AD/AT/22 Amdt 2 - Wing Centreline Splice Joint

The Directive’s primary technical concepts are fatigue management of a specific wing spar lower cap, safe‑life limits, non‑destructive testing (eddy‑current inspection), a structural modification alternative, configuration‑based exemptions, and cross‑referenced manufacturer/engineering documentation.

• Fatigue location and failure mode: The background explains the safety trigger , an in‑flight wing separation on a Model AT‑400A, traced to fatigue failure of the lower aluminium spar cap across a 3/8‑inch hole six and a half inches outboard of the fuselage centreline at the centre splice connection. The Directive’s inspections target the outboard holes in the lower spar cap to detect cracks before catastrophic failure. This is the operational risk the AD addresses (Background paragraph).

• Eddy‑current inspection: The AD prescribes eddy‑current inspection methodology and acceptance of Snow Engineering Co. Process Specification 197 (revised 4 June 2002) or later FAA‑approved revisions (Requirement 1). Eddy‑current is the mandated NDT method for detecting surface and near‑surface cracks at the specified hole location. The AD sets the periodicity (initial and repetitive) for that inspection.

• Safe‑life limits and scheduled replacement: The Directive sets safe‑life hour thresholds for removing and replacing the wing spar lower cap. The thresholds vary by model and configuration: 5,000 hours for certain reciprocating‑engine AT‑300/301 aircraft without the centre splice modification (Requirement 2.a), extension to 7,000 hours if the centre splice modification is installed under specified timing and inspection preconditions (Requirement 2.b), and a 4,450‑hour threshold for AT‑302, AT‑400/400A aluminium‑cap models and turbine‑converted AT‑300/301 airframes (Requirement 2.d). After each replacement the eddy‑current inspection regime continues until the modification or replacement requirement is satisfied.

• Structural modification pathway: The Snow Engineering Co. wing spar centre splice joint modification (Service Letter 55, specified revisions) is recognised as an engineering alternative to extend safe life. The AD requires that the modification be accomplished no earlier than 4,600 hours and no later than 5,000 hours on the wing spar lower cap, and that an eddy‑current inspection be done immediately prior to the modification (Requirement 2.b(i-ii)).

• Configuration‑based exemption: Aircraft fitted with steel web plates (part numbers 20990‑1 or 20990‑2) or steel spar caps are not subject to the eddy‑current inspections mandated by Requirement 1. The inspection requirement explicitly applies only to aircraft "without steel web plates ... or steel spar caps" (Requirement 1 opening).

• Interaction with previous ADs and FAA actions: The AD notes Amendment 1 followed inspection results from FAA AD 2002‑13‑02, and Amendment 2 responds to a new FAA AD prompted by cracks found below the previously specified modification time. The AD also contains a "Note: FAA AD 2008‑09‑10 Amdt 39‑15491 refers." These references highlight that CASA’s directive aligns with FAA findings and directives.

Mechanically, these concepts change maintenance scheduling inputs: operators must track "total hours time in service on the wing spar lower cap", schedule eddy‑current inspections at 3,500 hours (or credit existing recent inspections), then every 450 hours until modification or replacement, and perform replacement at specific thresholds unless the Snow Engineering modification is installed within the specified window and preconditioned by an eddy‑current inspection.

The Directive applies narrowly but materially. It affects operators, owners, maintenance organisations, and maintenance personnel for the listed Airtractor models and serial ranges with aluminium spar caps. The applicability text is specific and must be read literally:

• Models and serials: "Models AT‑300, AT‑301, AT‑302, and AT‑400A aircraft, all serial numbers, that have aluminium spar caps; Models AT‑400 aircraft, serial numbers 400‑0244 through 400‑0415, that have aluminium spar caps; Models AT‑300 and AT‑301 aircraft, all serial numbers, that have aluminium spar caps, and have been converted to turbine power." The AD thus targets a set defined by model, serial range (for some AT‑400s) and spar cap material, and includes reciprocating and turbine‑converted variants differently (Applicability heading).

• Operators and owners: Owners/operators of affected aircraft are the parties who must schedule and pay for the eddy‑current inspections, replacements, or modifications. The Directive imposes time‑in‑service thresholds and repetitive inspection intervals; the economic burden (parts, labour, downtime, potential ferry or replacement aircraft) falls on the operator/owner.

