AD/AT/26 - Horizontal Stabiliser Attachment Eyebolts

The Directive builds on three principal technical concepts expressly set out in the source: torque retention, fatigue/life-limited parts, and recurring inspection/maintenance. The torque-retention concept appears in the directive to "tighten" eyebolts to specified torque values (see Requirement 1 referencing SL No. 129, revised 21 October 2004). The safety concern identified is loss of torque in service as a likely causal mechanism for eyebolt failure. The fatigue or high-cycle stress concept appears in the "Background", which reports that failures resulted from "high cycle, low-nominal stress" and that eyebolts have cumulative hours time in service limits that govern replacement (Requirement 2 references SL No. 129 for those limits).

Recurrence is embedded as an operational concept. The AD prescribes an initial action and then recurring actions: tightening at intervals not to exceed 12 calendar months, and replacement thereafter at intervals specified in SL No. 129. The replacement timing is governed by cumulative hours in service; the AD defers to SL No. 129 for the exact numeric life limits and torque values, effectively incorporating that service letter by reference. The AD also contains an installation control: it prohibits installing particular eyebolt part numbers if those items "exceed the corresponding cumulative hours time in service specified in Requirement 2 of this Directive", thereby enforcing the life-limit concept at installation time as well as at removal.

The AD cross-references other safety instruments. It explicitly notes FAA AD 2005-13-12 Amdt 39-14149 as a related instrument. It also cites Snow Engineering SL No. 129 in two forms: the original issue date of 26 September 1994 and the revision on 21 October 2004. Those technical documents supply the torque values and the numerical life limits that the AD makes mandatory; the AD text does not reproduce the numeric torque or hour values.

Mechanically, the AD imposes obligations on aircraft operators and maintenance providers to: (a) identify affected aircraft (the "Applicability" block lists specific Airtractor models and serial-number limits), (b) obtain and follow SL No. 129 for torque and replacement schedules, (c) perform tightening within the prescribed window and at recurring intervals, (d) perform replacement within the prescribed windows and at recurring intervals, and (e) prevent the installation of specific part-number eyebolts that have exceeded their allowed cumulative hours. The safety mechanism asserted is the removal of worn or torque-lost eyebolts, and frequent retorquing to prevent fatigue progression to failure.

The AD affects operators, owners, maintainers and potentially parts suppliers connected to the Airtractor aircraft models identified under "Applicability". The applicability language lists "Models AT-300, AT-301, AT-302, AT-400, and AT-400A aircraft; all serial numbers. Model AT-401/AT-402 aircraft, with serial numbers up to and including 401-0700." The cover heading includes AT-300, 400 and 500 series wording, but the explicit applicability block does not list AT-500 models. That internal divergence in the document is material for legal and operational teams because compliance will depend on the listed applicability text not on the cover heading.

Primary affected parties are:

• Aircraft owners and operators of any Airtractor aeroplanes encompassed by the applicability block. Those entities must ensure inspections, torque checks and replacements are carried out within the prescribed timelines and that no prohibited eyebolts are installed beyond their allowed hours.

• Maintenance organisations, engineers and licensed maintenance personnel who physically carry out the tightening and replacements, who must use the torque values and replacement schedules set out in Snow Engineering SL No. 129 (SL No. 129 revised 21 October 2004), which the AD makes mandatory.

• Parts controllers and supply-chain staff responsible for verifying part numbers and cumulative hours-in-service before installation, because the AD prohibits installing specific part-number eyebolts if they have exceeded the cumulative hours specified in Requirement 2.

• Registries and regulators responsible for continuing airworthiness oversight. The AD is issued under CASA authority (signed by a CASA delegate) and references an FAA AD; it therefore will be of interest to regulators tracking compliance for aircraft on national registers.

It does not specify other stakeholders such as insurers or lessees, but those parties will be indirectly affected through contractual obligations and downtime risks. The AD imposes recurring maintenance burdens which will fall on operators to schedule and fund; the AD's "Compliance" timings create an operational cadence that can affect aircraft availability. Operators will need maintenance records showing cumulative hours on each eyebolt to determine whether replacement is required or whether an eyebolt may be installed. The Directive does not provide the numeric cumulative-hour thresholds in the text; it delegates those details to SL No. 129, so affected parties must obtain that service letter to determine exact replacement lifetimes and torque values.

Duties

• Identify affected aircraft. The "Applicability" component lists the models and serial numbers that the AD covers. The AD's actions apply to those aircraft.

