AD/AIRCON/14 Amdt 4 - Zonal Drying System Regeneration Air Duct Overheat

Key concepts in the AD are applicability, immediate mitigation, terminating action, and documentation for compliance. Applicability is defined by P/N and STC installation: the AD applies to the specified CTT Systems AB Zonal Drying System P/Ns when installed under one of the STCs listed in Table 1. The AD cautions the STC list in Table 1 "may be not exhaustive" and states that absence of an aircraft from the tables does not exempt an aircraft having an affected P/N from compliance (Applicability text). This places the onus on operators to check actual P/N installations on their airframes rather than rely on aircraft model lists.

Immediate mitigation is the operational action mandated by the AD: "Switch off the Zonal Drying System installed in accordance with one of the STCs listed in Table 1 ... and set system inoperative by pulling and securing the affected circuit breaker in accordance with relevant maintenance instructions" (Requirement). The AD prescribes the method of deactivation (pull and secure circuit breaker) and ties the physical action to "relevant maintenance instructions", implying that the maintenance organisation or engineer must follow approved procedures.

Terminating action is explicitly provided and narrowly framed. Modification of the affected Zonal Drying System by installing a new dryer unit, approved via the STCs listed in Table 2, "corrects the unsafe condition and terminates the requirements of this AD" (Background and Terminating Action sections). Table 2 lists the STC numbers, issuing authorities (EASA), the STC holder (principally Lufthansa Technik AG and CTT Systems AB, plus Jet Aviation), and the applicable airframes for which the STC provides terminating action. The AD also permits the use of later approved revisions of the referenced documents (CTT Information Letters and the Lufthansa service letter) for compliance.

Technical cause and remedy are stated in the Background. One reported incident involved premature material failure of regeneration air ducting downstream of a Zonal Drying System P/N 3000-002 installed in a Boeing 747-400. Investigation "showed that such failure was caused by combustion of contaminants in the dryer unit" (Background). The AD notes that a "new dryer unit" was developed which "removes this combustion risk" and that the new unit's installation, via listed STCs, is the approved terminating action.

Authorities and cross-references appear through STC issuing bodies and linked guidance: STCs and approvals are issued by bodies including LBA (Germany), EASA, FOCA (Swiss), and RLD, and the AD references EASA AD 2006-0140R4 (Note 4). The AD itself is issued under the delegation of the Civil Aviation Safety Authority (delegate signature and date are included). The compliance imperative is absolute in timing: "Before further flight after the effective date ... unless previously accomplished" (Compliance).

The AD thus operates as an immediate operational restriction targeted by part numbers and STC installations (a hardware/installation-specific safety directive), coupled with a narrowly defined engineering remedy (install STCs that embody the redesigned dryer unit) and documentary guidance (CTT and Lufthansa information letters and service information). The AD also documents amendment iterations, indicating an evolving scope of termination approvals and revisions to STCs.

The AD affects several distinct categories of actors. First and primarily, aircraft operators and owners who have one of the identified CTT Systems AB Zonal Drying System P/Ns installed on their aircraft are affected. The Applicability section lists P/Ns and notes installation on a range of aircraft types "known to be installed in, but not limited to, Airbus A330-243 and A330-322; Boeing 737-700, 737-800, 747-400, and 767-300 series; Bombardier CL-600-2B19; and McDonnell Douglas MD-11 series aeroplanes." The AD then specifies STCs under which the units are installed (Table 1). Operators of those airframes must check whether the affected P/N is present, irrespective of whether their aircraft model is explicitly listed, because the AD warns the Table 1 approval list "may be not exhaustive" and that aircraft not listed are not exempt if they have an affected P/N installed (Applicability).

Maintenance organisations and licensed aircraft maintenance engineers are directly affected by the procedural requirement to set the Zonal Drying System inoperative by "pulling and securing the affected circuit breaker in accordance with relevant maintenance instructions" (Requirement). That language requires maintenance action taken in accordance with approved maintenance documentation and suggests involvement of an appropriately certificated maintenance organisation or personnel.

