AD/EC 135/20 - Main Transmission Mast Bearing Retainer Bolts

The AD rests on three linked technical and regulatory concepts: a specific mechanical failure mode, mandated corrective/preventive maintenance actions, and compliance timeframes tied to flight hours and operational triggers.

Mechanical failure mode

• The AD identifies the failure mechanism as loosening of attachment screws that secure the outer race of the roller bearing of the lower hub-shaft bearing support. The observed symptom was unusual vibrations during the main rotor start phase that ceased after applying torque. Maintenance troubleshooting revealed six of eight attachment screws had loosened on the affected unit; similar looseness was detected in other fleet transmissions on inspection. The AD notes that, should all attachment screws become loose, the outer race might separate from the housing, which would be an unsafe condition even though axial guidance of the rotor hub-shaft would still be ensured (Background).

Mandated actions and document linkage

• The AD does not provide bespoke engineering steps; instead it prescribes compliance by following Section 3.B of ECD ASB No. EC135-63A-013 (various revisions are cited). The required modification is the installation of locking washers and inspection of the attachment hardware. The AD explicitly accepts the use of later approved revisions of the ASB as satisfying the AD’s requirements (Requirement). It also disallows the installation of a main transmission unless it has been modified in accordance with the referenced Section 3.B (Requirement).

Timed compliance regime

• The AD combines event-driven inspection with fixed flight-hour windows. If unusual vibrations are detected during start or end of main rotor running, inspection and installation of locking washers must occur within three flight cycles after detection (Compliance). Separately, there are thresholds expressed as flight hours: an initial window "within the next 50 flight hours but not later than 31 March 2009", and a further action "within 400 flight hours (+/- 10%) after the effective date" (Compliance). The AD’s effective date is 22 September 2008; the temporal limits therefore create hard deadlines for operators to plan work and parts procurement (Compliance).

Documentary and regulatory references

• The AD links Australian regulatory action to European material, referencing EASA Emergency AD 2008-0175-E and mandating conformity with a Eurocopter alert service bulletin. The AD is issued under the authority of the Civil Aviation Safety Authority (CASA) by a delegated official (title and signature present), indicating an Australian-directed mandatory safety requirement that operationally mirrors EASA’s emergency action (Requirement; Note; signature).

Safety rationale

• The documented safety rationale is that the loosened screws present a credible path to separation of the roller bearing’s outer race, which could compromise the transmission’s structural integrity and create an unsafe condition. The installation of locking washers is therefore a targeted intervention to secure the outer race against screw loosening (Background).

Mechanisms to ensure compliance

• Rather than prescribing a new part number or lockwire method, the AD relies on the manufacturer’s ASB Section 3.B procedure and permits the use of later approved revisions. The AD enforces the modification by (a) requiring inspection and locking washer installation within set timeframes; and (b) prohibiting the installation of unmodified main transmissions after the effective date (Requirement; Compliance).

The AD identifies a narrow population of affected aircraft and thus a focused set of stakeholders. The primary directly affected parties are operators and maintainers of the listed Eurocopter EC 135 variants, and owners who manage the airworthiness of those helicopters. The AD lists precise model variants: EC 135 P1(CDS), EC 135 P1(CPDS), EC 135 P2(CPDS), EC 135 P2+, EC 135 T1(CDS), EC 135 T1(CPDS), EC 135 T2(CPDS) and EC 135 T2+ (Applicability). Any entity operating, maintaining, or installing main transmissions on these variants must comply.

Specific roles and responsibilities implied by the AD text

• Aircraft operators: must detect and report unusual vibrations as an operational trigger, organise inspections and modifications within the given flight-cycle/flight-hour windows, and ensure no unmodified main transmission is installed from the effective date (Compliance; Requirement).
• Maintenance organisations and engineers: must perform the inspections and the installation of locking washers in accordance with Section 3.B of the Eurocopter ASB cited (Requirement). They must also ensure that the correct ASB revision is used or an approved later revision, and keep records to show compliance with the AD’s timelines and prohibitions (Requirement; Compliance).
• Owners who procure and install main transmissions: are constrained by the prohibition on installing transmissions that have not been modified per Section 3.B of the ASB from the effective date (Requirement).

