AD/AT/21 Amdt 3 - Wing Lower Spar Cap Safe Life - 2

The directive rests on a small set of operational and engineering concepts:

• Safe life as a mandatory operational limit: The AD sets a deterministic retirement life in hours TIS for the wing lower spar cap component for each affected serial-number cohort (Requirement 1 table). The aircraft owner/operator must "retire at or before reaching the safe lives listed" (Requirement 1). That retirement life is the central compliance metric that drives replacement, inspection and record updates.
• Serial-number differentiation: The safe life varies by specific serial-number ranges, reflecting production or modification history. For example, AT-501 s/n 0002-0061 have 4,531 hours TIS while AT-501 s/n 0062 and subsequent have 7,693 hours TIS (Requirement 1 table). AT-502B serials are split across multiple safe-life bands (1,650 hrs through 9,800 hrs) depending on serial number (Requirement 1 table).
• Interaction with STCs and modifications: The AD recognises that the presence of Marburger winglets (installed under STC SA00490LA) changes loading and therefore the effective safe life. The directive provides a winglet usage factor table to recalculate life and delegates the computational detail to FAA AD 2006-24-10 Appendix 1 (Requirement 4 and the winglet usage factor table).
• Replacement parts and modification standards: If replacement lower spar caps (part numbers 21058-1 and 21058-2) have been installed, the AD directs use of 9,800 hours TIS as the safe life (Requirement 3). The actual replacement/modification work must follow Snow Engineering Drawing No. 21050 and specific Snow Engineering Service Letters (Requirement 7).
• AMOC pathway: The AD permits an Alternative Method Of Compliance, based on inspection (eddy current) and modification, to extend safe life for certain aircraft. This is not automatic; the AD requires compliance with the inspection schedule in the AMOC (Requirement 8 and cross-reference to Appendix 2 of FAA AD 2006-24-10). The Compliance notes set out which previously approved AMOCs remain acceptable and which do not (Note 2).
• Administrative controls: The AD requires a specific logbook entry wording to record the life limitation, thereby creating an administrative control that should be discoverable during airworthiness checks and audits (Requirement 6).

These concepts combine an engineering response (inspection, modification, replacement) with administrative controls (logbook entries, serial-number-based limits) and a regulatory reliance on FAA AD apparatus and Snow Engineering technical documents. The directive does not itself provide detailed engineering procedures beyond delegating to the Snow Engineering drawings/service letters and to FAA AD appendices; the AD therefore functions as a compliance nexus that references external technical material.

The primary economic actors identified by the directive are:

• Aircraft owners and operators of the affected Airtractor models and serial numbers: the AD explicitly applies to Model AT-501, AT-502, AT-502A, AT-502B, and AT-503A aircraft within the serial-number ranges listed in Requirement 1 and the winglet usage-factor table. These operators must determine the appropriate safe life for their aircraft’s wing lower spar cap, perform required inspections/modifications and update aircraft records (Requirements 1-9).
• Maintenance organisations and engineers: specified replacement work must be done in accordance with Snow Engineering Drawing No. 21050 and Snow Engineering Service Letters No. 197, No. 205 and No. 244 where applicable (Requirement 7, Requirement 9). Eddy current inspections must follow the AMOC procedures where used (Requirement 8). These technical documents create a compliance workload for maintenance providers.
• STC holders and modification proposers: the AD explicitly affects the installation of Marburger winglets under STC SA00490LA. Some aircraft are explicitly "not eligible to have STC SA00490LA installed without additional fatigue data being provided to the FAA" (Requirement 5). The STC holder and any party seeking to install the STC on those serials must either supply fatigue data or obtain an FAA AMOC where applicable.
• Regulatory authorities: CASA, through the Delegate, is the issuer of the AD. The AD also ties Australian compliance to FAA ADs and FAA-issued AMOCs: it instructs operators to use FAA AD 2006-24-10 Appendix 1 and Appendix 2 for recalculation and AMOC procedures and identifies which FAA AMOCs are considered approved for this AD (Requirement 4; Note 2; Note 1).
• Parts suppliers: replacement spar caps (part numbers 21058-1 and 21058-2) are singled out in Requirement 3 and in the background/Applicability notes; suppliers of these parts and of repair/upgrade kits will be engaged by operators seeking to extend safe life to the 9,800 hours TIS band or to meet the replacement requirement.
• Insurers, lessors and purchasers: although not named, the AD’s record and life-limit requirements (Requirement 6) create an obligation that will be visible to financiers, lessors and insurers when assessing airworthiness and residual value.