• Maintenance organisations and technicians: The eddy‑current inspection method requires suitable equipment and qualified technicians. The AD references Snow Engineering Process Specification 197 and Snow Engineering Service Letter 55; maintenance organisations must have access to those procedures or later FAA‑approved revisions to comply (Requirement 1 and 2.b). Where replacements are required "in accordance with the applicable aircraft maintenance manual", the repair/replace actions will fall to certificated maintenance providers able to carry out structural replacements and modifications.

• Aircraft modders/conversion shops: For AT‑300/AT‑301 aircraft considering turbine conversion or those already converted, the AD imposes different replacement thresholds (Requirement 2.d). Shops undertaking conversions must be cognisant of how conversion history affects future compliance obligations and safe‑life triggers.

• Parts suppliers: The AD names specific parts (steel web plates part numbers 20990‑1 or 20990‑2, steel spar caps). Suppliers of those parts may see demand if operators prefer installation of steel components to avoid the eddy‑current inspection regime. Conversely, suppliers of replacement aluminium lower spar caps will be affected by the replacement schedules.

• Regulators and inspectors: CASA’s airworthiness branch and delegates will be responsible for issuing, tracking and enforcing compliance. The AD is signed by a CASA Delegate and made effective 8 May 2008, so national regulators are the issuing authority.

Populations not affected include Airtractor types with non‑aluminium spar caps already fitted with the specified steel components, and models not listed. The AD allows credit for recent inspections performed within 450 hours prior to 8 May 2008 (Requirement 1.b), so operators who inspected recently may avoid an immediate repeat.

Duties imposed by the AD are prescriptive and technical; the document does not state rights in legalistic terms but practical rights can be inferred from the AD’s provisions.

Primary duties

• Eddy‑current inspection duty: Operators of affected aircraft without steel web plates or steel spar caps must "eddy‑current inspect the left and right wing spar lower cap outboard holes for cracks in accordance with Snow Engineering Co. Process Specification 197, revised 4 June 2002, or later FAA approved revision" (Requirement 1). The AD defines initial inspection timing and repetitive intervals (initial at 3,500 hours or within 10 hours after 8 May 2008, then every 450 hours until modification or lower cap replacement).

• Replacement duty: The AD requires replacement of each wing lower spar cap in accordance with the applicable aircraft maintenance manual at defined safe‑life points (Requirement 2). Specific timelines are provided by model/configuration: 5,000 hours for certain AT‑300/AT‑301 reciprocating models (Requirement 2.a); 7,000 hours if the Snow Engineering modification is incorporated correctly and within the 4,600-5,000‑hour window (Requirement 2.b and 2.c); 4,450 hours initial and then 7,000 hours for AT‑302/AT‑400/400A aluminium‑cap models and turbine‑converted AT‑300/301 (Requirement 2.d).

• Pre‑modification inspection duty: If electing the Snow Engineering centre splice modification to extend safe life, operators must perform an eddy‑current inspection "immediately before incorporating the modification" (Requirement 2.b(ii)).

• Immediate grounding replacement duty: If cracks are detected during any required inspection, the wing lower spar cap must be replaced "before further flight" (Requirement 1).

Rights and operational flexibilities

• Credit for prior inspection: The AD explicitly permits operators to take credit for an eddy‑current inspection performed within the last 450 hours time in service before 8 May 2008. That is, a prior inspection can satisfy the initial inspection requirement and reset the 450‑hour timing (Requirement 1.b).

• Alternative compliance via configuration change: Operators have a procedural option to change the aircraft configuration by installing steel web plates (part numbers 20990‑1 or 20990‑2) or steel spar caps; the eddy‑current inspection requirement applies only to aircraft without those steel parts, so installing steel components effectively alters the compliance pathway (Requirement 1 opening). The AD does not specify the installation procedure for those steel parts, but their presence is a de facto exemption from the inspection scope.

• Service‑letter modification option: Operators can elect the Snow Engineering centre splice joint modification, within the timing window, to extend the safe life from 5,000 to 7,000 hours for certain models. That extension is conditional: the modification must be accomplished no earlier than 4,600 hours and no later than 5,000 hours on the wing spar lower cap, and require an eddy‑current inspection before modification (Requirement 2.b(i-ii)).