Indirectly affected parties

• Parts suppliers and logistics providers: will be engaged because locking washers and possibly other installation hardware might be required fleet-wide within short timeframes. The AD does not list parts by catalogue number; operators must follow the ASB to discover the specific kit or hardware (Requirement).
• Insurers and operators’ compliance units: will be affected because the AD creates mandatory maintenance work with tight windows; failure to comply may expose operators to increased risk (the AD does not mention insurance) but it does create an enforceable regulatory requirement (Compliance).
• Aeromedical, emergency services, and charter operators using these EC 135 variants: their availability of aircraft may be affected while inspections and modifications are completed within the specified intervals.

Regulatory actors

• The AD was issued by a delegate of the Civil Aviation Safety Authority (Charles Lenarcic), which places this directive within the Australian regulatory framework (signature block). The AD also refers to EASA Emergency AD 2008-0175-E, so European regulatory action is a cross-reference and may have driven the Australian directive (Requirement; Note).

Who is not affected

• Helicopters outside the listed EC 135 variants are not included in the AD’s applicability statement; the AD’s measures therefore do not apply to other models unless explicitly listed (Applicability). The AD is limited in scope to the main transmission mast bearing retainer bolts on the listed EC 135 variants.

Geographical and operational reach

• The AD text itself is an Australian document (signed by CASA delegate) and therefore affects operations within the jurisdiction where CASA’s ADs are applicable; however the AD explicitly references an EASA emergency AD. Operators who are subject to CASA’s jurisdiction or who operate in multiple jurisdictions should note the cross-reference and ensure compliance with both the Australian AD and any relevant foreign regulator directives (Requirement; Note).

The AD imposes clear duties on operators and maintainers of the specified EC 135 variants and establishes constraints on installation practices. It does not expressly articulate rights or appeal mechanisms in the text provided; the AD sets technical and timing obligations.

Primary duties

• Inspect and install locking washers. The core duty is to inspect the main transmission attachment hardware and install locking washers in accordance with Section 3.B of ECD ASB No. EC135-63A-013 (Requirement). The AD references specific revisions (02.2, 02.3 and, for helicopters already modified prior to the effective date, Revision 02.4) in describing how the inspection should be carried out, while also permitting later approved ASB revisions for compliance (Requirement).
• Do not install unmodified main transmissions. From the effective date of the AD, it is mandatory not to install a main transmission on a helicopter unless it has been modified in accordance with Section 3.B of ECD ASB No. EC135-63A-013 Revision 02 (Requirement; Compliance).
• Observe the specified compliance windows. The AD creates event-driven and flight-hour driven timing obligations: inspections after vibration detection within three flight cycles, an action within 50 flight hours but no later than 31 March 2009, and another within 400 flight hours (+/- 10%) after the effective date (Compliance).

Interpreting document references and revisions

• Operators must follow the Section 3.B procedure in the cited ASB revision(s). The AD’s text includes conditional references to different revisions depending on prior modification status and allows later approved revisions to be used. Therefore, an operational duty is to confirm which ASB revision applies to the helicopter’s modification history and whether any later ASB revisions have been approved for compliance with the AD (Requirement).

Record-keeping and evidence

• While the AD does not include an explicit record-keeping clause, compliance duties logically imply that operators should retain evidence that inspections and locking-washer installations were carried out within the required windows and in accordance with the ASB. The AD’s prohibition against installing unmodified transmissions from the effective date similarly implies a duty to verify modification status before installation (Requirement).