Who pays: the directive establishes costs primarily for the aircraft owner/operator: inspection costs (eddy current), potential replacement spar caps or modification work, downtime, and labour to perform required work and to update aircraft records. Who decides: CASA issued the AD and specified compliance timing; however, for AMOC issues the FAA’s approval or guidance is decisive because the AD cross-references FAA appendices and AMOCs and explicitly states which FAA AMOCs are acceptable (Requirement 8; Note 2). Behaviour changes required include retiring or replacing spar caps at specified TIS, refraining from installing the STC on ineligible serials without additional data, obtaining FAA AMOC where over-sized bolt holes or other non-conforming conditions exist, and making the specified logbook entry.

Duties (expressed in the AD text and compliance table)

• Determine applicability and safe life: Operators must identify their aircraft by model and serial number and apply the safe life specified in the AD’s Requirement 1 table. The AD is serial-number specific, so accurate fleet records are necessary.
• Retirement at or before safe life: The central duty is to "Retire at or before reaching the safe lives listed" for the wing lower spar cap (Requirement 1). This is an express operational obligation.
• Use turbine serial-number limits for piston-to-turbine conversions: If a piston-powered aircraft has been converted to turbine power, the operator must use the limits applicable to the corresponding turbine serial number (Requirement 2).
• Treat replacement spar caps as 9,800 hours TIS: Aircraft modified with lower spar caps part numbers 21058-1 and 21058-2 "should have a safe life of 9,800 hours TIS" (Requirement 3).
• Recalculate safe life when winglets have been installed: For aircraft that incorporate or have incorporated Marburger winglets, operators must use the winglet usage factor table in the AD, the safe life specified in Requirement 1 and the instructions in FAA AD 2006-24-10 Appendix 1 to determine a new safe life (Requirement 4).
• Logbook amendment: Operators must incorporate the specific statement into the aircraft logbook: "In accordance with AD/AT/21 Amdt 3, (AD/AT/21 Amdt 2 or AD/AT/20 Amdt 4) the wing lower spar cap is life limited to ___" (Requirement 6). This is an administrative duty designed to make the life limit discoverable.
• Perform replacement/modification according to Snow Engineering materials: Replacement/modification must follow Snow Engineering Drawing No. 21050 and Snow Engineering Service Letter Nos. 197 or 205, as applicable (Requirement 7).
• Eddy current inspection/AMOC compliance: Operators who use the AMOC to extend safe life must carry out the eddy current inspection and modification in accordance with the AMOC procedures included in Appendix 2 to FAA AD 2006-24-10 (Requirement 8).
• Cold work specific bolt holes for certain serials: For AT-502B s/n 502B-0643 and 502B-0655 through 502B-0692, cold work the two outboard wing centre splice block bolt holes in the lower spar cap per Snow Engineering Service Letter No. 244 dated 25 April 2005 (Requirement 9).
• Comply with specified timing: The Compliance section prescribes when each requirement becomes due (e.g. Requirement 6 within 10 hours TIS after certain dates depending on previous AD coverage; Requirement 9 before accumulation of 2,000 hours TIS or within 100 hours TIS after 15 February 2007, whichever is later). Where the logbook entry places the component over or within 50 hours of its safe life, operators are allowed an additional 50 hours TIS to complete replacement/modification (Compliance).