STC holders and modifiers are an affected group with remediation responsibilities and commercial opportunity: the AD identifies STC holders (CTT Systems AB, Lufthansa Technik AG, Jet Aviation, SR Technics, and others) in Tables 1 and 2. Table 2 lists the STCs that, when installed, provide terminating action; these STCs are principally held by Lufthansa Technik AG and CTT Systems AB (with Jet Aviation present for a particular Boeing 737-100 installation). Where an operator elects the terminating action, the STC holder (or an appropriate maintenance/engineering organisation approved to carry out the modification) will be engaged to perform the modification and reactivation work.

Regulators and certification authorities are implicated. The AD records STCs that were issued by national and regional authorities (LBA, EASA, FOCA, RLD). The AD itself is issued by a delegate of the Civil Aviation Safety Authority (James Coyne), addressing Australian-regulated operations; the AD references EASA AD 2006-0140R4 (Note 4) and EASA approvals for terminating STCs, indicating coordination or correspondence with European certification activity. Where STCs issued by foreign authorities have been used to show terminating action, the AD recognises those approvals in Table 2.

Flight operations personnel and safety/compliance staff in airlines and operators are affected operationally: the AD requires that the system be inoperative "before further flight", which aviation operators will have to manage in dispatch, flight release, and maintenance scheduling processes. That includes updating aircraft status records (to reflect a circuit breaker that has been pulled and secured), flight crew awareness (if applicable), and maintenance release documentation. The AD also directs operators to CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03 and Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR for technical instructions and allows later approved revisions for compliance, thereby affecting document control and engineering records.

Finally, the AD affects insurers, lessors and asset managers indirectly: availability of an STC-based terminating action and the requirement to de-activate until modification may influence aircraft downtime and maintenance planning, which are considerations for parties managing aircraft assets. The AD itself does not address contractual relationships, but its mandatory operational requirement will be material to those stakeholders.

Duties

• Immediate de-activation: The central duty imposed by the AD is operational and immediate: operators must "Switch off the Zonal Drying System installed in accordance with one of the STCs listed in Table 1 of the Applicability section of this AD and set system inoperative by pulling and securing the affected circuit breaker in accordance with relevant maintenance instructions" (Requirement). Compliance timing is mandatory: "Before further flight after the effective date of this AD, unless previously accomplished" (Compliance). The effective date is specified as 2 July 2009 (Compliance section).

• Use of approved maintenance instructions: The physical act of pulling and securing the circuit breaker must be done "in accordance with relevant maintenance instructions" (Requirement). That imposes a duty to follow approved maintenance procedures and document the action as required by standard maintenance practice.

• Reliance on STC/engineering modifications for termination: Operators choosing to restore system functionality must arrange for installation of the approved terminating STCs listed in Table 2. The AD states that "On aircraft where the following STCs are installed, the actions detailed in this AD are no longer applicable and the Zonal Drying System accordingly modified can be re-activated" (Terminating Action). The AD also identifies the new dryer unit and that its installation "is now approved through the EASA STCs listed in Table 2" (Background). Thus operators have the duty to procure and install the approved modification if they want to terminate the de-activation requirement.

• Use of specified technical documents: The AD points to CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03 and Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR, and permits later approved revisions for compliance (Relevant documents section). There is a duty to consult and apply those documents as part of compliance.

Rights and clarifications

• Right to use later approved revisions: The AD explicitly allows the use of later approved revisions of the referenced documents: "The use of later approved revisions of these documents is acceptable for compliance with the requirements of this AD" (Relevant documents). That provides operators, maintenance organisations and modifiers the right to rely on updated approved documents where those documents have later approval.

• Termination path: Operators have the right to regain system functionality by implementing the terminating action specified in Table 2 (installation of designated STCs), at which point "the actions detailed in this AD are no longer applicable and the Zonal Drying System accordingly modified can be re-activated" (Terminating Action). The AD therefore provides a clear remediation route rather than an indefinite ban.