• Documentation alignment: Replacement actions must be carried out "in accordance with the applicable aircraft maintenance manual", which gives operators a defined maintenance standard to follow and creates an audit trail for compliance (Requirement 2 opening).

Duties of maintenance personnel

• Use specified technical documents for inspection and modification: Snow Engineering Process Specification 197 (revised 4 June 2002) and Snow Engineering Service Letter 55 (revised 23 October 2002 or 4 October 2004, or later FAA‑approved revisions) are the documents the AD ties into. Maintenance personnel must follow those documents to satisfy the AD.

The AD does not set out a penalty schedule within the text. Compliance is required "as specified in the Requirement section of this Directive" and the AD becomes effective on the stated date; enforcement and consequences for non‑compliance are not listed in the Directive itself. The operator’s duties are focused on inspection at precise hours, replacement at safe‑life thresholds, pre‑modification inspection, and prohibition on further flight with detected cracks until replacement.

The Directive itself contains no explicit penalty schedule or enforcement mechanics in its text. What the AD does provide are mandatory technical obligations, an effective date ("This Amendment becomes effective on 8 May 2008"), and the signature of the CASA Delegate making the AD an operative regulatory instrument (signed by David Villiers, Delegate of the Civil Aviation Safety Authority, dated 6 May 2008). From the document text the enforcement picture emerges indirectly rather than as a set of penal provisions.

Operational enforcement mechanisms implied by the Directive’s structure

• Mandatory compliance deadline: The AD sets specific compliance times and repetitive intervals (Requirement 1 and Requirement 2). Those timings are precise and measurable: initial inspection at 3,500 total hours on the wing spar lower cap or within 10 hours after 8 May 2008; repetitive inspections at intervals not to exceed 450 hours; replacement at safe‑life thresholds (5,000; 4,450; 7,000 hours as applicable). These measurable triggers create objective checkpoints for regulators to verify compliance in maintenance logs and aircraft records.

• "Before further flight" restriction: The Directive requires replacement "before further flight" if cracks are detected during any required inspection (Requirement 1). That language creates an immediate safety constraint: an aircraft with a defective wing lower spar cap identified under the AD cannot legally continue to operate until the replacement is completed and the aircraft is returned to an airworthy condition.

• Delegated authority stamping: The issuance and signature by a CASA Delegate indicates the AD is an authorised airworthiness directive. While the AD does not enumerate penalties, such directives typically become part of the regulatory framework that underpins airworthiness enforcement (the AD here provides the technical requirement that CASA enforces via its broader regulatory powers). The document should be read together with the Civil Aviation safety regulatory regime that provides powers to enforce compliance (note: those statutory enforcement powers are not set out in the AD text).

Practical enforcement levers available to regulators (not detailed in the AD)

• Examination of maintenance records during audits or inspections to confirm eddy‑current inspection dates, replacement records, and modification evidence.

• Grounding or grounding notices for aircraft found to be non‑compliant with mandatory inspections or replacement obligations, especially where cracks were detected and not remediated "before further flight".

• Administrative or civil penalties under the wider civil aviation regulatory framework for contraventions of airworthiness directives, though specific penalties are not included in this document.

What is not present in the AD

• The AD does not specify fines, licence consequences, or criminal sanctions in its text. It does not detail how CASA will monitor or audit compliance, nor how an operator disputes an interpretation of the AD. Those enforcement processes and penalties would reside in CASA policy and the broader legal framework, which are not reproduced in this Directive.

In short, the Directive prescribes mandatory inspection, replacement and modification duties with objective timing triggers and an immediate "no further flight" rule when cracks are detected. The AD is effective as of 8 May 2008 and signed by a CASA Delegate; enforcement will rely on CASA’s established statutory powers and oversight mechanisms, which are outside the text of this Directive.

The Directive itself cross‑references and aligns with other aviation safety instruments and manufacturer engineering documents; it does not reproduce broader statutory powers or enforcement mechanisms. The document explicitly links to FAA ADs and Snow Engineering Co. service/process documents, creating an interaction chain between CASA, the FAA, the airframe’s manufacturer/engineering firm, and operator maintenance obligations.