Rights and options contained in the AD

• AMOC option to extend safe life: The AD provides an alternative compliance pathway. Operators who can meet the eddy current inspection and modification procedures in the FAA AD 2006-24-10 Appendix 2 AMOC may extend safe life up to the AMOC-prescribed retirement life (Requirement 8, Background and Amendment 2 description).
• Recalculation process allowance for winglets: The AD supplies a method to recalculate life when winglets have been or are installed (Requirement 4, winglet usage factor table).
• Use of previously approved AMOCs in certain cases: The AD identifies which AMOCs previously approved under earlier FAA ADs are considered approved for this directive and which are not (Note 2). This grants operators continuity of previously authorised compliance approaches in some circumstances.

Administrative mechanics and decision nodes

• CASA issues the base requirement (this AD), but the AMOC path and many technical appendices are supplied through FAA AD 2006-24-10 and the FAA AMOC approval process. The AD therefore creates a dual-decision structure: CASA requires compliance and references FAA technical materials; the FAA grants AMOCs or additional data approvals that materially affect whether an operator may extend life or fit winglets on certain serials (Requirement 4; Requirement 8; Note 2).
• The directive mandates particular technical documents (Snow Engineering Drawing No. 21050; Snow Engineering Service Letters Nos. 197, 205 and 244) as the procedural standard for replacement/modification and for cold working of bolt holes (Requirement 7; Requirement 9).

The AD does not itself set out civil penalties or enforcement procedures in the text provided; it sets mandatory technical and record-keeping duties and specific compliance times. The AD also preserves certain options through the AMOC process and through recalculation rules for winglet installations.

The AD text provided does not contain an express schedule of civil or criminal penalties or a section describing enforcement mechanisms or sanctions. It sets mandatory technical requirements, prescriptive timing and logbook duties that are directed to operators and maintenance providers, and it states that "This Amendment becomes effective on 15 February 2007" (Compliance). The instrument is signed by the Delegate of CASA (David Villiers) and includes the required technical and administrative requirements.

Observed enforcement-relevant features in the AD text

• Mandatory time limits and actions: The AD prescribes definitive actions with precise timing , for example, the logbook entry timing for Requirement 6 ("within 10 hours time in service after 20 March 2003" for aircraft previously affected by FAA AD 2002-26-05, or "within 10 hours time in service after 15 February 2007" for aircraft not previously affected) and the replacement deadline rules (Compliance). These definitive times are enforcement-relevant because they allow an auditor or regulator to check compliance against a clock and against aircraft logbooks.
• Administrative traceability: The required logbook entry (Requirement 6) provides an auditable record that an aircraft's wing lower spar cap is life-limited to a specific number of hours TIS. That logbook wording is explicit and mandatory in the AD. An absence of that entry would be plainly observable upon record inspection.
• Delegation and signature: The AD is issued by CASA and signed by its delegate (signed 4 January 2007), which is the authority for the instrument within the Australian regulatory framework. The document explicitly references FAA ADs and AMOCs and instructs operators to use FAA appendices for specific technical compliance steps (Requirement 4; Requirement 8; Note 1 and Note 2), creating a two-jurisdiction compliance model.

What the AD does not state (and which should be noted)

• No civil penalty schedule is included in the text provided; the AD itself requires actions rather than prescribing fines or specifying enforcement processes.
• The AD does not set out an administrative enforcement mechanism or the consequences of non-compliance in this text. It is a regulatory technical directive that anticipates oversight through airworthiness compliance processes, but the textual instrument does not, in itself, list penalties.

Implications for enforcement practitioners and compliance officers

• The directive’s clear timing and record-keeping obligations facilitate standard regulatory oversight and audit: inspectors can check logbooks against the prescribed wording and against aircraft TIS to determine compliance.
• Where an operator seeks to use an AMOC, the AD makes clear which FAA AMOCs are considered acceptable and which are not, so enforcement officers can assess whether an operator’s AMOC is in the list of acceptable approvals (Note 2).
• In the absence of an express penalty table, enforcement action would proceed on the broader statutory/regulatory powers vested in the aviation regulator and under the Civil Aviation Safety Act and associated regulations (not included in this AD text). The AD itself creates compliance obligations that can be referenced in any enforcement action, but the mechanics of civil sanctioning are not detailed in the AD text provided.

