects utilisation and may cascade into crew and passenger scheduling impacts.

In summary, the AD primarily affects aircraft operators of CFM56-5B engines with the listed HMUs, their maintenance providers, and spare-part suppliers, with enforcement and oversight by CASA as indicated by the AD’s issuance and effective date.

The AD sets out a concise set of duties for operators and maintenance organisations, and specifies limited procedural rights about acceptable compliance methods.

Key duties imposed by the AD

• Immediate records check: Requirement 1 mandates that operators check engine records "for any occurrence of ECAM 'ENG X START FAULT- ENG X STALL' and/or 'ENG X COMPRESSOR VANE' warnings since entry into service of the HMUs" (Requirement 1). The Compliance row requires that this check be completed "Within 24 hours after the effective date of this AD" unless already done.

• Mandatory replacement when warnings have already occurred: Requirement 2 states that "For engines which have displayed before the effective date of this AD, any of the ECAM warnings described in Requirement 1 of this AD, replace the affected HMU with a serviceable unit." The Compliance row requires this replacement "Before further flight" unless already accomplished.

• Mandatory replacement upon occurrence for previously clean engines: Requirement 3.a covers engines that have never displayed the warnings before the AD. It imposes an immediate duty that "after any ECAM warnings described in Requirement 1 ... Replace the affected HMU with a serviceable unit." The replacement must be completed "Before further flight."

• Replacement when both engines have affected HMUs: Requirement 3.b states that "If both engines on the same aeroplane are found with HMUs S/N as identified within paragraph 1.A of SB CFM56-5B S/B 73-0238 Revision 1, replace at least one HMU with a serviceable unit." The Compliance row sets the deadline "Within 30 days after the effective date of this AD."

• Method of replacement: Every replacement must be "done in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Replacement instruction).

Optional rights and acceptable alternatives

• Optional terminating action: The AD allows an alternative corrective action that terminates the AD’s requirements by modifying both HMUs according to one of three CFM SBs: 73-0208, 73-0239 or 73-0180. The AD explicitly permits "the use of later approved revisions of these documents" for compliance (Optional Terminating Action). This gives operators a clear compliance pathway that can be treated as a permanent fix if carried out on both HMUs.

• Use of approved service bulletin revisions: The AD’s language on later approved revisions is a procedural right that allows operators to adopt an updated manufacturer procedure so long as it is an approved revision. The AD does not specify the approval authority or method for approving later revisions, but it does state that their use is acceptable to meet the AD requirements (Optional Terminating Action).

Timing obligations

• Requirement 1 must be completed within 24 hours after the AD is effective. Requirement 2 and 3.a both require replacement "before further flight." Requirement 3.b requires at least one HMU replacement "within 30 days after the effective date of this AD." These timings create immediate and short-term duties for operators once the AD is effective (Compliance).

Procedural delegations and work specification

• The AD delegates the technical details of replacement and modification to manufacturer service bulletins, and specifically to paragraph 3.C of SB 73-0238 for replacement. Thus the duty is both substantive (replace specified HMUs) and procedural (do so following the SB instruction).

What the AD does not state as rights or duties

• The AD does not define "serviceable unit" beyond requiring replacement with one; it does not lay out inspection techniques, nor does it specify penalties for non-compliance within the text provided. It does not list a formal method to request alternative means of compliance, nor does it provide an explicit record-retention period within the AD text supplied.

Notice and effective date

• The AD sets an effective date of 27 July 2009 and is signed by a CASA delegate (James Coyne, 23 July 2009). Those dates and the signature show when the duties crystallise and identify the issuing authority (signature and effective date lines).

In practice, operators have clear duties to inspect records promptly, to carry out replacements or modifications within short windows, and the right to use specified SB modifications, including later approved revisions, as a terminating action. The AD ties the how-to directly to manufacturer service bulletin paragraphs.

The AD text supplied sets compliance obligations, deadlines and an issuing authority, but does not itself articulate specific penalties or enforcement procedures within its wording. The document does, however, provide the formal regulatory instrument, effective date, and the compliance timetable that frame enforcement activity.

Elements in the AD that support enforcement activity

• Issuing authority and signature: The AD is presented as an official CASA instrument and is signed by "James Coyne, Delegate of the Civil Aviation Safety Authority" with the signature dated 23 July 2009. The AD's effective date is stated as 27 July 2009. Those items establish the AD as a formal directive that operators are required to meet (signature and effective date lines).

