AD/AC-SNOW/26 - Vertical and Horizontal Stabilisers

The directive revolves around several key technical and regulatory concepts. An airworthiness directive is a legally enforceable instrument issued by CASA under the Civil Aviation Act 1988 and Civil Aviation Safety Regulations 1998 that mandates corrective action on aircraft or components that have an unsafe condition. This AD adopts the substance of FAA AD 2007-26-01 (Amdt 39-15303) but is a stand-alone Australian requirement. The empennage is the tail structure of the aircraft; the vertical and horizontal stabilisers provide directional and pitch stability. The attach fittings are metal brackets that connect the stabilisers to the fuselage structure; part numbers are specified to identify the exact components subject to the directive. A visual inspection is defined by the referenced service bulletin; it involves looking for corrosion (chemical degradation of metal) and cracking (mechanical fracture) without the use of nondestructive testing methods unless specified. The compliance times are expressed in hours time in service (airframe hours since manufacture or last overhaul) and calendar time. The directive uses three distinct timeframes: (1) a 50-hour initial window for the first inspection; (2) a recurring 100-hour interval for subsequent inspections until the replacement is done; (3) a final 2,000-hour or 2-year cap for the replacement action if no cracks were found. The concept of a special flight permit allows an aircraft that is not fully airworthy to be flown under strict conditions to a maintenance facility; here the permit is limited to ferry flights with an empty hopper, reduced maximum speed (Vne), and a prohibition on flying in known turbulence. The AD also references service bulletin SB-AG-45 Revision B, which provides detailed instructions for the inspection, replacement, and repair processes. Importantly, the directive allows use of a later FAA approved revision of that bulletin, meaning operators can adopt updated instructions issued after June 2007 without needing a separate AD amendment. The applicability condition links directly to the FAA AD's serial number list; the operator must check federally the affected serial numbers for models S2R through S2R-T660, including those with a "DC" suffix. The part numbers mentioned (e.g., 40301-7, 95266-3) are critical because the AD is type-specific; a wrong part number identification could lead to unnecessary work or missed compliance.

The directive directly binds every person who operates, owns, or maintains an aircraft listed in the applicability section of FAA AD 2007-26-01, provided the aircraft is fitted with metal empennage part numbers 40220 or 95400. This covers a range of Ayres Thrush models: S2R, S2R-R1340, S2R-R1820, S2R-T11, S2R-T15, S2R-T34, S2RHG-T34, S2R-T45, S2R-T65, S2RHG-T65, S2R-G1, S2R-G5, S2R-G6, S2R-G10, and S2R-T660. Because the AD adopts serial numbers from the FAA directive, any aircraft with a serial number appearing on that FAA list is subject. Aircraft with serial numbers not on the FAA list but sharing the same part numbers are not captured; operators should confirm the exact serial number range. The directive affects primarily agricultural operators who use these aircraft for crop dusting, as the Thrush S2R is a single-engine agricultural plane. Maintenance organisations that perform 100-hour or annual inspections must include the AD tasks in their work scope. Licensed aircraft maintenance engineers (LAMEs) and authorised persons must carry out the inspection and replacement in accordance with the service bulletin. Part suppliers are affected indirectly: the AD mandates replacement of attach fittings with new part numbers (95266-3, 95267-5) and a new bolt (NAS6207-68), creating demand for these components. The repair or replacement of the vertical fin aft spar (part number 40261-24 or 95253-1) may require manufacturing or ordering of those parts. The directive also impacts insurance and sale transactions; an aircraft without AD compliance may be unairworthy and hence unsaleable or uninsurable. Because the AD has a long compliance tail (2,000 hours or 2 years for replacement of unaffected aircraft), it creates a schedule that operators must track meticulously. The special flight permit provisions affect operators who need to ferry an aircraft to a maintenance facility; they must apply to CASA for the permit and ensure the flight conditions are met. In effect, the AD imposes a compliance cost on the entire fleet of affected aircraft, with the heaviest burden falling on operators who have aircraft with many hours accumulated near the 2,000-hour cap or those that have already cracked.