• No automatic exemption by omission: The AD clarifies that omission from the tables does not confer an exemption: "The approval list for the installation of the affected Zonal Drying System in Table 1 of this AD may be not exhaustive. In case the relevant aircraft are not listed therein, this does not exempt any aircraft having an affected Zonal Drying System P/N installed from compliance with this directive." That is a clarification, not a right, but it shapes operator responsibilities to verify equipment by P/N rather than model listing.

Operational implications implicit in duties

• Immediate operational effect: The principal duty will lead to immediate change in aircraft status , the Zonal Drying System must be inoperative until the terminating STC modification is installed. The AD does not specify interim operational mitigations beyond the circuit-breaker method; it requires de-activation as the immediate safety mitigation.

• Requirement to coordinate maintenance work: The duty to act "in accordance with relevant maintenance instructions" and the existence of an STC-based terminating action mean that maintenance organisations, STC holders and operators must coordinate to validate and execute the necessary maintenance and modification actions.

The AD is prescriptive about what must be done, how it must be de-activated, and the engineering route that terminates the requirement, while allowing use of later officially approved documentation for compliance. The AD does not itself create contractual or commercial rights beyond those practical rights to pursue terminating action and rely on updated documents; it is an operational regulatory requirement focused on equipment safety.

The text of the AD as provided sets out mandatory compliance actions and timing, but it does not itself specify civil or criminal penalties, enforcement instruments, or sanctioning mechanisms within its text. The AD prescribes the required actions ("Switch off ... and set system inoperative ...", Requirement) and specifies that compliance must be accomplished "Before further flight after the effective date of this AD" (Compliance). It is signed by a Delegate of the Civil Aviation Safety Authority and dated (James Coyne, 22 May 2009), and it records the effective date (2 July 2009) and amendment history.

What the AD contains on enforcement-related matters

• Mandatory compliance instruction: The AD requires immediate action and precise steps (pull and secure circuit breaker per maintenance instructions). This constitutes the operative command the regulator expects operators to follow.

• Effective timing: Compliance is mandatory prior to further flight after the effective date of the AD (Compliance). That timing is operationally binding in the AD's terms.

What the AD does not contain (as provided)

• No penalty clauses or enforcement procedure language: The provided AD text does not contain statutory penalty provisions, specified fine amounts, or explicit administrative sanction procedures tied to non-compliance within the AD itself. It likewise does not include procedural information about notices, rectification timeframes after detected non-compliance, or enforcement pathways such as prohibition notices or grounding that might be used in practice.

• No mention of right of appeal or review processes specific to the AD: The AD does not set out administrative review or appeal mechanisms in its text.

Implication for practitioners

Because the AD is issued by a delegate of the Civil Aviation Safety Authority, it is presented as a regulatory airworthiness directive with a clear compliance requirement. In practice, enforcement and penalties for non-compliance with airworthiness directives sit within the broader regulatory framework administered by CASA and relevant civil aviation law and regulations, which are not reproduced in the AD text provided. The AD itself therefore functions as the instrument stating the requirement, but the instruments describing enforcement powers, penalties, and procedures are not contained in the text supplied.

For operational and compliance advisers, the practical takeaway from the AD's enforcement-relevant content is that the de-activation action and compliance timing are absolute within the AD, and that operators will need to document actions taken to show compliance to the regulator. The AD points to technical documents to be used for compliance work but does not specify how CASA will manage enforcement or sanctions, nor does it include any explicit penalty schedule. Any enforcement consequences would be determined by reference to the regulator's powers under aviation law and associated regulations, which are outside the AD text provided.

The AD cross-references and interacts with other regulatory certifications and documentation processes, as shown in the AD text. The AD recognises STCs and approvals issued by other aviation authorities and references EASA ADs and approvals, evidencing cross-jurisdictional regulatory interaction between CASA and other certifying bodies.

Explicit cross-references in the AD

• EASA approvals and ADs: The AD explicitly references EASA approvals and an EASA AD. Table 2 contains STC numbers issued by EASA (for example EASA.A.S.03502 Revision 1, EASA.A.S.03504 Revision 1 etc.), and Note 4 states "EASA AD 2006-0140R4 dated 17 April 2009 refers." The Background also notes that the installation of the new dryer unit is "approved through the EASA STCs listed in Table 2" (Background). This evidences reliance on EASA certification for the terminating modification and signals that operators should engage with EASA-approved STCs when implementing the terminating action.