Inter‑authority interaction

• FAA ADs referenced: The AD states "Note: FAA AD 2008‑09‑10 Amdt 39‑15491 refers" and the Background recounts that Amendment 1 responded to inspection results from FAA AD 2002‑13‑02, and that Amendment 2 is issued in response to a new FAA AD prompted by recent cracks discovered at hours below previous modification time. These references show CASA is taking actions that mirror or follow FAA airworthiness findings; the AD therefore functions in coordination with FAA directives addressing the same structural deficiency. The AD requires compliance in accordance with FAA‑approved revisions where specified (e.g. Process Specification 197 or Service Letter 55 or later FAA‑approved revisions).

Interaction with manufacturer/engineering documentation

• Snow Engineering Co. Process Specification 197 and Service Letter 55 are normative for compliance with the AD. The eddy‑current inspection must be carried out "in accordance with Snow Engineering Co. Process Specification 197, revised 4 June 2002, or later FAA approved revision" (Requirement 1). The wing spar centre splice joint modification is specified using Snow Engineering Co. Service Letter 55, revised 23 October 2002 or 4 October 2004, or later FAA‑approved revisions (Requirement 2.b). This makes the AD dependent on specific engineering documentation for both inspection procedures and modification instructions, and it creates a legal‑procedural link between the regulator’s requirement and the manufacturer/engineer’s technical instructions.

Interaction with maintenance manuals and parts standards

• Replacements must be carried out "in accordance with the applicable aircraft maintenance manual" (Requirement 2 opening). Consequently, the AMM and approved parts/assemblies required by the AMM determine how replacement is performed and recorded.

• The AD names specific part numbers (20990‑1 or 20990‑2) for steel web plates whose presence affects the applicability of the eddy‑current inspection requirement. That ties compliance to parts provenance and installation records.

Legal/regulatory context (scope within the AD)

• The AD does not recite CASA’s enabling statute or the penalties available under the broader Civil Aviation Safety Act or regulations. It is a technical directive issued by CASA’s delegate; its overlay with statutory enforcement mechanisms and potential administrative or civil sanctions is implicit, not set out in the AD text.

Potential cross‑jurisdictional implications

• Operators flying internationally may have to reconcile CASA’s AD with other civil aviation authorities’ directives and with the FAA ADs referenced. The AD’s references to FAA‑approved revisions and FAA ADs facilitate coordination for aircraft that are operated under multiple jurisdictions or that were manufactured/modified under FAA oversight.

Areas where the AD changes obligations relative to other instruments

• The AD modifies previous AD requirements (it is Amendment 2). It changes replacement times and adds repetitive eddy‑current inspections in response to FAA findings (Background). Operators must therefore track both the current AD and prior AD versions, and they must follow the current requirements where they differ.

In sum, the AD operates as a node in a network of aviation safety documents: it is anchored to specific Snow Engineering and aircraft maintenance manual procedures, it references FAA ADs, and its enforceability depends on CASA’s regulatory framework which is external to the AD text. Compliance therefore implicates parts, documentation, cross‑referenced FAA approvals, and record‑keeping that interacts with other regulatory and manufacturer obligations.

The Directive contains an explicit amendment trace in its Background section that identifies the AD’s evolution and the impetus for changes.

• Original issue: The original Airworthiness Directive "became effective on 24 July 2002." The AD does not reproduce the full text of that original issue here, but establishes that the issue at hand has been under regulatory attention since 2002 (Background final sentence).

• Amendment 1: The Background states "Amendment 1 further reduced the mandatory wing spar lower cap replacement times; in response to inspection results from FAA AD 2002‑13‑02." That identifies two things: (a) operators previously had different replacement times, and (b) FAA inspections and findings under FAA AD 2002‑13‑02 drove the CASA change to shorten replacement intervals. The AD does not give the earlier numerical thresholds, but it records the causal attribution , inspection results from the FAA prompted CASA to reduce mandatory replacement times.

• Amendment 2 (this document): The AD text explicitly indicates that Amendment 2 (this current amendment) is "issued in response to a new FAA AD, which was prompted by a recent report of cracks found on a Model AT‑301 aircraft at hours below the specified modification time. A repetitive eddy‑current inspection is added to the AD requirements." In other words, this amendment adds a repetitive eddy‑current inspection regime because cracks were discovered earlier than the existing modification-based safe‑life was intended to cover. The Background thus records that a field finding (cracks found below the previous modification time) triggered the addition of repetitive inspections.