The AD explicitly interlocks with external technical directives and approvals, creating a compliance mesh whose key strands are FAA ADs, supplemental type certificates (STCs), and Snow Engineering technical documents. The AD’s interactions are contractual and administrative rather than purporting to change substantive private law.

Cross-jurisdictional linkage to FAA ADs

• FAA AD 2006-24-10: The AD requires use of FAA AD 2006-24-10 Appendix 1 for recalculating life when Marburger winglets are or have been installed, and FAA AD 2006-24-10 Appendix 2 for the AMOC eddy-current inspection and modification procedures (Requirement 4; Requirement 8). The AD therefore imports FAA-prescribed technical computations and inspection protocols into CASA’s compliance framework.
• Prior FAA ADs and AMOCs: The AD describes which AMOCs previously approved under earlier FAA ADs are considered approved for AD 2006-24-10 and which are not. The compliance notes state: "AMOCs approved for FAA AD 2001-10-04 and/or AD 2000-14-51 are not considered approved for AD 2006-24-10 (this Directive). AMOCs approved for AD 2001-10-04 R1, AD 2002-11-03, AD 2002-11-05, AD 2002-11-05 R1, or AD 2002-26-05 are considered approved for AD 2006-24-10 (this Directive)" (Note 2). This is a direct interaction rule for approvals: an AMOC that was previously acceptable under one FAA AD may or may not be portable to this AD.

Interaction with STCs

• The AD refers to Marburger Enterprises Inc. winglets installed under STC SA00490LA. It requires operators to use a winglet usage factor and FAA Appendix 1 instructions to recalculate safe life if the aircraft "incorporate or have incorporated Marburger winglets" (Requirement 4). Moreover, specific aircraft are stated to be "not eligible to have STC SA00490LA installed without additional fatigue data being provided to the FAA" (Requirement 5). Thus the AD governs the interplay between STC installation and airworthiness limits.

Interaction with Snow Engineering technical documents

• The AD requires replacement/modification to be done in accordance with Snow Engineering Drawing No. 21050 and Snow Engineering Service Letters No. 197 or No. 205 (Requirement 7) and requires cold working of certain bolt holes under Snow Engineering Service Letter No. 244 (Requirement 9). Those documents are therefore normative for compliance under this AD.

Record-keeping and audit law interface

• The AD’s specific logbook wording requirement (Requirement 6) creates an evidentiary and administrative link between technical compliance and record-keeping regimes that inform sales, leasing, insurance and regulatory oversight. That wording is to be inserted verbatim into the aircraft logbook.

Limitations and delegated technical decisions

• The AD delegates significant technical decision-making to the FAA AMOC process and to the Snow Engineering documentation. CASA sets the requirement but relies on FAA-approved AMOCs and FAA appendices for the technical mechanisms to extend safe life or to calculate winglet-adjusted lives (Requirement 4; Requirement 8). Parties seeking relief or different compliance methods will typically engage with FAA processes as identified in the AD.
• The AD depends on external documents not reproduced in the text (FAA appendices and Snow Engineering materials), meaning the full legal-technical obligations and the precise inspection intervals for AMOC-based life extension are located in those external materials (Note 1).

What the AD does not do

• The AD does not purport to amend or override statutes outside the aviation regulatory apparatus. It is an instrument within the airworthiness directive regime, and the scope of its authority is the technical and administrative demands for airworthiness. The AD does not by its text create or modify contract rights between private parties, except insofar as airworthiness limits and certificate conditions influence those rights in practice.

Contactability and source documents

• The AD provides a contact route and a source for FAA appendices (Note 1, including the FAA website and a CASA airworthiness directives email address), signalling that the AD assumes access to FAA technical materials is necessary to fulfil obligations.