The AD imposes several mandatory duties on aircraft operators. First, the duty to conduct a visual inspection of the specified components within 50 hours time in service after 23 January 2008, and again at intervals not exceeding 100 hours, until the replacement actions are completed. Second, the duty to replace the vertical stabiliser attach fitting, horizontal stabiliser attach fitting, and attachment bolt with new parts if any corrosion or cracking is found, and to do so before further flight. Third, the duty to replace the same components even if no corrosion or cracking is found, within 2,000 hours or 2 years after the effective date, whichever occurs first. Fourth, the duty to inspect the vertical fin aft spar when no cracks were found, and to repair or replace it if corrosion or cracks are present. Fifth, the duty to use Thrush Service Bulletin SB-AG-45 Revision B or a later FAA approved revision to accomplish all work. Sixth, the duty to keep records of compliance, including logbook entries and component history cards. Seventh, the duty to ensure that any special flight permit for ferry only operates under the specified limitations: empty hopper, reduced Vne, and no flight into known turbulence. Eighth, the duty to ensure that non-compliance does not occur; the aircraft may not be operated except in accordance with the AD or a special flight permit. Operators also have certain rights arising from the AD. They have the right to use a later FAA approved revision of the service bulletin, which may simplify or improve the process. They have the right to apply for a special flight permit from CASA to ferry the aircraft to a maintenance facility without first completing all the AD requirements, subject to the stated conditions. They have the right to repair (rather than replace) the vertical fin aft spar if corrosion or cracking is found, provided the repair conforms to the service bulletin. They have the right to replace components earlier than the mandatory deadlines if convenient. They have the right to seek an alternative means of compliance or an extension from CASA under the general provisions of CASR Part 21, though the AD does not explicitly mention this. They also have the right to delegate the performance of the inspection and replacement to a qualified maintenance organisation, provided the organisation is properly authorised. These duties and rights together create a structured compliance regime that prioritises safety while offering limited operational flexibility.

The AD itself does not specify monetary penalties or imprisonment terms, but it derives its legal force from the Civil Aviation Act 1988 and the Civil Aviation Safety Regulations 1998. Under CASR Part 21, a person who operates an aircraft in contravention of an airworthiness directive commits an offence. The maximum penalty for an individual is typically 50 penalty units (currently about AU$18,780) and for a body corporate up to 250 penalty units (about AU$93,900), along with possible imprisonment for more serious breaches under the Act. CASA can also suspend or cancel the aircraft's certificate of airworthiness for non-compliance, effectively grounding the aircraft. The AD contains built-in enforcement mechanisms: if corrosion or cracking is found, the aircraft cannot be flown again (before further flight) until the corrective action is completed. This creates an immediate financial disincentive to delay inspection, as discovering a crack without parts on hand could ground the aircraft. Even if no cracks are found, failure to replace the fittings within 2,000 hours or 2 years is a breach that can lead to grounding. CASA inspectors may check AD compliance during routine audits, ramp checks, or when reviewing maintenance records. The special flight permit provision is a limited exception; if an operator ferries the aircraft outside the permit conditions (e.g., with a full hopper or into turbulence), that operation is unauthorised and exposes the operator to penalties. The AD also references FAA AD 2007-26-01; because the Australian AD adopts the FAA's applicability and service bulletin, any variation from the FAA directive could also affect bilateral airworthiness agreements. In practice, enforcement is often reactive: a crash caused by empennage failure would trigger investigations and potentially criminal charges for wilful non-compliance. However, the AD's layered compliance times reduce the risk of inadvertent non-compliance for operators who record hours carefully. The most significant enforcement risk is for operators who never inspect or replace within the 2-year/2,000-hour window; such non-compliance over time could lead to a fleet-wide grounding order.

This AD operates within a hierarchy of aviation safety regulation. It is issued under the Civil Aviation Act 1988 and the Civil Aviation Safety Regulations 1998, specifically Part 21 (Certification and airworthiness) which empowers CASA to issue ADs. The AD must be read together with CASR 21.175 which requires that no person operate an aircraft unless it complies with applicable ADs. It also interacts with the continuing airworthiness requirements in CAR Part 42 and CASR Part 42, particularly the obligation to maintain the aircraft in an airworthy condition and to record all maintenance actions. The AD specifically references FAA AD 2007-26-01; compliance with the FAA directive is not a substitute for compliance with this Australian AD, but the similarity means operators who have already complied with the FAA AD for aircraft imported from the US are considered compliant here (assuming the serial numbers and parts match). The AD refers to Thrush Aircraft Inc. Service Bulletin SB-AG-45 Revision B. That service bulletin is a manufacturer's instruction, not itself legislation, but the AD makes compliance with it mandatory. If Thrush later issues Revision C with FAA approval, the AD allows use of that revision, so the AD dynamically incorporates future amendments without a new rulemaking. The special flight permit provisions are governed by CASR 21.197 and 21.199; operators must apply under those regulations. The AD does not affect other ADs that may apply to the same aircraft; for example, AD/AC-SNOW/23 on engine fuel systems would impose separate requirements. Operators must ensure compliance with all applicable ADs; the inspection and replacement under this AD may be scheduled to coincide with other maintenance intervals, but the AD's 50-hour initial inspection could conflict with an operator's customary 100-hour cycle. The AD also interacts with state and territory workplace health and safety laws that apply to maintenance facilities; the corrosion and cracking detection may involve chemicals and processes regulated by those laws. If the aircraft is used for commercial agricultural operations, the Civil Aviation Safety Regulations on aerial application (Part 137) continue to apply, and the hopper-empty condition on special flight permits is consistent with those rules. Insurance policies often require compliance with all mandatory ADs; failure to comply could void coverage. The AD may also have implications under the Air Navigation Act for aircraft registered in Australia but operating overseas; extra-territorial enforcement is possible.