Rights and process notes (what the AD does not say)

• The text of the AD does not set out procedural rights such as review, extension, or exemption processes, nor does it state civil penalties or enforcement mechanisms. It does, however, state that later approved revisions of the ASB are acceptable for compliance, which affords operators a degree of flexibility in the technical means of compliance if later approved revisions are issued (Requirement). The AD also refers to an EASA Emergency AD, indicating parallel or originating regulatory action elsewhere, but the AD itself does not describe how conflicts between documents are to be resolved (Requirement; Note).

Delegated authority and approval

• The AD is issued by a delegate of the Civil Aviation Safety Authority, which signals that regulatory authority under CASA is the issuing body (signature block). The AD’s acceptance of later approved ASB revisions indicates that an approval process (for ASB revisions) governs which later revisions are acceptable for compliance, but the AD does not enumerate who grants that approval or the criteria for approval (Requirement).

Operational discretion

• The AD prescribes what must be done and when, but leaves technical execution to the referenced manufacturer ASB (Section 3.B) and to approved maintenance personnel. The AD therefore grants operational discretion only to the extent that the ASB and the operator’s maintenance organisation determine the specific steps, tools, and parts needed to satisfy Section 3.B’s instructions, subject to acceptance of later approved revisions (Requirement).

The AD text itself does not specify penalties, fines, or a detailed enforcement mechanism. The document sets mandatory maintenance requirements and compliance timelines but contains no explicit sanction schedule, civil or criminal penalties, or an enforcement process described within the text provided (Requirement; Compliance). The AD’s legal force is conveyed through its issuance by a delegate of the Civil Aviation Safety Authority (signature block) and by setting mandatory requirements and prohibitions (for example, the ban on installing unmodified main transmissions from the effective date), but the AD does not articulate the statutory enforcement pathway, inspector powers, or penalty amounts.

What the AD does establish that is relevant to enforcement

• Mandatory compliance obligations. The AD prescribes mandatory maintenance and inspection obligations and a prohibition on installation of unmodified parts; such directives are typical instruments regulators use to manage unsafe conditions. The AD’s internal wording makes these duties obligatory for operators of the listed aircraft (Requirement; Compliance).
• Timed and event-based triggers. The AD creates short compliance windows for action after specific triggers (e.g., the requirement to act within three flight cycles after detecting unusual vibrations) and prescriptive flight-hour deadlines (e.g., within 50 FH but not later than 31 March 2009). These timeframes are enforceable features because they remove discretion about when inspections and modifications must be completed (Compliance).

What the AD does not specify but is operationally relevant

• The AD does not state which authority will verify compliance or how compliance will be documented and audited. It does not say whether CASA will require submission of compliance reports, nor does it specify penalties for non-compliance, suspension of certificates, groundings, or other administrative actions (text silent).
• There is no mention of a process for obtaining an extension, a special flight permit, or an alternative means of compliance within the AD text. The AD does, however, permit "later approved revisions" of the ASB to be used for compliance, which implicitly recognises an approval or acceptance mechanism for ASB revisions (Requirement).

Practical enforcement implications for operators

• Because the AD imposes mandatory obligations without listing penalties, operators must assume that regulatory non-compliance could lead to action by the regulator consistent with its broader statutory powers. The AD’s explicit prohibition on installing unmodified main transmissions places a direct operational constraint: any party installing a transmission must verify the modification status per the ASB (Requirement).
• The operator should therefore ensure there is a documented trail demonstrating inspections and washer installations were performed in accordance with the ASB and within the prescribed windows. Although the AD does not lay out reporting requirements, retained evidence will be the practical means by which an operator demonstrates compliance if an enforcement question arises.

Cross-jurisdictional enforcement note

• The AD references EASA Emergency AD 2008-0175-E. Where an AD parallels an EASA emergency directive, operators who fly internationally or who are also subject to European oversight will need to be mindful of any enforcement regimes in those jurisdictions. The AD itself does not describe how cross-border enforcement or reciprocal recognition is to operate (Requirement; Note).