The directive text itself includes an explicit amendment summary and history for AD/AT/21:

• Original AD: The "original issue of this Airworthiness Directive became effective on 10 July 2002" (Background).
• Amendment 1: "Amendment 1 further reduced the safe life of the Models AT-502, AT-502B, and AT-503A aircraft, and added aircraft recently manufactured to the applicability of this Directive" (Background). The AD does not provide the Amendment 1 date in this text.
• Amendment 2: "Amendment 2 allowed the use of an AMOC which involves the use of inspections and modifications and a new extended retirement life (8,000 Hours TTIS), in lieu of the safe lives specified in this AD" (Background). This establishes that Amendment 2 formalised an AMOC-based extension pathway, including a specific extended retirement life (8,000 hours TTIS) being available under conditions set out in the AMOC.
• Amendment 3 (this instrument): The document states that "Amendment 3 is issued in response to a new FAA AD. Since the issue of FAA AD 2002-26-05 (AD/AT/21) and 2002-11-05 R1 (AD/AT/20), apart from retaining the actions of these Directives, the FAA determined that additional aircraft should be added to the applicability section and also determined the safe life for new production aircraft and replacement spar caps should be extended. The FAA also developed an AMOC to the requirements of 2006-24-10 (this Directive). This Directive incorporates the requirements of AD/AT/20 Amdt 4" (Background). Amendment 3 became effective on 15 February 2007 (Compliance). The AD is dated and signed 4 January 2007.
• Cross-reference of ADs: The text repeatedly cross-references earlier CASA ADs (AD/AT/20 Amdt 4, AD/AT/21 Amdt 2) and FAA ADs (FAA AD 2006-24-10 and earlier FAA ADs such as AD 2001-10-04, AD 2002-11-03, AD 2002-11-05, AD 2002-26-05). The Compliance notes set out the temporal carryover of compliance obligations for aircraft previously affected by earlier FAA ADs: for example, "For aircraft previously affected by FAA AD 2002-26-05, within 10 hours time in service after 20 March 2003" is the timeframe repeated in Requirement 6’s compliance scheduling.

Practical consequences of the amendment history

• Progressive tightening and differentiation: Over successive amendments, safe-life numbers have been lowered for some cohorts and extended for others, new production serials were added, and AMOC pathways were developed and calibrated. This history shows an iterative regulatory response to accumulating data (see Background: reports of earlier-than-expected cracks) and to technical work by FAA and Snow Engineering.
• AMOC institutionalisation: Amendment 2 institutionalised the AMOC pathway that allows inspection/modification in lieu of earlier deterministic retirement lives, specifically providing a mechanism to extend life (8,000 hours TTIS under Amendment 2’s description) subject to meeting inspection/modification protocols. Amendment 3 incorporated FAA-developed AMOC elements for AD 2006-24-10.

Records and effective dates

• The AD becomes effective 15 February 2007 (Compliance). The AD retains earlier compliance deadlines for aircraft previously affected by FAA AD 2002-26-05, and sets new deadlines for those not previously affected (Compliance).

Sources and referencing

• The AD text notes where to obtain FAA appendices (Note 1) and supplies the CASA airworthiness directives email as a contact. The AD relies on external documents for technical detail (Snow Engineering service letters and FAA appendices), which are part of the amendment architecture and necessary for compliance.

The directive text supplied does not record any litigation history. It contains no references to court proceedings, judicial reviews, enforcement proceedings, appeals or determinations arising from this AD. The document is descriptive of regulatory action and amendment chronology; it does not include or cite any case law, administrative appeals, or judicial outcomes.

What is known from the document

• No judicial or tribunal decisions are mentioned in the AD text. The background and amendment notes describe regulatory actions and FAA inputs but do not record disputes or litigation.
• The AD relies on FAA technical documents and AMOC decisions rather than on adjudicated legal interpretations, so the operative interactions are administrative and technical.

Implications for researchers and counsel

• Absence of litigation in the AD text means one cannot infer from this instrument alone whether there were subsequent appeals, enforcement actions, or litigation around compliance. To assess whether the AD has been challenged or whether enforcement actions were taken, researchers must consult administrative records, CASA enforcement logs, tribunal and court databases, or industry publications external to this AD text.
• For compliance counsel, the AD’s administrative requirements and inter-authority references (FAA) are the operative documents for advising clients. If clients ask about legal risk beyond regulatory non-compliance (for example indemnity disputes, contractual breaches, or claims in tort arising from failure to comply), those would need to be assessed against other sources not included in this AD text.