• Effective date of Amendment 2: The AD states plainly "This Amendment becomes effective on 8 May 2008." The signature block shows the AD was signed on 6 May 2008 with the same effective date listed earlier in the document.

• Cross‑references: The AD also contains a "Note: FAA AD 2008‑09‑10 Amdt 39‑15491 refers." That indicates Amendment 2 has a corresponding FAA action recorded under the FAA AD numbering and amendment sequence, and that the CASA AD should be read alongside that FAA AD.

What the amendment history signals operationally

• Progressive tightening: The record indicates a sequence where FAA inspections and field findings led to shorter replacement intervals and the addition of repetitive NDT inspection. For operators this means the AD has evolved from scheduled replacement to a combination of scheduled replacement and repeated monitoring where early‑life cracks were discovered.

• Engineering pathway continuity: The AD keeps the same Snow Engineering change pathway (Service Letter 55) as an alternative to repeated replacement, but tightens the timing and adds inspection preconditions to ensure the modification is applied at an appropriate life stage (no earlier than 4,600 hours and no later than 5,000 hours for extension to 7,000 hours).

• Temporal markers for credit: Amendment 2 creates a crediting rule for recent inspections done within the 450 hours preceding 8 May 2008 , a pragmatic concession to operators who had recently inspected their lower spar caps (Requirement 1.b). That crediting mechanism directly relates to the amendment’s effective date.

Limitations in the AD’s amendment history

• The AD does not provide a full redline of what the pre‑amendment text said, so a reader must compare previous AD issuances elsewhere to identify precisely which textual provisions changed and by how much. The Background summarises the drivers and direction of change (shortening replacement times; adding repetitive eddy‑current inspections) but does not reproduce numerical history.

• The AD references FAA actions and Snow Engineering documents by revision date; operators must locate the correct revisions and the FAA‑approved later revisions if applicable. The AD permits the use of "later FAA approved revision" for Process Specification 197 and Service Letter 55, but those later revisions are not included in the AD text and must be obtained separately.

In short, the amendment history records a movement from scheduled replacement to a combined regime of scheduled replacement, an optional engineering modification (with constraints), and now mandatory repetitive eddy‑current inspections added in response to field crack detections and FAA findings. Amendment 2 (effective 8 May 2008) is the operative issuance.

The AD document does not contain or refer to any litigation, court proceedings, tribunal decisions, or legal challenges in relation to this Directive or its predecessors. The Background recounts inspection findings and a specific in‑flight separation incident, but it does not reference any legal proceedings arising from that event or any challenge to the AD.

What the AD does establish as factual for litigation context

• Incident as factual trigger: The Background describes that "the FAA received a report of an in‑flight separation of a wing from a Model AT‑400A aircraft" and that investigation "revealed that fatigue caused the lower aluminium spar cap to fail across the 3/8‑inch hole ... in the centre splice connection." If litigation arises in the future, that factual investigation would be a primary evidentiary matter. The AD ties the technical requirement for inspection and replacement directly to that factual predicate.

• Administrative decision record: The AD, with its Background and signed effective date, constitutes an administrative act and a contemporaneous record of CASA's safety determination and choices. In any hypothetical judicial review or enforcement challenge, the AD and its Background would be primary administrative materials demonstrating the regulator’s safety reasoning and technical choices relating to inspection intervals, safe‑life limits, and modification conditions.

Absence of litigation in the AD

• No appeals, stays or variances are recorded: The AD does not document any appeals against prior ADs, stays of operation, or special flight permits. It does not reference any requests for alternative methods of compliance (AMOCs) or exemptions, nor any decisions granting or denying such requests. That absence does not mean such applications could not or did not occur outside the text of the AD; it only records that no litigation is reported within the text.

Practical implications for interested parties

• Evidence trail for enforcement or dispute: The AD and cited Snow Engineering and FAA documents would be central to any compliance dispute, enforcement action, or civil claim arising from a failure to comply or from an accident involving a wing separation. The AD’s precise timing language (e.g. initial inspection at 3,500 hours; replacement at 5,000 or 7,000 hours; 450‑hour inspection intervals) provides objective metrics for assessing whether an operator complied.