The source provided does not indicate any amendments to this AD since its effective date of 23 January 2008. The AD appears to be the original issue, with the notation "3/2008 DM" likely indicating it is the third airworthiness directive issued in 2008 and approved by a delegate of CASA (David Villiers). There is no indication of subsequent revisions, rescissions, or supersedure within the source. However, the AD's condition that operators may use a "later FAA approved revision" of the service bulletin means that the procedural requirements of the AD can be updated without amending the AD itself. Because the AD is type-specific and based on a FAA directive that may itself be amended over time (for example, the FAA could issue a new AD restating or replacing 2007-26-01), Australian operators should check whether the FAA reference has been superseded. CASA typically tracks FAA AD changes and may issue a new Australian AD to adopt the revised requirement, but that would be a separate amendment. In this specific source, no amendment history is provided, so the safest assumption is that the AD as published in January 2008 remains in effect unless CASA has subsequently replaced it. Practitioners should verify current status through the CASA Airworthiness Directive database because many older ADs are eventually superseded by more comprehensive directives or by the integration of the modification into the type design (e.g., via a supplemental type certificate). If the replacement parts (95266-3, 95267-5, NAS6207-68) become standard production for all new aircraft or are mandated by a later airworthiness directive, this AD might be cancelled. But based solely on the source, the amendment history is nil.

The source document does not refer to any litigation concerning this AD, and no case law is named. Airworthiness directives rarely generate direct litigation because operators typically comply or risk grounding; disputes are often resolved through administrative processes such as applications for alternative means of compliance or extensions. However, if an operator refused to comply and CASA took enforcement action, the operator could appeal under the Administrative Appeals Tribunal Act or seek judicial review of the AD's validity under the Administrative Decisions (Judicial Review) Act 1977. Such cases would be rare. More commonly, litigation arises from accidents where empennage failure is alleged; if an operator had not complied with this AD, civil negligence claims could follow. The AD could be used as evidence of a known safety defect. No reported cases linking directly to AD/AC-SNOW/26 were provided. It is also possible that the underlying FAA AD 2007-26-01 was litigated in the United States, but that is outside this source. In Australia, a similar AD might be referenced in an accident investigation report by the Australian Transport Safety Bureau (ATSB) if an empennage failure occurred on an affected aircraft, but no such report is cited. The absence of litigation history suggests either high compliance rates or that any non-compliance was handled administratively. The legal risk for operators remains low if they follow the directive; the risk of tort liability for failure to comply is a distinct but unlitigated concern. Practitioners should be aware that the AD's requirements are mandatory and any deviation (e.g., using uncertified parts) could expose the operator and engineer to liability. The lack of case law does not equate to immunity.