The AD explicitly references and interoperates with manufacturer guidance and a foreign regulator’s emergency AD, which creates layers of technical and regulatory interaction. The document shows how Australian regulatory requirements are being aligned with manufacturer instructions and with an EASA emergency directive.

Interaction with manufacturer documentation

• The AD bases technical compliance on Section 3.B of ECD ASB No. EC135-63A-013, including several specific revisions. It allows the use of later approved revisions of that ASB to satisfy the AD’s requirements (Requirement). This creates a practical dependency: compliance with the AD is achieved by following manufacturer-prescribed procedures, and any updates the manufacturer makes (subject to approval) can be used to satisfy the AD. The AD therefore interacts as a regulatory overlay directing adherence to a manufacturer’s service bulletin rather than substituting a different engineering solution.

Interaction with EASA emergency AD

• The AD expressly notes that "EASA Emergency AD 2008-0175-E dated 16 September 2008 refers" (Requirement; Note). This indicates that the Australian AD aligns with or follows the safety concern identified by EASA. Operators subject to both regulators’ jurisdictions should therefore consider compliance obligations under both directives; in practice, the referenced EASA AD is part of the global context for the unsafe condition. The AD does not set out a procedure for reconciling differences between ASB revisions and the EASA AD, beyond permitting later approved ASB revisions for compliance.

Regulatory authority and delegation

• The AD is signed by a delegate of the Civil Aviation Safety Authority (Charles Lenarcic), which situates the instrument within CASA’s administrative framework (signature block). The AD text does not quote the legislative provision under which the delegate issued the AD, and it does not include any cross-reference to Australian aviation statutes or regulations. The practical consequence is that the AD should be read as a regulatory instrument issued by CASA, referencing manufacturer and EASA material, but the text itself does not spell out the legislative or administrative procedure used to promulgate the AD.

Permissible alternative compliance

• The AD states that "The use of later approved revisions of this document is acceptable for compliance with the requirements of this AD" (Requirement). That phrase implies that there is an approval mechanism that can validate later revisions of the ASB for compliance purposes. The AD does not identify the approving body or process in the text provided, but the presence of that allowance interacts with the legal environment by enabling future manufacturer changes to be incorporated into an operator’s compliance programme, subject to approval.

Installation prohibition compared with parts and maintenance law

• From the effective date, the AD prohibits installation of a main transmission unless modified per the ASB (Requirement). This interacts operationally with standard maintenance and parts procurement practices: any maintenance release or parts installation acceptance must confirm modification status before installation proceeds. The AD does not specify how a maintainer should verify modification (stamped logbook entry, parts tag, conformity statement), leaving that to existing maintenance procedures and regulator expectations.

Absence of explicit conflict resolution language

• The AD does not set out how to resolve conflicts between the AD, the ASB, or foreign directives. It simply references the EASA emergency AD and defers to the manufacturer’s ASB procedures. This leaves procedural interactions to the broader regulatory system and to operators’ processes for implementing mandatory service bulletins.

The AD itself includes references to multiple revisions of the manufacturer’s Alert Service Bulletin (ASB No. EC135-63A-013), which are invoked to determine the correct inspection and modification procedure. The AD cites Revision 02 as the baseline for prohibiting installation of a main transmission unless modified per Section 3.B, and it also mentions Revision 02.2, Revision 02.3 and Revision 02.4 in various conditional contexts (Requirement). The AD expressly allows the use of later approved revisions of the ASB for compliance (Requirement).

What the AD records as of its issuance

• Effective date. The AD states: "This Airworthiness Directive becomes effective on 22 September 2008" (Compliance). The signature date on the AD is 17 September 2008, signed by Charles Lenarcic, Delegate of the Civil Aviation Safety Authority (signature block).
• Revisions referenced. The AD cites multiple ASB revisions:
  • Section 3.B of ECD ASB No. EC135-63A-013 Revision 02 (explicit prohibition on installing unmodified transmissions).
  • Revision 02.2 and Revision 02.3 referenced in the initial Requirement text for inspection and locking washer installation.
  • Revision 02.4 referenced for helicopters that have already been modified prior to the effective date in accordance with earlier ASB issue or Revision 01 (Requirement).