• Tighten eyebolts. Requirement 1 directs: "Tighten the four eyebolts that attach the front and rear spar of the horizontal stabiliser to the respective stabiliser strut using the torque values referenced in Snow Engineering Co. Service Letter (SL) No. 129, revised 21 October 2004." This is a mandatory technical task; the AD sets the torque standard by reference to SL No. 129.

• Replace eyebolts. Requirement 2 directs: "Replace eyebolts that attach the front and rear spar of the horizontal stabiliser to the respective stabiliser strut, in accordance with SL No. 129, Issued 26 September 1994, Revised 21 October 2004." The AD mandates replacement according to the life limits and procedures in that service letter.

• Observe installation prohibition. The AD states: "Do not install any 5/16-inch eyebolt (part number AN44-17A or AN44-21A), 7/16-inch eyebolt (AN47-22A or AN-47-30A), or 9/16-inch eyebolt (part number 30774-1 or 30775-1) that exceeds the corresponding cumulative hours time in service specified in Requirement 2 of this Directive." That imposes a duty on maintenance personnel and parts controllers to verify cumulative hours before installation.

• Meet timing windows. Under "Compliance", tightening must occur within 100 hours time in service or 12 calendar months after 1 September 2005, whichever occurs first, unless already done. Thereafter tighten at intervals not to exceed 12 calendar months. Replacement must be done upon reaching the applicable hours in SL No. 129 or within 50 hours after 1 September 2005, whichever is later, and thereafter at intervals specified in SL No. 129.

Rights

• The AD does not set out procedural rights, appeal mechanisms or exemptions within the text provided. It is a prescriptive safety instrument that establishes mandatory maintenance obligations and timing. The text does not contain mitigation, alternative means of compliance (AMOC) provisions, or express rights of review; none of these are present in the source. Parties wishing to assert alternatives would need to consult CASA policy or administrative mechanisms outside the text provided.

• Operators retain the right to rely on SL No. 129 for the technical specifications. The AD incorporates SL No. 129 by reference for torque values and replacement intervals; the incorporated service letter is therefore the technical standard that implementers must follow.

Operational implications of the duties

• Maintenance planning: operators must plan initial and recurring tasks and maintain records of cumulative hours on each eyebolt to meet the installation prohibition and replacement intervals.

• Sourcing and part control: replacement will require supply of eyebolts that have not exceeded allowable hours; stock control must track hours to prevent prohibited installations.

• Technical compliance: maintainers must apply the torque values from SL No. 129; failure to follow the specified torque or replacement schedule would be non-compliant with the AD.

The AD text itself is prescriptive and specifies effective date and mandatory tasks, but it contains no express penalties or enforcement procedures within the provided text. The document states: "This Airworthiness Directive becomes effective on 1 September 2005." It is signed by a CASA delegate, David Villiers, dated 20 July 2005. The AD cross-references FAA AD 2005-13-12 Amdt 39-14149 and Snow Engineering SL No. 129 for technical requirements. The instrument does not, in the text supplied, set out sanctions, fines, or the administrative enforcement steps CASA will take for non-compliance.

What the source does provide that is relevant to enforcement:

• Command language. The AD uses mandatory language: "Tighten", "Replace", and "Do not install", supported by definitive compliance windows. Mandatory wording implies regulatory enforceability, and the stated effective date marks when the obligations commence.

• Delegation. The AD is signed by a CASA delegate, which identifies CASA as the issuing authority and therefore the body likely to oversee compliance. The document does not, however, state the mechanisms CASA will use to verify compliance, how non-compliance will be processed, or what penalties may apply.

Practical enforcement considerations implied by the AD text

• Record-keeping will be necessary to demonstrate compliance with tightening schedules, replacement intervals and the installation prohibition because the AD conditions actions on hours-in-service and calendar time. The AD’s reliance on cumulative hours makes maintenance records central to demonstrating compliance.

• Cross-reference to FAA AD. The note "FAA AD 2005-13-12 Amdt 39-14149 refers" suggests that parallel or corroborative enforcement or guidance may exist from the FAA. The AD does not detail whether the FAA’s enforcement will have direct legal effect in Australia, but operators subject to multiple jurisdictions often must consider both regulators’ instruments.

• Absence of explicit AMOC process. The text does not specify an alternative compliance pathway or an approval process for alternate means of compliance. The lack of an AMOC clause in the text supplied means that, based on the AD alone, the prescriptive maintenance actions are the standard to meet.

Because the AD does not set out penalties or enforcement procedures in the text provided, parties seeking clarity on sanctions, inspection protocols, or dispute resolution will need to consult CASA’s wider regulatory framework, legislative instruments or administrative guidance outside the AD text supplied here.