Several provisions in this AD can trip up unwary operators. First, the 50-hour initial inspection window is shorter than the typical 100-hour inspection cycle for agricultural aircraft that may operate seasonally. An operator who interprets "within 50 hours time in service" as a 50-hour interval (i.e., inspect every 50 hours) misreads the requirement; it is a one-time initial deadline, after which the next inspection is at 100 hours. Missing the 50-hour window is a breach even if the aircraft only has 10 hours since the effective date. Second, the AD requires replacement of the attach fittings and bolt even if no corrosion or cracks are found, within 2,000 hours or 2 years. Some operators may assume that if the first inspection-clears them, they only need to inspect the fins for cracks and can postpone fitting replacement indefinitely. That is wrong; the replacement is mandatory under Requirement 3a, regardless of condition. The 2,000-hour/2-year cap applies to that replacement. Third, the serial number applicability refers to the FAA AD list, which may be updated; operators must check the current FAA list, not assume their serial number is included based on a one-time check. Fourth, the "or later FAA approved revision" of the service bulletin creates a trap: if a later revision changes the inspection intervals or part numbers, operators who only use Revision B may be out of compliance. They must actively monitor for FAA-approved revisions. Fifth, the special flight permit conditions are strict; "no flight into known turbulence" is a vague condition that can be breached inadvertently if unexpected turbulence is encountered; operators should plan routes carefully. Sixth, the "hopper must be empty" condition includes residual chemical residues; operators must clean the hopper thoroughly, not just empty it. Seventh, the Vne reduction to 126 mph (109 knots) means normal airflow limits are lower; exceeding that speed, even if the aircraft is capable, breaches the permit. Eighth, the AD applies to serial numbers "with or without a 'DC' suffix", so a DC suffix does not exempt the aircraft. Ninth, the requirement to repair the vertical fin aft spar "in accordance with Thrush SB-AG-45 Revision B" means the repair method must be exactly as prescribed; deviating without FAA approval is non-compliant even if a different repair is structurally sound. Tenth, the AD does not specify a grace period for the 2-year calendar limit; it is strictly from 23 January 2008, so an aircraft that accumulates hours slowly (e.g., 100 hours per year) must still replace fittings by January 2010, not at 2,000 hours. Eleventh, the AD is type-specific to metal empennage part numbers 40220 or 95400; if a later modification replaced the empennage with a different part number, the AD may no longer apply, but the operator must document that.

Compliance requires a systematic approach. Step one: determine whether your aircraft is affected. Check the aircraft model plate against the list: S2R through S2R-T660. Obtain the serial number and verify it against the serial number list in FAA AD 2007-26-01 (the AD does not reproduce the list, so operators must source the FAA AD). Also confirm that the aircraft is equipped with metal empennage part numbers 40220 or 95400; if the empennage has been replaced with a non-metal type or a different part number, the AD may not apply, but document the basis. Step two: obtain a copy of Thrush Aircraft Inc. Service Bulletin SB-AG-45 Revision B (dated 1 June 2007) or any later FAA approved revision. Check the Thrush website or contact the manufacturer for the latest version. Step three: schedule the initial visual inspection within 50 hours time in service after 23 January 2008. Coordinate with a qualified maintenance organisation or LAME. The inspection must follow the service bulletin exactly, covering the vertical stabiliser attach fitting (40301-7), horizontal stabiliser attach fitting (40303-1/-4/-7 or 95267-1), attachment bolt (NAS1105-68), and vertical fin aft spar (40261-24 or 95253-1). Record findings. Step four: analyse results. If corrosion or cracking is found on any fitting or the bolt, stop the aircraft from further flight (Requirement 2). Replace the vertical stabiliser attach fitting with new part number 95266-3, the horizontal stabiliser attach fitting with new 95267-5, and the bolt with NAS6207-68 per the service bulletin. If corrosion or cracks are found on the vertical fin aft spar, repair per the bulletin or replace with new 40261-24 or 95253-1. Complete this before further flight. If no corrosion or cracking is found on any component, you are in Requirement 3 territory. You still must replace the fittings and bolt with the new parts, but you have up to 2,000 hours or 2 years (whichever first) from 23 January 2008 to do so. However, you must also perform a visual inspection of the vertical fin aft spar for corrosion and cracking at the time of the replacement (Requirement 3b). If that inspection finds damage, repair or replace per the bulletin. Step five: until the replacement is done, repeat the full visual inspection of all components at intervals not exceeding 100 hours time in service (Requirement 1). Keep records of each repeat inspection. Step six: complete the replacement of the fittings and bolt by the deadline (2,000 hours or 2 years) even if no cracks are ever found. After replacement, no further inspections under this AD are required because the new parts are not subject to the AD. Step seven: make logbook entries referencing AD/AC-SNOW/26, the serial number, hours at compliance, part numbers removed and installed, and the service bulletin used. Keep copies of the service bulletin, inspection sheets, and invoices. Step eight: if you need to ferry the aircraft to a maintenance facility without completing all AD requirements, apply for a special flight permit from CASA. The permit will require the hopper empty, Vne 126 mph, and no flight into known turbulence. Step nine: monitor for any later FAA approved revision of SB-AG-45; if a revision is issued, update your procedures accordingly. Step ten: if you are unable to comply by the deadline (e.g., parts unavailable), apply to CASA for an alternative means of compliance or an extension under CASR Part 21; do not simply continue operating. Following these steps exactly will result in full compliance with the AD. Non-compliance invites enforcement action and safety risk.