Interpreting the AD’s internal amendment trace

• The AD traces a sequence of ASB revisions and ties different compliance scenarios to different ASB revisions. For helicopters already modified before the AD’s effective date under the ASB original issue or Revision 01, the AD requires an inspection/install action in accordance with Section 3.B of Revision 02.4 (Requirement). The AD therefore documents a chain of manufacturer documentation changes and establishes which revision applies depending on the helicopter’s prior modification history.

No register of AD amendments in the text

• The AD text provided does not list any subsequent amendments to the AD itself, nor does it include a history of prior ADs modifying this particular AD. The AD appears to be a discrete instrument dated September 2008, and the document’s internal references to multiple ASB revisions are the only “history” contained in the text (Requirement).

Implication for future amendments and operators

• The AD’s allowance for "later approved revisions" of the ASB to be used for compliance creates a pathway for the AD to be implemented in light of future manufacturer revisions without requiring immediate amendment of the AD. Operators therefore need to track both the AD and any later approved revisions of the ASB to maintain an up-to-date compliance posture (Requirement).

Cross-reference to EASA AD

• The AD references an EASA Emergency AD (2008-0175-E), which itself may have its own amendment history and revisions. The Australian AD does not reproduce a cross-reference log beyond noting that the EASA emergency AD refers (Requirement; Note). Operators should be aware that any subsequent changes to EASA or Eurocopter guidance after the AD’s effective date may be treated as “later approved revisions” where approval has been granted.

The AD text does not record any litigation history, court challenges, or judicial interpretation of this directive. There are no cases, appeals, or legal proceedings mentioned in the document. The AD therefore carries no litigation narrative in the source material (text silent).

What the absence of litigation history implies operationally

• Because the AD contains no recorded litigation or contested administrative history, there is no precedent within the document to clarify disputes over interpretation, timing, or technical detail. That leaves interpretive questions , such as which exact ASB revision applies in marginal cases, the precise meaning of the installation prohibition, or the form of acceptable evidence of compliance , to be resolved operationally by maintainers, operators, and, if necessary, the regulator (text silent; Requirement).

Practical implications for operators and maintainers

• Operators and maintenance organisations should not rely on judicially-refined interpretations; they must rely on the AD’s plain text and on communications with the regulator or manufacturer for clarifications. Because there is no recorded litigation guiding how the AD is to be interpreted, the practical route to resolving disputes will generally be administrative: queries to CASA or to Eurocopter for clarification, or formal approval processes for alternative means of compliance (the AD allows later approved ASB revisions but does not describe the approval body or process) (Requirement).

Risk management in the absence of case law

• Where the AD leaves technical discretion to a manufacturer’s ASB and allows later approved revisions, operators should document decision-making and approvals used to comply. In the absence of litigation history, documentation is the main method to demonstrate an operator has acted in accordance with the AD and with accepted industry practices.

The AD appears straightforward in purpose but contains several practical and interpretive pitfalls that operators, maintainers and parts suppliers should note. These are factual and procedural issues derived from the AD’s text.

Ambiguity and multiple ASB revisions

• The Requirement paragraph references several revisions (02.2, 02.3, 02.4 and Revision 02 as a baseline) in ways that are not purely chronological and may be confusing when mapping an individual helicopter’s compliance obligation. For example, the AD says to "inspect ... in accordance with Section 3.B of ECD ASB No. EC135-63A-013 Revision 02.2" and then states "Unless accomplished as required by Requirement 1 of this AD, inspect ... in accordance with Section 3.B ... Revision 02.3." It also provides a distinct instruction for helicopters already modified prior to the effective date in accordance with the original issue or Revision 01, directing inspection per Revision 02.4 (Requirement). Operators must take care to match the helicopter’s modification history to the specific revision referenced, rather than assuming a single revision applies universally.