This AD contains several detail-sensitive items and traps for the unprepared operator or maintenance provider. Each "gotcha" is grounded in the AD’s text and timing provisions.

Serial-number and part-number specificity

• The safe life varies by serial-number cohorts, and multiple different safe lives exist even within a single model (Requirement 1 table). Applying the wrong serial-number band will lead to non-compliance. For example, AT-501 s/n 0002-0061 is 4,531 hours TIS while AT-501 s/n 0062+ is 7,693 hours TIS (Requirement 1 table). AT-502B has bands from 1,650 to 9,800 hours depending on serial number (Requirement 1 table).
• Replacement parts change the life rule: aircraft modified with replacement spar caps part numbers 21058-1 and 21058-2 "should have a safe life of 9,800 hours TIS" (Requirement 3). If replacement spar caps are installed but the operator does not record that fact, the wrong safe life could be applied.

Marburger winglets and STC eligibility traps

• The presence, current or historical, of Marburger winglets triggers recalculation using a winglet usage factor and FAA instructions (Requirement 4). Operators who do not account for previously installed/removed winglets may miscalculate their component life.
• Certain AT-502B serials are not eligible to have STC SA00490LA installed "without additional fatigue data being provided to the FAA" (Requirement 5). Attempting to install the STC without supplying the data required by FAA will be non-compliant with the AD as written.

AMOC equivalency pitfalls

• Not all previously approved FAA AMOCs carry forward: the AD says "AMOCs approved for FAA AD 2001-10-04 and/or AD 2000-14-51 are not considered approved for AD 2006-24-10 (this Directive)" whereas AMOCs approved for other listed FAA ADs are considered approved (Note 2). Assuming that any prior AMOC is automatically acceptable for this AD is an error. Operators need to check the AD’s AMOC equivalency list carefully.
• For aircraft with oversized outboard bolt holes at the splice block following a two-part modification, the AD requires an FAA AMOC to determine applicable inspection intervals (Compliance table note). Where operators have "over-sized outboard bolt holes", they must obtain an AMOC from FAA , a procedural dependency that can delay or condition continued operation.

Timing and logbook entry nuances

• Requirement 6 has a bifurcated timetable: aircraft previously affected by FAA AD 2002-26-05 had to comply with Requirement 6 within 10 hours TIS after 20 March 2003; aircraft not previously affected must comply within 10 hours TIS after 15 February 2007. Failure to apply the correct date bracket can produce late compliance (Compliance).
• If, at the time the logbook entry is made, the wing lower spar cap is over or within 50 hours of the safe life, "an additional 50 hours time in service is allowed to accomplish the replacement/modification" (Compliance). Misunderstanding this allowance could either lead to premature grounding or inadvertent overrun.

Inspection and modification procedural dependencies

• The AD delegates procedural specificity to Snow Engineering drawings/service letters and FAA Appendix materials. Operators who do not have immediate access to Snow Engineering Service Letters Nos. 197, 205 and 244 or to FAA AD 2006-24-10 appendices will lack the complete procedural picture. Note 1 of the AD provides a web link and a CASA email for obtaining FAA appendices, but the AD’s full technical obligations are in those external documents.
• The AMOC-based extended life depends on eddy current inspection criteria and inspection schedules that are found in Appendix 2 to FAA AD 2006-24-10. Practically, this means operators must rely on an FAA-approved inspection/modification protocol to avail themselves of any extended safe life. That is a dependency on another authority.

Cold work requirement timing

• For specific AT-502B serials, the cold-work procedure must be accomplished "before the accumulation of 2,000 hours time in service or within 100 hours time in service after 15 February 2007, whichever occurs later" (Compliance). Operators with these serial numbers must schedule and complete this work by that deadline or show compliance via an approved AMOC. Missing the deadline risks non-compliance and creates immediate maintenance pressure.