• National certification authorities and STC issuers: Table 1 and the subsequent tables list STC numbers with issuing bodies including LBA (Germany), RLD, FOCA (Switzerland) and RLD. The AD recognises STCs issued by foreign authorities and lists STC holders (Lufthansa Technik AG, CTT Systems AB, Jet Aviation, SR Technics). That indicates that CASA's AD is framed to accept modifications and approvals issued under other competent authorities where the appropriate STC exists.

• Manufacturer/Service/Information Letters: The AD mandates or directs operators to consult CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03, and for Lufthansa STCs the Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR. The AD explicitly allows the use of later approved revisions of those documents for compliance. These letters and service information form part of the technical regulatory regime connected to the AD and are treated as authoritative technical guidance for carrying out the required actions.

What the AD does not specify about broader legal interaction

• The AD text does not set out how it interacts with statutory civil aviation legislation (for example, the Civil Aviation Act or specific regulatory instruments). It is issued by a CASA delegate, but the underlying statutory grant of power, enforcement mechanisms, and appeals process are not recited in the AD itself.

• The AD does not provide instruction on how the AD interacts with private contractual obligations (leases, insurance), although its operational requirements (de-activation till remedied) will have practical contract and commercial implications.

Practical interface points for operators and maintainers

• Certification and approvals path for remedy: The AD requires a remedy that is implemented via STCs approved by EASA or other recognised authorities. The interplay here is that CASA accepts the EASA-approved STC as the basis for terminating the AD's requirements for Australian-registered aircraft, indicating a mutual recognition or acceptance in the context of addressing the safety issue.

• Document hierarchy and updates: The AD requires compliance "in accordance with relevant maintenance instructions" and references manufacturer/service letters; operators will need to reconcile these documents with their approved maintenance program, continuing airworthiness documentation, and possibly with supplemental instructions from STC holders. The AD permits the use of later approved revisions of the referenced documents, which is an important interaction with document control and regulatory approval channels.

• Cross-jurisdictional repairs and parts supply: Because terminating action depends on STC installation approved by EASA (and STCs issued by other national bodies), operators may need to engage with foreign STC holders or obtain approvals/installation support from organisations holding approvals under those foreign authorities. That creates practical interaction with import/export controls, parts procurement and qualification processes outside of the AD text.

In sum, the AD operates within an international certification ecosystem, recognising EASA and national STC approvals and tying terminating action to those approvals. The AD does not restate the statutory enforcement architecture, but it situates CASA's directive within existing certification and service-information channels.

The AD includes a recorded amendment history and dates within its text. The instrument presented is AD/AIRCON/14 Amendment 4 (Amdt 4). The AD text itself summarises the amendment progression and the effective dates for the various amendments and the original AD. The summary in the instrument states:

• The current document is Amendment 4. Amendment 4 "includes additional STCs that are approved as terminating action to this AD and updates the revision status of STC EASA.A.A.02547 in Table 2 of this AD." This describes the mechanical change effected by Amendment 4: it expands the list of STCs which, when installed, terminate the AD, and updates the revision status of a particular STC entry.

• Amendment 3 previously "included additional STCs that are approved as terminating action to this AD." No further details of Amendment 3's contents appear in the excerpt beyond that statement.

• Amendment 2 became effective on 20 November 2008. Amendment 1 of this AD became effective on 26 May 2006.

• The original issue of this AD became effective on 19 May 2006.

The Compliance section of the current amendment states that "This Amendment becomes effective on 2 July 2009." The AD is signed and dated by James Coyne as Delegate of the Civil Aviation Safety Authority on 22 May 2009.

Tables and notes reflecting amendment content

• Table 2 (Required action and Compliance Time section) contains the list of STCs deemed to provide terminating action. The presence of entries such as "EASA.A.S.03502 Revision 1", "EASA.A.S.03504 Revision 1", and updated entries for EASA.A.S.02547 Revision 2 reflect the amendment activity to add approved STCs and update revision status. The AD expressly notes that Amendment 4 updates the revision status of STC EASA.A.S.02547 in Table 2.