Event-triggered short windows

• The AD requires inspection "within 3 flight cycles after detection" where unusual vibrations are observed during the start or end phase of the main rotor’s running (Compliance). A flight cycle is a short unit of operation; three flight cycles is a tight window. Maintenance teams must be operationally ready to perform inspections at short notice, and flight operations must be prepared to ground aircraft for inspection within that narrow timeframe if vibrations are detected.

Multiple deadlines and date cap

• The AD uses both flight-hour thresholds and a calendar cap: "Within the next 50 flight hours after the effective date of this AD, but not later than 31 March 2009" (Compliance). Because the effective date is 22 September 2008, the calendar cap of 31 March 2009 may be earlier or later than 50 FH depending on use rates. Operators must ensure they meet both the flight-hour and the calendar deadline, whichever comes first, and plan maintenance slots accordingly.

Prohibition on installation without explicit verification steps

• The AD states "Do not install a main transmission on a helicopter, unless it has been modified in accordance with Section 3.B of ECD ASB No. EC135-63A-013 Revision 02" (Requirement). The AD does not define what constitutes adequate proof that a transmission has been modified as required , for example, whether a maintenance release, logbook entry, parts tag, or conformity certificate suffices. Operators should therefore establish clear documentation procedures before installing transmissions to avoid inadvertent non-compliance.

"Later approved revisions" requires confirmation of approval

• The AD allows usage of later approved revisions of the ASB for compliance, but the AD does not specify who approves such revisions or how to verify approval (Requirement). Operators must obtain explicit confirmation that a later ASB revision is "approved" for compliance with this AD, which may require contacting the regulator or the manufacturer.

Dependence on manufacturer’s Section 3.B

• The AD delegates the detailed technical steps to Section 3.B of the manufacturer ASB. That means an operator who has not retained or cannot access the correct ASB revision may not be able to determine the precise parts, torque settings, or procedural steps required. Ensuring timely access to the appropriate ASB is therefore essential.

Potential for duplicate or conflicting directives

• The AD references an EASA Emergency AD (2008-0175-E). While cross-references are helpful, operators subject to multiple jurisdictions should check for any differences in requirements or timing between the CASA AD and EASA AD to avoid conflicting implementation steps. The AD itself makes no guidance on resolving conflicts between documents (Requirement; Note).

No penalties or enforcement detail in the AD

• Because the AD does not state enforcement procedures or penalties, an operator cannot rely on the AD text to understand enforcement risk or the regulator’s likely response to non-compliance. Absent that guidance, conservative compliance and thorough documentation are the safest practical position.

Logistics and parts supply timing

• The initial 50 flight-hour window and, in some cases, immediate action within three flight cycles, mean parts (locking washers) and qualified maintenance personnel must be available quickly. Smaller operators or those with centralised maintenance chains may face supply or scheduling bottlenecks; the AD does not provide temporary relief pathways.

The AD requires specific inspections and installations tied to the manufacturer’s Alert Service Bulletin and sets short compliance windows. Below are practical, source-grounded compliance steps derived from the AD text.

1. Confirm applicability

• Identify whether each helicopter in your fleet matches the AD’s Applicability statement: EC 135 P1(CDS), EC 135 P1(CPDS), EC 135 P2(CPDS), EC 135 P2+, EC 135 T1(CDS), EC 135 T1(CPDS), EC 135 T2(CPDS), EC 135 T2+ (Applicability). Only helicopters listed are subject to the AD.

2. Establish records of prior modification

• Review maintenance and modification logs to determine whether the main transmission has already been modified in accordance with any earlier issue of ECD ASB No. EC135-63A-013 or Revision 01 before the effective date of this AD. The AD treats helicopters already modified prior to the effective date differently and directs inspection per specific ASB revisions in those cases (Requirement).