The directive expressly references two external documents that provide the technical standards it mandates: Snow Engineering Co. Service Letter No. 129 (both the original 26 September 1994 issue and the revision dated 21 October 2004) and FAA AD 2005-13-12 Amdt 39-14149. The AD makes SL No. 129 the source of the torque values and the cumulative hours time in service thresholds that determine replacement intervals, and it notes FAA AD 2005-13-12 as a relevant referent. Therefore, operational compliance under this AD requires coordination with both the service letter and the FAA AD.

Mechanisms of interaction evident in the text

• Incorporation by reference for technical detail. The AD itself does not replicate numerical torque settings or life limits; it delegates those details to SL No. 129. That means Snow Engineering’s service letter becomes the operative technical standard for torque and replacement intervals. In practice, an operator must possess and apply SL No. 129 to comply with the AD.

• Cross-jurisdictional alignment. The AD references a contemporaneous FAA AD. That indicates the safety concern was addressed by FAA rulemaking and that CASA’s Directive aligns with or mirrors the FAA requirement. The AD does not spell out whether compliance with the FAA AD alone satisfies the CASA AD, but the citation indicates relevant cross-jurisdictional coordination.

No direct interaction clauses with domestic aviation statutes or regulations appear in the AD text supplied. The AD does not itself cite enabling legislation, penalties, or formal interactions with continuing airworthiness regulations, maintenance licence requirements, or civil aviation safety rules beyond the signature by a CASA delegate. The AD does, however, create obligations that intersect with standard continuing airworthiness duties: maintaining records of hours-in-service, carrying out scheduled maintenance, and ensuring only permitted parts are installed. Those duties typically interact with broader maintenance and airworthiness regulatory regimes, even though the AD does not reproduce those regimes.

Potential operational frictions the text implies

• The AD’s reliance on SL No. 129 for numeric limits means that any inconsistency between SL No. 129 and other technical documentation for an aircraft (for example, manufacturer manuals or previous service letters) must be reconciled by following the AD-mandated SL No. 129 values; the AD takes precedence for the tasks it prescribes.

• The cover heading includes "AT-300, 400 and 500 Series", while the "Applicability" block does not list AT-500 models. This divergence between heading and applicability creates potential uncertainty about scope; under the AD language, the precise "Applicability" entry should control. Implementers must read the applicability block rather than rely on the cover heading.

• Since the AD does not state an AMOC or exemption process in the text supplied, any interaction with alternative technical standards or novel repairs will require engagement with CASA outside the AD text.

In short, conforming to this AD requires operational alignment with Snow Engineering SL No. 129 and awareness of the related FAA AD; the AD is silent in itself on how it integrates with other domestic aviation legislation beyond its issuance by CASA.

The AD document supplies a concise chain of dates and revisions relevant to its operative requirements. These dates constitute the amendment and antecedent documentary history expressly cited within the AD.

• Snow Engineering SL No. 129: The AD references SL No. 129 in two forms: "Issued 26 September 1994" and "Revised 21 October 2004." The AD directs implementers to use the torque values and replacement intervals specified in the revised 21 October 2004 version of SL No. 129 for the tightening task (Requirement 1 references SL No. 129, revised 21 October 2004), and it references SL No. 129 (issued 26 September 1994, revised 21 October 2004) for replacement instructions (Requirement 2).

• FAA AD: The AD contains a note that "FAA AD 2005-13-12 Amdt 39-14149 refers." The presence of that reference connects this CASA AD to the FAA action published around the same time; the AD does not reproduce the FAA AD text but indicates referential alignment.

• AD issuance and effective dates: The document is signed by David Villiers as "Delegate of the Civil Aviation Safety Authority" on 20 July 2005, and it explicitly states: "This Airworthiness Directive becomes effective on 1 September 2005." The AD also carries the identifier "AD/AT/26" and the effective designation "9/2005."

• Service Letter incorporation dates influence replacement windows. The Compliance section sets initial and recurring windows with explicit dates tied to the AD's effective date. For example, the initial tightening must occur "within 100 hours time in service or 12 calendar months after 1 September 2005, whichever occurs first" and replacement must be "within 50 hours time in service after 1 September 2005" if the SL's hours threshold has already been exceeded.

There is no later amendment or revocation recorded in the material supplied. The AD does not itself indicate any subsequent amendments or supersessions. If a practitioner needs the latest position, the AD text supplied should be treated as the state of the instrument as of its effective date, with the 21 October 2004 revision of SL No. 129 providing the technical baseline. The AD’s explicit references and dates form the amendment history the text itself relies upon; no additional legislative amendments, revisions to the AD, or subsequent service-letter updates are recorded in the supplied document.