• Express compliance deadlines: The AD sets express and short compliance windows: the records check "Within 24 hours after the effective date" (Requirement 1), replacements "Before further flight" in certain cases (Requirement 2 and 3.a), and at least one HMU replacement "Within 30 days after the effective date" where both engines on an aeroplane have the affected HMUs (Requirement 3.b). The presence of these clear, time-stamped obligations enables an overseeing regulator to determine compliance or non-compliance during surveillance and inspections (Compliance table).

• Delegation to manufacturer procedures: The AD requires that replacements be done "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B SB 73-0238 Revision 1" (Replacement instruction). This facilitates inspection verification because the work scope and method are specified in known manufacturer documentation.

What the AD does not state

• The AD text provided does not specify civil or criminal penalties, monetary fines, or administrative enforcement measures for failure to comply. It also does not set out an explicit process for requesting an alternative means of compliance (AMOC) or a formal approval channel for deviations. The document likewise does not set a record-retention period for evidence of compliance or describe audit processes.

Implications for oversight and enforcement practice

• Because the AD creates a clear inspection and replacement duty with precise deadlines, enforcement in practice would rely on CASA’s broader regulatory powers and oversight mechanisms under the national aviation law and regulations. The AD’s inclusion of a CASA delegate’s signature and effective date provides the legal imprimatur that national regulators use to direct enforcement action. The AD’s specification that replacement must follow a particular SB paragraph provides an auditable standard to which maintenance records and work-orders can be compared.

Reference to related regulatory action

• The AD notes that "EASA AD 2009-0154-E dated 17 July 2009 refers" (Note 3), indicating parallel regulatory action by the European Aviation Safety Agency. That cross-reference is relevant in any multinational oversight or operator-of-record determination, but the AD text supplied does not elaborate how EASA’s actions interact with CASA enforcement procedure.

Practical consequence

• The immediate practical enforcement consequence built into the AD is operational: if the required replacement is not accomplished "before further flight" in cases where that is specified, the aircraft would not meet the AD’s airworthiness requirement and, in ordinary regulatory practice, would be grounded until compliance is demonstrated. The AD itself stops short of enumerating the downstream compliance-enforcement tools or penalties; those are ordinarily the subject of the regulator’s statutory enforcement framework rather than the AD text itself.

In sum, the AD creates measurable duties and timeframes that enable enforcement, but it does not in this text set out penalties or the detailed enforcement process; enforcement would rely on CASA’s existing regulatory powers and oversight procedures outside the literal words of this AD.

The AD primarily operates through reference to manufacturer service bulletins and cross-reference to contemporaneous foreign airworthiness directives. The text supplied contains limited explicit discussion of interaction with other statutes, regulations or instruments, so the interaction must be read from the AD’s internal cross-references.

Reference to manufacturer service bulletins as compliance mechanism

• The AD requires replacement "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" and allows an Optional Terminating Action by modifying both HMUs per CFM SBs 73-0208, 73-0239 or 73-0180, with later approved revisions acceptable (Requirement text; Optional Terminating Action). Through those references the AD incorporates manufacturer-sanctioned procedures into regulatory compliance. In practice that means that the technical content of the SBs becomes the operative method to meet the AD requirements.

Cross-reference to the European AD

• Note 3 in the AD states "EASA AD 2009-0154-E dated 17 July 2009 refers." This establishes a contemporaneous link to an EASA instrument. The wording suggests that EASA issued an AD addressing the same matter around the same time, but the AD text supplied does not set out the legal consequences of compliance with EASA ADs for Australian operators, nor does it say whether compliance with the EASA AD automatically satisfies CASA. The AD does, however, make the cross-reference an adjunct to operator awareness of internationally coordinated action.

Tension between manufacturer SBs and regulatory approval

• The AD accepts "later approved revisions" of the listed SBs for compliance, which implies an approval process exists whereby revised SBs become acceptable for AD compliance. The AD does not specify the approval authority or the administrative route to obtain approval for a later revision, but explicitly permits their use.

Absence of explicit interaction with civil aviation regulations in the AD text

• The AD does not quote or reference particular provisions of the Civil Aviation Regulations or Acts. It does not specify administrative enforcement powers, penalties or the procedural mechanics for AMOCs within the AD text provided. Those interactions are therefore implicit: the AD is the instrument that feeds into the broader statutory and regulatory framework governing continuing airworthiness and enforcement, but the AD itself leaves those linkages external and unenumerated.