• Potential forum and remedies: The AD does not describe how disputes over interpretation or compliance are to be resolved administratively; those processes would be governed by CASA procedures and relevant civil aviation legislation, which are external to the AD text.

Because this Directive contains no litigation history, operators and advisors must treat the AD as an administrative instrument with compliance obligations and record‑keeping requirements that could be relevant in subsequent enforcement or tort litigation, but any actual litigation or judicial reasoning about the AD is not recorded in the document.

The AD contains several technical and procedural traps for operators, maintenance organisations and compliance teams that could lead to non‑compliance if overlooked. These are concrete operational pitfalls grounded in the AD’s phrasing and timing rules.

1. Hour‑counting is component‑centric, not just aircraft hours

• The AD ties inspection and replacement triggers to "total hours time in service on the wing spar lower cap." This requires tracking hours on the specific lower spar cap component, including resets on replacement. After each replacement, the clock for initial inspection restarts: "After each replacement, initially inspect upon reaching 3,500 total hours time in service on either wing spar lower cap" (Requirement 1.a). Failure to track spar‑cap life separately from airframe hours risks missing an inspection or replacement deadline.

2. Credit rule limited by date and 450‑hour window

• The AD permits credit for an eddy‑current inspection "accomplished on the wing spar lower cap within the last 450 hours time in service before 8 May 2008" (Requirement 1.b). Operators who had inspections earlier than that 450‑hour window cannot claim credit and must comply with the initial inspection timing. The credit applies only to inspections within that specified historical window tied to the Amendment effective date, so bookkeeping must align precisely with AD dates.

3. Configuration matters , aluminium vs steel spar caps and web plates

• The inspection requirement applies only to aircraft "without steel web plates, part numbers 20990‑1 or 20990‑2, or steel spar caps installed" (Requirement 1 opening). Operators with steel parts installed avoid the eddy‑current inspection requirement as specified, but must be able to prove parts provenance, part numbers and installation history. Unclear documentation of parts can expose an operator to disagreements with a regulator about whether the aircraft is within the AD’s scope.

4. Different regimes for reciprocating vs turbine‑converted aircraft and across models

• The AD imposes different safe‑life limits and initial replacement times depending on model and whether the aircraft was turbine‑converted. For example, AT‑300/AT‑301 reciprocating models without the centre splice modification have a 5,000‑hour safe life (Requirement 2.a), but those that are, or have ever been, converted to turbine power fall under the 4,450‑hour initial replacement rule (Requirement 2.d). Record‑keeping of conversion history is therefore essential; a past conversion (even if reversed) can change compliance obligations.

5. Modification timing constraint

• If relying on the Snow Engineering centre splice joint modification to extend safe life to 7,000 hours, operators must accomplish the modification "no earlier than 4,600 total hours" and "no later than 5,000 total hours time in service on the wing spar lower cap" (Requirement 2.b(i)). Performing the mod outside that window either loses the safe‑life extension or is non‑compliant with the AD. Additionally, an eddy‑current inspection is required "immediately before incorporating the modification" (Requirement 2.b(ii)). Logistics must arrange inspection and modification within a tight allowable window.

6. Replacement does not extinguish inspection immediately

• Even after replacement, the AD requires continuation of the inspection regime until the wing spar centre splice joint modification or the required wing spar lower cap replacement is done. The AD repeatedly states that "After each replacement, inspect as specified in Requirement 1 until the wing spar centre splice joint modification or the required wing spar lower cap replacement is done." That means replacements do not necessarily terminate the inspection obligation unless they achieve the safe‑life replacement specified for the particular model/configuration.

7. "Before further flight" requires immediate remedial action

• The AD requires that if cracks are found "before further flight, replace the wing lower spar cap" (Requirement 1). This is an absolute operational stop condition. If a crack is detected during an inspection, the aircraft must be grounded and the replacement completed prior to any further flight , with the attendant cost and scheduling impact.

8. Normative documents and revision control

• The AD requires compliance with documents described by revision dates and allows "later FAA approved revisions." Operators must maintain access to the correct Snow Engineering Process Specification 197 revision and Snow Engineering Service Letter 55 revisions or any FAA‑approved later revisions. Relying on the wrong revision, or failing to obtain the FAA‑approved later revision used by a maintenance shop, risks non‑compliance.