The supplied AD includes no litigation history. The document does not reference any judicial proceedings, administrative appeals, or contested enforcement actions, nor does it cite any case law. The background material describes incidents and engineering analysis that prompted the AD, but the text does not report legal challenges or disputes about the Directive.

What the text does provide that is relevant to litigation risk

• Factual basis. The AD reports the FAA received "numerous reports of failures of horizontal stabiliser attachment eyebolts" and states the engineering analysis concluded failure was due to "high cycle, low-nominal stress" with "most likely due to the loss of torque during service." The AD frames the safety risk as a potential loss of pitch control and/or departure from controlled flight. Those factual statements would form part of the administrative record if enforcement or review proceedings were initiated, but the AD itself does not indicate any legal contest has occurred.

• Administrative origin. The AD is a regulatory instrument issued by CASA (signed by a delegate). If litigation were to be commenced about the AD’s validity, it would proceed under the applicable administrative law and aviation statutes. However, none of that is recorded in the AD text.

Absence of litigation history in the source means practitioners seeking information about any legal challenges must look beyond this AD text, for example to court or tribunal records, CASA publications, or later legislative materials. The AD as provided should be read as a technical regulatory directive rather than a document that narrates any enforcement or judicial disputes.

This AD contains several items that create practical hazards for implementers if not carefully read. The "gotchas" are points where the AD’s drafting, cross-references or compliance mechanics can produce operational confusion, record-keeping pitfalls or unintended non-compliance.

1. Cover heading versus applicability discrepancy. The document header names Airtractor AT-300, 400 and 500 series, but the "Applicability" block does not list AT-500 models. The precise "Applicability" block lists "Models AT-300, AT-301, AT-302, AT-400, and AT-400A aircraft; all serial numbers. Model AT-401/AT-402 aircraft, with serial numbers up to and including 401-0700." For compliance purposes the applicability language controls; operators should rely on the detailed applicability block rather than the cover heading.

2. Essential technical numbers are not reproduced. The AD mandates torque values and cumulative hours limits but does not include the numeric torque settings or numeric cumulative-hour thresholds in the text. Those essential figures are contained in Snow Engineering SL No. 129 (revised 21 October 2004). Failure to obtain SL No. 129 will prevent correct execution of the AD.

3. Installation prohibition depends on hours history. The "do not install" prohibition applies to specific part numbers "that exceed the corresponding cumulative hours time in service specified in Requirement 2." To apply this rule, maintenance staff must know the cumulative hours for the specific eyebolts proposed for installation. That requires reliable maintenance and part-history records for each eyebolt. If hours tracking is incomplete or missing, an eyebolt may become effectively unfit for installation under the AD even if physically undamaged.

4. Ambiguous interplay of replacement alternatives. The replacement timing is phrased as: "Replace initially upon accumulating the applicable number of hours time in service specified in SL No. 129, revised 21 October 2004, or within 50 hours time in service after 1 September 2005, whichever occurs later." Practically, where SL No. 129 indicates a long hours life that operators have not yet reached, the AD still sets a mandatory replacement within 50 hours of 1 September 2005 if that later date is triggered. Interpreting "whichever occurs later" correctly is critical to avoid premature or delayed replacement.

5. Recurrence cadence requires both time and hours tracking. Tightening is required initially within 100 hours or 12 calendar months after 1 September 2005, whichever occurs first, and thereafter "not to exceed 12 calendar months." Replacement recurrence is governed by SL No. 129 hours thresholds. Operators will therefore need to manage two different recurrence logics: one calendar-based (tightening) and one hours-based (replacement), with both affecting scheduling and aircraft availability.

6. Part-number specificity. The AD lists specific part numbers for the 5/16-inch, 7/16-inch and 9/16-inch eyebolts. Those part numbers must be checked exactly as written: "AN44-17A", "AN44-21A", "AN47-22A", "AN-47-30A", "30774-1", "30775-1". Note the slight inconsistency in hyphenation ("AN-47-30A" appears hyphenated where others are not). Maintain a careful approach to part-number matching to avoid misapplication of the prohibition.

7. No stated AMOC or appeal process in the text. The AD does not include an explicit alternative means of compliance pathway. If a maintainer believes an alternative procedure is acceptable, the AD provides no in-text mechanism for approval. Parties should therefore assume the AD’s specified actions are the required path unless they obtain separate regulatory clearance.

8. Effective date versus signature date. The AD is signed on 20 July 2005 and states it "becomes effective on 1 September 2005." The initial compliance windows are anchored to that effective date. Operators must align their scheduling with the effective date and the windows defined (for example, "within 100 hours time in service or 12 calendar months after 1 September 2005").