Practical implications for multi-jurisdictional operators

• Because the AD references EASA and CFM SBs, operators that operate internationally will typically need to reconcile CASA’s AD with parallel foreign directives and manufacturer recommendations. The AD’s acceptance of later approved SB revisions suggests that CASA intends to align technical compliance paths with manufacturer updates, which can facilitate international fleet maintenance planning. The AD does not specify whether compliance with the EASA AD is in itself sufficient in Australia; operators should check CASA’s published guidance or seek formal confirmation if needed.

Net effect

• The AD creates a regulatory obligation that is satisfied by performing tasks specified in manufacturer service bulletins, and sits alongside related international action taken by EASA. It does not, within its own text, restate the regulatory provisions under which CASA enforces compliance or provide a standalone legal pathway for alternate compliance beyond the optional terminating action specified.

The source text supplied contains the initial issuance details for this AD and does not record subsequent amendments or revisions within the text provided.

Issuance details in the document

• The AD identification is AD/CFM56/32, appearing in the header block with the title "Engine Hydromechanical Unit" and dated "10/2009 TX". The signature block indicates the delegate of CASA, James Coyne, signed on 23 July 2009. The AD states an effective date: "This Airworthiness Directive becomes effective on 27 July 2009" (signature and effective date lines; header).

References connecting to other regulatory actions

• The AD contains a Note 3 which states "EASA AD 2009-0154-E dated 17 July 2009 refers." That indicates that EASA issued a related AD approximately ten days earlier than the effective date of this AD; the supplied text does not treat that EASA AD as an amendment to the CASA AD, but as a related instrument.

Service bulletin revision references

• The AD mandates actions per "CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Applicability; Replacement instruction). It also identifies alternative terminating SBs: "CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180" and states that "The use of later approved revisions of these documents is acceptable for compliance with the requirements of this AD" (Optional Terminating Action). The AD therefore accommodates future manufacturer revisions in principle, but the AD text itself does not record any later approved revisions or amendments to the AD.

No recorded modifications or supersessions in the supplied text

• The AD text provided contains no amendment log, no "supersedes" language, and no later-date alterations recorded within the document. There is no mention of prior CASA ADs that are superseded or varied by this AD. The AD also does not list serial-number corrections, compliance relaxations or extensions.

Implication for users of this AD

• Based on the supplied text, the AD as issued in July 2009 stands in its initial form. Operators relying on this AD should, however, verify the current regulatory status with CASA (outside the bounds of this AD text) to ascertain whether subsequent amendments, supersessions, or acceptances of later service bulletin revisions have been formally recorded after the effective date.

Summary

• The document records initial issuance dates, references relevant SB revisions as the operative technical documents, and cross-references an EASA AD. The supplied AD does not contain any subsequent amendment entries or history beyond initial issue.

The text of the AD supplied contains no litigation history. It does not mention any court proceedings, appeals, or administrative challenges related to the AD’s issuance or content.

What the AD states about legal process

• The AD is a regulatory instrument issued by the Civil Aviation Safety Authority via its delegate, and it contains the effective date and the required technical and compliance actions. It offers an optional terminating action by following specified manufacturer service bulletins and permits the use of later approved revisions of those documents (Optional Terminating Action). The AD does not set out any dispute resolution or appeal mechanism within the text that is provided.

What is absent

• There is no mention of any party having challenged the AD, nor any recording of outcomes of legal challenges, enforcement actions in courts, administrative reviews, or tribunal decisions. There are no citations to judicial or administrative decisions, and the AD name-list does not include case citations.

Implication for research and compliance

• Because no litigation history is provided in the AD text, legal practitioners and compliance officers seeking to understand any disputes about the AD will need to look beyond this document to external records, court dockets or CASA administrative records. The AD’s own text does not supply any information about whether its requirements were contested or upheld in any judicial or administrative forum.

In short, the supplied instrument contains no litigation history or litigation-related references; it is silent on appeals or contested enforcement in the material provided.

The AD contains concise requirements, but there are several practical and interpretive points within the text that can trip up operators and maintenance organisations if not read closely.

Exact serial-number list is external to the AD

• The AD’s applicability hinges on serial numbers "as identified within paragraph 1.A of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Applicability). The AD itself does not reproduce the serial-number list. Operators must consult paragraph 1.A of SB 73-0238 Rev 1 to determine whether their HMUs are included. Failure to consult that paragraph could lead to misclassification of applicability.