9. Serial‑number specificity for AT‑400

• The AD’s applicability includes Models AT‑400 "serial numbers 400‑0244 through 400‑0415, that have aluminium spar caps." Operators must check serial numbers carefully. An incorrect reading of serial numbers can either over‑apply the AD or miss its application to an aircraft that is within the listed range.

10. Potential for misinterpretation around "have been converted to turbine power"

• The applicability language includes AT‑300/AT‑301 aircraft "that are, or have ever been converted to turbine power." This captures airframes that underwent conversion in the past even if they are currently in a different configuration, unless documentation proves otherwise. Operators and maintenance recordkeepers must be precise about conversion history.

11. Parts availability and workshop capability

• The AD prescribes eddy‑current inspections and a structural modification. Availability of qualified eddy‑current technicians, the equipment and Snow Engineering‑approved shop capability may be limited, causing scheduling delays. The AD does not provide permitted alternative inspection methods or a route for temporary ferry flights under certain conditions; such operational requests would require engagement with CASA.

12. Ambiguity about the effect of installing steel parts after inspections

• The AD exempts aircraft with steel web plates or steel spar caps from the inspection requirement, but it does not state whether installing those steel parts mid‑life changes the replacement safe life or interacts with timing windows. Operators considering part installs to alter obligations should seek technical and regulatory clarification to avoid unintentionally creating non‑compliance.

Operators must therefore treat the AD’s technical wording as prescriptive and maintain precise component life records, parts provenance, and access to the named engineering documents. Small administrative errors (miscounting hours, missing the 450‑hour window, mis‑stating serial numbers, or failing to obtain the correct revision of Snow Engineering documents) can translate to regulatory non‑compliance.

This section provides an operational checklist and practical steps, grounded in the Directive text, to bring an affected Airtractor into compliance. Compliance actions must be traceable in maintenance records and aligned with the Snow Engineering and aircraft maintenance manual instructions named in the AD.

1. Confirm applicability

• Check the aircraft model, serial number and spar cap material. The AD applies to:
  • Models AT‑300, AT‑301, AT‑302, and AT‑400A aircraft, all serial numbers, that have aluminium spar caps;
  • Models AT‑400 aircraft, serial numbers 400‑0244 through 400‑0415, that have aluminium spar caps;
  • Models AT‑300 and AT‑301 aircraft, all serial numbers, that have aluminium spar caps, and have been converted to turbine power (Applicability heading).
• If a steel web plate (part number 20990‑1 or 20990‑2) or steel spar cap is installed, the eddy‑current inspection requirement in Requirement 1 does not apply. Verify parts and installation records to determine whether the airframe falls within the inspection obligation.

2. Establish component hours and records

• Record the "total hours time in service on the wing spar lower cap." The AD’s inspection and replacement triggers are component‑hour based. For each lower spar cap (left and right), record the accumulated hours and the date of any previous eddy‑current inspection or replacement. After any replacement, restart tracking for that new cap (Requirement 1.a and repeated phrases).

3. Eddy‑current inspection scheduling and performance

• If the aircraft is within the AD scope and lacks the specified steel parts, arrange eddy‑current inspections per Snow Engineering Co. Process Specification 197, revised 4 June 2002, or a later FAA‑approved revision (Requirement 1).
• Initial inspection timing: For all affected aircraft, perform the initial eddy‑current inspection upon reaching 3,500 total hours time in service on the wing spar lower cap or within 10 hours time in service after 8 May 2008, whichever occurs later (Requirement 1.a). If an eddy‑current inspection was carried out within 450 hours prior to 8 May 2008, credit can be taken for that inspection and the 450‑hour countdown runs from the time of that inspection (Requirement 1.b).
• Repetitive inspection timing: After the initial inspection, perform inspections at intervals "not to exceed 450 hours time in service" until the wing spar centre splice joint modification or the required replacement is accomplished (Requirement 1.a). Keep precise records of inspection dates and hours to demonstrate compliance.

4. Actions on crack detection

• If cracks are detected during any required eddy‑current inspection, replace the wing lower spar cap "before further flight" (Requirement 1). Plan for parts availability and shop scheduling to address a detected crack immediately; aircraft must not return to service until the replacement is completed and the aircraft is restored to airworthiness.