• Note 3 indicates that RLD STC SA 0009 NL was superseded by EASA.A.S.02547 on 1 March 2006, an item that reflects administrative history between approvals.

Service letters and information letters referenced

• The AD attaches technical guidance and compliance documents (CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03 and Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR). The AD expressly states that later approved revisions are acceptable for compliance. This suggests that as engineering knowledge and documentation evolved, the AD's compliance references were designed to accommodate updated technical documents without needing further AD amendment.

Why the amendment trail matters for practitioners

• The recorded amendment history shows the AD has been responsive to engineering developments: as a new dryer unit was developed and approved under STCs, those approvals were added as terminating actions through successive amendments. Amendment 4 is the final recorded update in the text provided, consolidating additional terminating STCs and revision updates.

• Operators and maintainers must pay attention to the STC revision status and to the AD version in force at any time, because which STC revision constitutes terminating action can change. The AD permits later approved revisions of the referenced documentation, but the list of STCs in Table 2 as published (and any further revisions) is the operative reference for a terminating action unless further amendments are issued.

• The effective dates for amendments and the AD itself are critical for compliance timing: the original AD and subsequent amendments have discrete effective dates; the current amendment explicitly becomes effective on 2 July 2009, and the Compliance clause requires action before further flight after that date.

The amendment history thus documents how the AD evolved from an initial immediate restriction to an instrument that identifies an approved engineering remedy, and shows how regulatory recognition of STC approvals was added incrementally and recorded through amendments.

The AD text as provided does not include any litigation history, legal challenges, or references to court or tribunal decisions. There are no case names, appeals, or judicial reviews cited in the instrument. The document records the AD issuance, amendment history, technical background, compliance requirements, relevant STCs, and the regulatory signature (James Coyne, Delegate of the Civil Aviation Safety Authority dated 22 May 2009), but it does not recount any legal proceedings or disputes arising from the AD.

Implications for advisers and stakeholders

• No recorded litigation in the AD: Practitioners reviewing the AD should note that the AD itself contains no account of legal challenges. If parties seek to understand any disputes or enforcement actions that may have arisen from this AD, they will need to search enforcement records, administrative logs, or court and tribunal dockets outside of the text provided.

• Administrative remedies and appeals not set out: The AD does not provide a description of any appeal or review mechanism specific to this directive. Any administrative review or appeal rights would be contained in the broader statutory and regulatory framework governing CASA decisions, which are not reproduced in the AD text.

• For due diligence: Where operators, lessors or insurers require assurance regarding potential legal exposures arising from the AD, they will need to consult CASA enforcement records or legal databases to determine whether any enforcement action or litigation followed from non-compliance with this AD. The AD itself does not document such activity.

In short, there is no litigation history included in the AD text provided, and any enquiry into disputes or court challenges would require external research beyond the AD.

The AD contains several operational and compliance pitfalls that must be foregrounded for operators, maintainers and compliance officers because the text places obligations and contains precautionary caveats that could easily be misunderstood.

1. "Table 1 may be not exhaustive" , do not rely on aircraft model lists alone

• The Applicability section cautions that "The approval list for the installation of the affected Zonal Drying System in Table 1 of this AD may be not exhaustive." It expressly states that "In case the relevant aircraft are not listed therein, this does not exempt any aircraft having an affected Zonal Drying System P/N installed from compliance with this directive." Practically, that means operators must verify the actual part numbers installed on each airframe rather than assuming exemption because their aircraft model is not listed. An audit of physical installations and maintenance records is necessary.

2. Method of de-activation is specific and maintenance-led

• The AD requires setting the system inoperative "by pulling and securing the affected circuit breaker in accordance with relevant maintenance instructions" (Requirement). The clause binds operators to use approved maintenance instructions and to take a physical action (pull and secure circuit breaker), not merely a cockpit switch-off. The "in accordance with relevant maintenance instructions" qualification means the circuit breaker action must be done under an approved maintenance process and documented accordingly.