"Since entry into service of the HMUs" extends the lookback window

• Requirement 1 instructs operators to check engine records for occurrences of the specified ECAM warnings "since entry into service of the HMUs" (Requirement 1). That wording requires a service-history review covering the full life of the HMU, not merely a recent window. Depending on record-keeping practices, reconstructing that history could be non-trivial.

ECAM wording must be matched exactly, with engine-number placeholder

• The AD specifies the ECAM message texts to search: "ENG X START FAULT- ENG X STALL" and "ENG X COMPRESSOR VANE" (Requirement 1). It clarifies that "X" is a placeholder for engine 1 or 2 (Note 2). Operators must search for either engine-number variant and be careful with spacing, hyphenation and message variants; the AD does not provide equivalence rules for similar but not identical messages.

Immediate compliance timelines create operational pressure

• Requirement 1 must be completed "Within 24 hours after the effective date of this AD" (Compliance). Requirement 2 and 3.a require replacement "Before further flight." These short deadlines impose scheduling and supply-chain pressure: obtaining a "serviceable unit," arranging maintenance and downtime must all be accomplished promptly. There is no grace period given in the AD text.

"Serviceable unit" is not defined in the AD

• The AD mandates replacement with a "serviceable unit" but does not define the term. Operators will need to rely on their maintenance procedures or the service bulletin (paragraph 3.C) to determine what meets the "serviceable" standard. Different maintenance organisations may interpret this differently unless the SB sets the acceptance criteria.

Both-engines rule may have operational consequences

• Requirement 3.b requires that "If both engines on the same aeroplane are found with HMUs S/N as identified ... replace at least one HMU with a serviceable unit" and sets the compliance time "Within 30 days after the effective date of this AD" (Requirement 3.b; Compliance). This can be an unexpected logistical constraint for airlines that have two affected HMUs but no prior ECAM warnings. The AD compels at least one replacement within 30 days even absent warning events.

Replacement method prescribed in a specific SB paragraph

• The AD mandates replacement "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Requirement 2 / Replacement instruction). Operators must ensure they have access to SB 73-0238 Rev 1 and specifically paragraph 3.C; lacking access or misreading the paragraph could lead to non-compliant work.

Optional terminating action limited to three SBs

• The AD accepts modifications per three specific CFM SBs (73-0208, 73-0239, 73-0180) as an alternative corrective action (Optional Terminating Action). The AD permits "later approved revisions," but the AD does not state who approves such revisions. Operators must verify that any later SB revision they intend to use is indeed officially approved for AD compliance.

EASA AD cross-reference implies parallel obligations

• Note 3 says "EASA AD 2009-0154-E dated 17 July 2009 refers." For operators that operate in multiple jurisdictions, there may be overlapping or slightly different requirements. The AD does not state whether compliance with the EASA AD satisfies CASA’s AD; practical fleet management should confirm this.

Potential ambiguity about applicability to modified HMUs

• The applicability excludes HMUs "which have not been modified in accordance with CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180" (Applicability). Operators who have carried out partial or non‑standard modifications may need to establish whether those modifications meet the AD’s exclusion criteria. The AD is specific only about those three SB modification references.

Record-keeping and evidence of compliance

• Although not spelled out, compliance with the AD will typically require retention of records showing the engine-record search, the date of replacement, and the SB references used. Because the AD requires immediate action, maintaining auditable documentation will be important for later inspector verification.

In short, the principal "gotchas" stem from reliance on off‑document serial-number lists in the manufacturer SB, tight compliance windows, precise ECAM message matching, and the need to follow specific SB paragraphs for replacement. Operators should plan logistics, parts availability, and maintenance access promptly to meet the AD’s deadlines.

The AD sets clear, short-timed tasks and prescribes the technical references governing how those tasks must be carried out. Below is a stepwise compliance checklist strictly derived from the AD text.

1. Identify affected engines and HMUs

• Consult your fleet records to identify engines of the model CFM56-5B that are fitted with Hydromechanical Units having VINs 8061-532 (P/N 1348M79P10), 8061-535 (P/N 1348M79P13), 8061-536 (P/N 1348M79P14). Then consult paragraph 1.A of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1 to determine whether the HMU serial numbers on your engines are listed there (Applicability). Note that the AD excludes HMUs "which have been modified in accordance with CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180" from the applicability list for non‑modified units (Applicability).