3. Obtain the correct ASB revision and verify approval status

• Obtain Section 3.B of ECD ASB No. EC135-63A-013 in the revision applicable to your helicopter’s history (the AD references Revision 02, Revision 02.2, Revision 02.3, and Revision 02.4 in different contexts). The AD also states that "The use of later approved revisions of this document is acceptable for compliance with the requirements of this AD" , therefore, if you intend to use a later ASB revision, obtain written confirmation that the later revision is an "approved" revision for compliance with this AD (Requirement).

4. Plan inspections and installations to meet timing requirements

• If unusual vibrations are detected during main rotor start or end of running, schedule the inspection and installation of locking washers within three flight cycles after detection (Compliance).
• For all affected helicopters not triggered by vibrations, arrange inspection and installation:
  • Within the next 50 flight hours after the effective date (22 September 2008), but no later than 31 March 2009 (Compliance).
  • Or, where the AD requires, within 400 flight hours (+/- 10%) after the effective date (Compliance).
• Ensure you interpret and apply the flight-hour windows conservatively and complete work at the earliest practical point to avoid non-compliance.

5. Perform the inspection and install locking washers per Section 3.B

• Carry out the inspection of the attachment hardware and install locking washers exactly as prescribed in Section 3.B of the applicable ASB revision. The AD delegates the technical execution to that Section; do not substitute alternative methods unless you have documented regulator approval or the AD explicitly accepts a later approved ASB revision (Requirement).

6. Verification before part installation

• From the AD’s effective date, before installing any main transmission on an affected helicopter, verify that the transmission has been modified in accordance with Section 3.B of ECD ASB No. EC135-63A-013 Revision 02 (Requirement). Maintain traceable evidence of the verification (maintenance release, signed logbook entry, parts conformity record).

7. Maintain documentation and evidence of compliance

• While the AD does not specify reporting formats, retain inspection reports, ASB revision copies used, parts installation records, and sign-offs demonstrating inspections and washer installations were carried out within the required timeframes. These records will be the primary evidence to demonstrate compliance if queried by the regulator.

8. Coordinate parts and maintenance resources

• Locking washers and any related installation hardware or kits specified in the ASB must be procured in time to meet the short compliance windows, particularly the three flight-cycle action after vibration detection and the 50 FH/31 March 2009 deadline (Compliance). Ensure maintenance personnel are trained and scheduled to perform these tasks promptly.

9. Monitor for subsequent approved revisions or further ADs

• The AD allows later approved ASB revisions to satisfy compliance. Monitor Eurocopter communications and regulatory notices to identify later ASB revisions and any additional ADs (including EASA activity referenced by the AD) that may refine or change the required actions. If you plan to rely on a later ASB revision, obtain and retain written confirmation that it is an approved means of compliance (Requirement).

10. Engage the regulator for clarifications or alternative compliance

• If there is uncertainty about which revision to comply with, what constitutes acceptable proof of modification, or whether an alternative procedure can be used, approach the Civil Aviation Safety Authority or the delegated approving authority for formal guidance or approval. The AD does not itself describe an alternative means of compliance process, but the practical way to manage ambiguity is to obtain regulator direction and record it.

11. Consider operational contingencies

• Because the AD includes an immediate-action element tied to in-service vibrations, create operational procedures so flight crews and operators report vibrations promptly and so aircraft are grounded or removed from service for inspection within three flight cycles when required (Compliance). Develop contingency schedules to cope with short-notice maintenance.

Adhering to these steps will align operator practices with the AD’s requirements as stated in the document: inspect the attachment hardware and install locking washers per Section 3.B of ECD ASB No. EC135-63A-013 (the appropriate revision), meet the AD’s timing triggers, and ensure no unmodified main transmissions are installed from the effective date (Requirement; Compliance).