3. System remains inoperative until terminating STC installed

• The AD's immediate action is de-activation, with re-activation only permitted once the aircraft is modified with one of the STCs listed in Table 2. Until such a terminating STC is installed and the modification completed, the Zonal Drying System must not be reactivated. Operational planning must account for the loss of function and scheduling of the engineering mod.

4. Terminating STCs and approvals are STC-holder and authority specific

• Table 2 lists the STCs that constitute terminating action and the issuing authority (EASA) and holder (primarily Lufthansa Technik AG and CTT Systems AB). Operators must ensure that the STC to be installed is the exact STC listed (including revision status where applicable) and that installation is carried out under an appropriate approval route (for example, via approved organisations or under approved procedures). The AD allows later approved revisions of the referenced documents, but STC identity remains a key check.

5. CTT and Lufthansa technical letters govern compliance method

• The AD references CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03; and Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR. These documents contain technical instructions and may specify parts, installation practices and limitations. Practitioners must ensure they use the approved revision or a later approved revision and document which revision was used.

6. Note on superseded STCs , check lineage

• Note 3 explains that RLD STC SA 0009 NL was superseded by EASA.A.S.02547 on 1 March 2006. This sort of supersession can complicate record searches: an STC number in earlier maintenance records may have an updated or superseding approval. Operators must trace the approval lineage to ensure the installed modification meets the terminating STC criteria.

7. No penalty or enforcement language in AD text , but the requirement is mandatory

• The AD does not include penalty language in the text, yet it imposes a strict "Before further flight" compliance obligation. Operators should not treat the absence of stated sanctions as allowing non-compliance; rather they should regard the AD as a binding regulatory instrument requiring action and document their compliance accordingly.

8. Operational impact not quantified

• The AD does not quantify operational consequences such as effects on cabin moisture control, maintenance cycles, or dispatch reliability. Operators must assess these impacts internally when de-activating the drying units and plan mitigation, recognising the AD does not provide those operational management details.

9. International approvals potentially required

• The terminating action relies on EASA STCs and approvals. For operators in jurisdictions outside EASA, the integration of an EASA-approved STC may require validation steps or bilateral acceptance. The AD is silent on validation procedures; operators should consult their national approval process or CASA guidance if cross-jurisdictional certification must be navigated.

10. Timing and amendment awareness

• The AD has multiple amendments and specific effective dates. Practitioners must ensure they follow the requirements in force at the time of compliance (the current AD becomes effective on 2 July 2009 in this iteration). Also, the AD allows later approved revisions of referenced documents; parties must track which document revisions are approved.

In practice, the "gotchas" reduce to three core operational checks: verify actual P/N installs rather than model lists; perform the circuit-breaker de-activation under approved maintenance instructions and document it; and pursue the terminating STC installation only through the exact STCs and approved revisions identified in Table 2 (or subsequent approved revisions). Failure to follow these precise steps will risk non-compliance, even if an operator has otherwise acted in good faith.

The AD sets out a narrow, stepwise compliance path. The following is a practical checklist rooted directly in the text and intended for operators, maintenance organisations, and compliance officers. Each step corresponds to language or tables in the AD.

1. Identify affected aircraft and installations

• Confirm whether the aircraft has any of the listed CTT Systems AB Zonal Drying System P/Ns installed: 1000-001; 2000-000; 2000-005; 2001-001; 2001-006; 3000-000 Rev.A; 3000-001; 3000-002 Rev.A; 3000-002 Rev.D (Applicability). Do not rely solely on the aircraft model; the AD warns the Table 1 approval list may not be exhaustive and absence from the list does not exempt an aircraft that has the affected P/N installed.

• Inspect maintenance and equipment records, parts usage logs, and physical aircraft installations to confirm the presence or absence of the specific P/N on each airframe. Trace STC lineage where installations were made via STC.

2. If an affected P/N is installed under an STC listed in Table 1, implement immediate mitigation

• Before further flight after the effective date of this AD (2 July 2009), switch off the Zonal Drying System and set it inoperative by pulling and securing the "affected circuit breaker in accordance with relevant maintenance instructions" (Requirement; Compliance). This is the mandated immediate action.