9. Reliance on external documents for evidence. Because key design and life-limit data are in SL No. 129 and the FAA AD, an operator’s ability to demonstrate compliance in an audit will depend on possession of those documents and any service records generated in performing the AD tasks.

Being alert to these "gotchas" will reduce the risk of inadvertent non-compliance and scheduling or parts-control errors.

The AD itself sets out the compliance actions and timing; below is a practical, source-grounded checklist and process distilled from the AD text to assist implementers. Each step derives directly from the AD’s "Applicability", "Requirement" and "Compliance" entries and the "Background" rationale.

1. Confirm applicability. Check the aircraft model and serial number against the AD’s Applicability block: Models AT-300, AT-301, AT-302, AT-400, and AT-400A, all serial numbers; and Model AT-401/AT-402 aircraft with serial numbers up to and including 401-0700. Do not rely on the cover heading for scope; use the explicit applicability table.

2. Obtain SL No. 129 (revised 21 October 2004). The AD requires use of Snow Engineering Co. Service Letter No. 129, revised 21 October 2004, for torque values and replacement intervals. Acquire that revised service letter and any referenced procedures in full before performing work.

3. Schedule initial tightening. The AD prescribes that the four eyebolts attaching the front and rear spar of the horizontal stabiliser to the stabiliser struts must be tightened "using the torque values" from SL No. 129. Perform this tightening within 100 hours time in service or 12 calendar months after 1 September 2005, whichever occurs first, unless the tightening has already been accomplished. Document the torque values applied and the technician who performed the work.

4. Establish recurring tightening cadence. After initial tightening, schedule retorquing at intervals "not to exceed 12 calendar months." Use maintenance records to ensure no interval exceeds 12 calendar months.

5. Determine replacement triggers. Using SL No. 129, establish the applicable cumulative hours time in service thresholds for eyebolt replacement. If any eyebolt has already exceeded the SL No. 129 hours threshold as of 1 September 2005, replace it "within 50 hours time in service after 1 September 2005" per the AD. If the SL-specified hours threshold has not yet been reached, replace upon accumulating that number of hours.

6. Perform initial replacements as required. Replace eyebolts that have reached or exceeded the SL No. 129 cumulative hours threshold, or within 50 hours after 1 September 2005 if that criterion applies, and thereafter at the intervals specified in SL No. 129. Record part numbers, serial numbers (if applicable), hours at removal and hours at installation.

7. Prevent prohibited installations. Before installing any eyebolt of the listed part numbers, verify that the particular eyebolt has not exceeded the cumulative hours limit set out in Requirement 2 (i.e., SL No. 129 thresholds). The prohibited part numbers are: 5/16-inch AN44-17A and AN44-21A; 7/16-inch AN47-22A and AN-47-30A; 9/16-inch 30774-1 and 30775-1. Do not install a listed eyebolt if its recorded cumulative hours exceed the SL No. 129 limit.

8. Keep accurate hours and parts records. The AD’s reliance on cumulative hours and the installation prohibition make scrupulous record-keeping necessary. Record hours-at-installation for each eyebolt, track their service hours, and maintain part-number traceability to demonstrate compliance.

9. Log maintenance entries. Enter the tightening actions, replacement actions and part installations into the aircraft maintenance logbooks or records in a manner consistent with airworthiness recordkeeping practices. Include dates, times-in-service values, torque values used, part numbers, and names/licence numbers of maintenance personnel.

10. Review related documents. Obtain and review the FAA AD referenced in the AD (FAA AD 2005-13-12 Amdt 39-14149) to understand any additional technical recommendations or context. Confirm that SL No. 129 revised 21 October 2004 is the controlling technical document for both torque values and replacement intervals.

11. Audit and scheduling. Integrate the tightening 12-month maximum interval and the SL No. 129 replacement intervals into recurring maintenance planning to minimise unexpected downtime. Coordinate parts provisioning so that replacement eyebolts that comply with allowable hours are available when required.

12. Seek regulatory clarification if necessary. The AD provides no in-text AMOC procedure and does not reproduce numeric torque or hours figures. If an operator cannot obtain SL No. 129, or if there is uncertainty about applicability (for example in respect of AT-500 models mentioned in the header), contact CASA or the issuing delegate for clarification prior to performing alternative actions.

Following these steps will align maintenance activity with the AD’s explicit requirements as set out in the "Requirement" and "Compliance" blocks. The AD’s effective date is 1 September 2005; scheduling should respect the deadlines anchored to that date.