Eddy current inspection immediate-to-replacement requirement

• The directive requires an eddy current inspection "immediately prior to the replacement/modification to detect and correct any crack in a bolt hole before it extends to the modified centre section of the wing" (Background paragraph). This is a sequencing requirement: the inspection must be performed at the time of replacement/modification, not at an earlier arbitrary interval, to ensure a crack is not present at the interface.

Document access and traceability

• The AD depends on external documents that are not included in full in the directive. Failure to procure and follow the Snow Engineering service letters and FAA appendices will result in incomplete compliance. Note 1 gives the FAA web site and a CASA AD email address as points of contact, but operators must obtain and follow those documents.

Operational impact and downtime

• The AD contains replacement and modification obligations that may require aircraft downtime. The narrow allowance of 50 additional hours in certain logbook situations is the only explicit operational relief; otherwise, retiring or modifying a spar cap will take aircraft out of service. Operators who do not plan for this may face operational disruption.

This section sets out a practical, source-grounded checklist and sequencing based on the AD provisions, the compliance table and the referenced technical documents. All quoted step names and compliance times are taken from the AD as reproduced.

1. Confirm model and serial number applicability

   • Check the aircraft’s model and serial number against the AD’s Applicability and Requirement 1 table. The AD applies to Models AT-501, AT-502, AT-502A, AT-502B and AT-503A in the serial-number ranges listed in the tables in Requirement 1 and the winglet usage-factor table. Identify which safe-life band from the Requirement 1 table applies to your aircraft.

2. Determine if piston-to-turbine conversion applies

   • If the aircraft was converted from piston to turbine power, apply the limits for the corresponding turbine serial number (Requirement 2).

3. Check for replacement spar caps

   • If the aircraft has replacement lower spar caps installed with part numbers 21058-1 or 21058-2, apply the 9,800 hours TIS safe life per Requirement 3.

4. Check Marburger winglet (STC SA00490LA) history

   • Determine whether Marburger winglets have been installed at any time. If so, use the winglet usage factor table and the base safe life from Requirement 1, and follow FAA AD 2006-24-10 Appendix 1 instructions to recalculate the effective safe life (Requirement 4). The AD’s winglet usage factor table shows factors (for example, 1.6 or 1.2 depending on model and serial ranges) that multiply into the computation in Appendix 1.

5. Confirm eligibility for STC installation (if considering new installation)

   • For Model AT-502B serial numbers 0643, 0655 and subsequent, and any aircraft modified with replacement spar caps part numbers 21058-1 or 21058-2, confirm whether additional fatigue data is required before installing STC SA00490LA; the AD states these aircraft "are not eligible to have STC SA00490LA installed without additional fatigue data being provided to the FAA" (Requirement 5). Contact FAA or the STC holder to provide or request the necessary fatigue data if planning an installation.

6. Make the mandatory logbook entry

   • Make the specified aircraft logbook entry: "In accordance with AD/AT/21 Amdt 3, (AD/AT/21 Amdt 2 or AD/AT/20 Amdt 4) the wing lower spar cap is life limited to ___" and insert the specific safe-life hours determined in steps 1-4 (Requirement 6). Observe the timing for Requirement 6:
     • For aircraft previously affected by FAA AD 2002-26-05: within 10 hours TIS after 20 March 2003.
     • For aircraft not previously affected by FAA AD 2002-26-05: within 10 hours TIS after 15 February 2007 (Compliance).

7. Plan replacement/modification work per Snow Engineering documents

   • Perform replacement or modification work in accordance with Snow Engineering Drawing No. 21050 and Snow Engineering Service Letter No. 197 or No. 205, both revised 26 March 2001, as applicable (Requirement 7). If the wing lower spar cap is already over or within 50 hours of safe life at the time of logbook entry, the AD allows an extra 50 hours TIS to accomplish the replacement/modification (Compliance).