5. Replacement timing and method

• Replace each wing lower spar cap according to the applicable aircraft maintenance manual, per Requirement 2. The safe‑life and replacement schedule depends on model/configuration:

  • AT‑300/AT‑301 with reciprocating engines, without the centre splice modification: replace upon reaching 5,000 total hours on either wing spar lower cap or within 25 hours after 7 May 2003 (Requirement 2.a). Replace each time the 5,000‑hour safe life is reached. After each replacement, continue inspections as per Requirement 1 until modification or replacement obligations are satisfied.
  • For those same aircraft, incorporation of the Snow Engineering centre splice joint modification (Service Letter 55, specified revisions) allows extension of the safe life to 7,000 hours, subject to timing and inspection preconditions (Requirement 2.b and 2.c).
  • For AT‑302, AT‑400 and AT‑400A with aluminium spar caps, and for AT‑300/AT‑301 that are or have been turbine‑converted: replace upon reaching 4,450 total hours on either wing spar lower cap or within 25 hours after 7 May 2003, then replace each time the 7,000‑hour safe life is reached (Requirement 2.d).

• All replacements must be completed "in accordance with the applicable aircraft maintenance manual". Maintain work orders and parts documentation.

6. Incorporate the Snow Engineering centre splice joint modification (if chosen)

• If electing the modification to extend safe life from 5,000 to 7,000 hours for the appropriate models, follow Snow Engineering Co. Service Letter 55, revised 23 October 2002 or 4 October 2004, or later FAA‑approved revisions (Requirement 2.b).
• Do not perform the modification earlier than 4,600 total hours and no later than 5,000 total hours on the wing spar lower cap (Requirement 2.b(i)). Perform an eddy‑current inspection "immediately before incorporating the modification" (Requirement 2.b(ii)). After the modification, monitor and maintain records of the modified stat using the AD’s post‑mod inspection guidance.

7. Document everything precisely

• Maintain inspection reports, eddy‑current NDT evidence, replacement invoices and parts certificates, records showing the hours on the replaced spar cap at the time of replacement, and any Snow Engineering modification paperwork. Demonstrate chain of custody for parts such as steel web plates or steel spar caps (part numbers 20990‑1/20990‑2) when these are relied upon to alter inspection applicability.

8. Engage qualified resources

• Ensure eddy‑current inspections are done by technicians with the appropriate training and equipment, and that structural replacements and the Service Letter modification (if used) are performed by an organisation authorised to do that work in line with the aircraft maintenance manual and Snow Engineering instructions.

9. Treat the "before further flight" rule as absolute

• If a crack is identified, do not permit the aircraft to fly until the replacement is completed and the aircraft is returned to service following the AMM and applicable airworthiness release procedures.

10. Keep abreast of FAA‑approved revisions

• The AD permits compliance with later FAA‑approved revisions of Snow Engineering Process Specification 197 and Service Letter 55. Monitor FAA and Snow Engineering publications for later approvals and adopt them when available and appropriate, keeping records of which revision was used for each inspection or modification.

11. Plan for downtime and parts lead time

• Given the tight modification window (4,600-5,000 hours) and the repetitive 450‑hour inspections, schedule inspections and potential modifications to avoid an operational cliff where multiple aircraft require attention simultaneously. Order replacement spar caps and any steel parts in advance to avoid extended grounding.

12. Seek regulatory clarification if in doubt

• If there is uncertainty about interpretation (for example, the effect of installing steel components mid‑life, or treatment of a prior conversion), raise the question with CASA or the delegated authority to obtain formal guidance or an authorised alternative method of compliance.

Following this checklist maps directly to the AD’s Requirement 1 and Requirement 2 language. The two procedural flexibilities available are: (a) taking credit for a qualifying pre‑8 May 2008 inspection within the 450‑hour window, and (b) adopting the Snow Engineering centre splice modification within its required window to obtain a safe‑life extension. Both require contemporaneous documentation in maintenance records.

Operators should treat compliance as both a safety obligation and a record‑keeping exercise capable of external verification by CASA or other authorities. The AD becomes effective 8 May 2008; all actions must be aligned with the timing and revision language specified.