• Ensure the circuit-breaker action is undertaken and documented by appropriately certificated maintenance personnel and in accordance with approved maintenance procedures. Record the action in the aircraft maintenance log and continuing airworthiness records, noting the AD reference, the P/N identified, the STC under which it was installed (if applicable), the person and organisation performing the action, and the date/time.

3. Consult and apply the technical documents referenced in the AD

• Use the CTT Systems AB Information Letters IL06-01, IL06-02 issue 2, IL06-03 and, for Lufthansa Technik STCs, Lufthansa Technik AG Service Information Letter LHT 06-001, Revision IR dated 12 May 2006 (Relevant documents). The AD permits use of later approved revisions of these documents for compliance. Ensure the documentation version used is approved and noted in records.

4. Plan and implement terminating action if re-activation is required

• The AD provides a terminating action path: "On aircraft where the [STCs listed in Table 2] are installed, the actions detailed in this AD are no longer applicable and the Zonal Drying System accordingly modified can be re-activated" (Terminating Action; Background). Table 2 lists the STCs (EASA-issued STCs such as EASA.A.S.03502 Revision 1, EASA.A.S.03504 Revision 1, EASA.A.S.03505 Revision 1, etc.) and the STC holders.

• If an operator elects to pursue re-activation, arrange for the installation of the appropriate STC modification via the STC holder or other appropriately approved organisation. Verify the exact STC number and revision match those listed (or later approved revisions where applicable). Obtain and keep records demonstrating the installation was performed under the correct STC approval and by an approved organisation.

• After completing the terminating STC modification and any post-installation tests required by the STC or by maintenance instructions, the Zonal Drying System can be reactivated in accordance with the STC and maintenance release.

5. Record-keeping and compliance evidence

• Document the initial de-activation action (circuit breaker pulled and secured), including reference to the AD number (AD/AIRCON/14 Amdt 4), date, and name of the person/organisation performing the work.

• Retain and index the CTT and Lufthansa technical information letters used for compliance, and record which revision was used.

• If a terminating STC is installed, retain all STC paperwork, approval documentation, modification records, post-modification test reports, and maintenance release documentation that demonstrate compliance and the basis for re-activation.

6. Operational procedures and dispatch

• Update aircraft technical logs, MEL/dispatch criteria, and operational procedures to reflect that the Zonal Drying System is inoperative where applicable, until the terminating action is completed and recorded.

• Ensure flight crew and operational dispatchers understand any operational implications of the de-activated system, and that flights are only dispatched when maintenance has declared the aircraft airworthy with the circuit breaker secured and the AD requirement documented.

7. Coordination with STC holders and authorities

• Engage with the STC holders listed in Table 2 (Lufthansa Technik AG, CTT Systems AB, Jet Aviation, etc.) to obtain timelines for availability of the terminating STC modification, parts, and approved installers. Because Table 2 STCs are EASA-approved, confirm any national validation or acceptance steps required by CASA for the modification as installed on Australian-registered aircraft.

• Where STC installation will be carried out by a third party, ensure the installer is authorised to carry out STC modifications under applicable approvals and that the installation will be performed in accordance with the STC documentation.

8. Monitor for further AD amendments or revision to the STC list

• The AD has a history of amendments that add terminating STCs or update STC revision status. Operators should monitor CASA AD notices for any subsequent amendments which may expand terminating action options or change compliance expectations.

Key caveats in compliance

• Do not assume an aircraft is outside the scope based on model alone; verify P/N presence.

• Do not re-activate the drying system until the terminating STC modification is installed and properly documented.

• Use only authorised maintenance procedures for the circuit-breaker action and for any subsequent STC modifications.

Following these steps will place an operator in alignment with the AD's stated requirements: immediate de-activation by circuit-breaker action before further flight after the AD's effective date, and a documented path to termination of the AD via installation of the STCs listed in Table 2 and use of the referenced technical documents (with later approved revisions accepted).