8. Consider AMOC (Alternative Method Of Compliance) where life extension is desirable

   • If you wish to extend the safe life instead of retiring at the deterministic limit, obtain and comply with the AMOC procedures in Appendix 2 to FAA AD 2006-24-10. An eddy current inspection and modification per that AMOC are required to extend safe life (Requirement 8).
   • Check which FAA AMOCs are treated as acceptable for this AD. The AD specifically states which previously approved AMOCs are considered approved and which are not (Note 2). If the aircraft has oversized outboard bolt holes from past modifications, you must obtain an AMOC from the FAA to determine applicable inspection intervals (Compliance table note).

9. Accomplish specific cold-work requirements for certain AT-502B serials

   • For Model AT-502B aircraft s/n 502B-0643 and 502B-0655 through 502B-0692, perform the cold-working of the left and right hand two outboard wing centre splice block bolt holes (four total) in the lower spar cap in accordance with Snow Engineering Service Letter No. 244 dated 25 April 2005 (Requirement 9). Timing: before accumulating 2,000 hours TIS or within 100 hours TIS after 15 February 2007, whichever occurs later (Compliance).

10. Perform eddy current inspection immediately prior to replacement/modification

    • The AD requires an eddy current inspection immediately prior to replacement/modification to detect and correct any crack in a bolt hole before it extends into the modified centre section (Background). If using an AMOC, follow the eddy current procedures and inspection schedule as specified in Appendix 2 to FAA AD 2006-24-10 (Requirement 8).

11. Retire part at or before safe life

    • Retire the wing lower spar cap at or before the safe-life hours calculated and entered into the logbook (Requirement 1 and Requirement 6). Do not operate beyond the life unless operating under an AMOC that explicitly permits a different inspection schedule and retirement life.

12. Document everything and maintain traceability

    • Keep copies of Snow Engineering Drawing No. 21050 and the applicable Snow Engineering Service Letter(s) on file as the AD requires the work be performed to those documents (Requirement 7 and Requirement 9).
    • Maintain copies of any FAA AMOC approvals that you rely on and ensure the AMOC references are consistent with the AD’s Note 2 equivalencies.

13. Use CASA and FAA contacts for technical documents

    • Note 1 states FAA AD 2006-24-10 Appendix 1 and Appendix 2 may be obtained from the FAA Internet web site (www.airweb.faa.gov/rgl) or by request to the CASA airworthiness.directives email (airworthiness.directives@casa.gov.au). Obtain the referenced appendices and Snow Engineering service letters before planning work.

14. Plan for operational impact and costs

    • Anticipate inspection costs (eddy current), part costs (replacement spar caps or repair/modification kits), labour, scheduled downtime and potential delays tied to obtaining AMOC approvals or extra fatigue data (for STC installations). The AD’s additional 50-hour allowance and AMOC pathways provide limited relief but do not eliminate downtime or costs.

15. Where in doubt, escalate to CASA or the FAA

    • The AD repeatedly ties technical judgments to FAA appendices and AMOCs; for technical questions about AMOC eligibility or acceptance of specific inspection schedules, seek the relevant FAA approval or CASA guidance as referenced in Note 1 and the compliance provisions.

Checklist summary (minimum essentials before flight after 15 February 2007 unless previously complied):

• Aircraft identified against serial-number table and safe-life hours recorded.
• Aircraft logbook amended with the exact required wording and the safe-life number (Requirement 6).
• Replacement/modification performed per Snow Engineering documents where applicable (Requirement 7).
• Eddy current inspection performed immediately before replacement or per AMOC if extending the life (Requirement 8; Background).
• Cold work performed for specified AT-502B serials within the AD’s deadlines (Requirement 9).
• STC installations checked against the ineligibility rule for certain serials and additional fatigue data supplied to FAA where required (Requirement 5).
• Keep copies of all supporting FAA AD appendices, Snow Engineering service letters and any AMOC approvals relied upon (Note 1; Note 2).

Following these steps aligns an operator with the prescriptive requirements and timings set out in AD/AT/21 Amdt 3.