2. Perform the initial records check within 24 hours

• Requirement 1 requires checking engine records "for any occurrence of ECAM 'ENG X START FAULT- ENG X STALL' and/or 'ENG X COMPRESSOR VANE' warnings since entry into service of the HMUs." Complete this check "Within 24 hours after the effective date of this AD" unless it has already been accomplished (Requirement 1; Compliance). Ensure searches include both engine-number variants because "X" is a placeholder for 1 or 2 (Note 2).

3. If records show prior occurrences, replace affected HMU before further flight

• If any engine has "displayed before the effective date of this AD, any of the ECAM warnings described in Requirement 1," Requirement 2 mandates replacing the affected HMU with a serviceable unit. The AD requires this replacement to be completed "Before further flight" unless already accomplished (Requirement 2; Compliance). Execute the replacement in accordance with paragraph 3.C of CFM SB 73-0238 Revision 1.

4. If no prior occurrences, react to any future occurrences immediately

• For engines that have never displayed the ECAM warnings prior to the AD (Requirement 3), the AD requires that "after any ECAM warnings described in Requirement 1 ... Replace the affected HMU with a serviceable unit." The replacement must be completed "Before further flight" (Requirement 3.a; Compliance). Maintain monitoring procedures to catch and act upon any future ECAM events promptly.

5. If both engines have affected HMU serial numbers but no warnings, replace at least one within 30 days

• If inspection reveals both engines on the same aeroplane have HMU S/Ns as identified in paragraph 1.A of the SB and neither engine has displayed the warnings, Requirement 3.b requires replacing at least one HMU "Within 30 days after the effective date of this AD." Plan maintenance accordingly to meet that window.

6. Use manufacturer SB procedures for replacement

• All HMU replacements must be performed "in accordance with paragraph 3.C of CFM Service Bulletin CFM56-5B S/B 73-0238 Revision 1" (Replacement instruction). Ensure the maintenance organisation has access to the correct SB revision and follows the procedures and acceptance criteria in paragraph 3.C. Document the SB revision used and the work order evidence.

7. Consider the Optional Terminating Action for a permanent fix

• The AD allows an optional terminating action: "Modification of both HMUs per one of the following SBs: CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180 is an alternative corrective action for the requirements of this AD." Using any of these modifications on both HMUs satisfies the AD, provided the SB revision used is an acceptable later approved revision (Optional Terminating Action). If electing this route, schedule modifications for both HMUs and record the SB revision(s) and approval status.

8. Record and retain evidence of compliance

• Although the AD text does not set a retention period, best practice derived from the AD’s duties is to keep records showing the initial ECAM search, dates and results, work orders for HMU replacement or modification, the SB(s) and paragraph numbers relied upon (for replacement, paragraph 3.C of SB 73-0238 Rev 1), and serial numbers of replaced HMUs. These records will be the primary evidence for an inspector to verify AD compliance.

9. Co-ordinate parts procurement and scheduling

• Because several replacement requirements are "before further flight" or within 24 hours/30 days, plan for rapid acquisition of "serviceable units" and ensure maintenance resources are available. Confirm what qualifies as a "serviceable unit" by reference to the SB instructions and your approved maintenance procedures.

10. Confirm interaction with international directives if operating overseas

• The AD notes that "EASA AD 2009-0154-E dated 17 July 2009 refers" (Note 3). If operating in jurisdictions beyond Australia, confirm whether compliance with the EASA AD or the specified CFM SBs satisfies local requirements, and ensure any SB revision used is approved for the relevant authority.

11. Verify that HMUs modified under the specified SBs are outside the AD’s applicability

• The AD excludes HMUs "which have been modified in accordance with CFM56-5A S/B 73-0208, CFM56-5B S/B 73-0239 or CFM56-5C S/B 73-0180" from the set of affected units (Applicability). If HMUs on your engines have previously been modified per those SBs, document the modification records to demonstrate non-applicability.

12. Stay alert for updated guidance

• The AD accepts later approved SB revisions for compliance. While the AD text does not itself provide a process for confirming approval status, operators should monitor CFM communications and CASA updates (outside the AD text) for any approved SB revisions or formal AMOCs.

This compliance checklist follows only the instructions and cross-references contained in the AD text. Operators should execute the tasks within the specified deadlines, document all steps, and ensure maintenance tasks are performed in accordance with the specified service bulletin paragraph(s).