{"id":"F2006B02214","name":"AD/AC-SNOW/23 - Vertical Fin Rear Spar","slug":"ad-ac-snow-23-vertical-fin-rear-spar","collection":"legislative_instrument","jurisdiction":"commonwealth","status":"in_force","isInForce":true,"actNumber":null,"makingDate":null,"administeringDepartment":null,"currentVersion":{"id":41444,"registerId":"commonwealth-F2006B02214-current","compilationNumber":null,"startDate":"2026-04-01","status":"InForce","reasons":null,"registeredAt":null},"sections":[{"sectionNumber":"1","sectionType":"section","heading":"AD/AC-SNOW/23 - Vertical Fin Rear Spar","content":"COMMONWEALTH OF AUSTRALIA (Civil Aviation Regulations 1998), PART 39 \\- 105 CIVIL AVIATION SAFETY AUTHORITY\n\nSCHEDULE OF AIRWORTHINESS DIRECTIVES\n\n![](image.001.png)  \nAyres Thrush (Snow) Commander Series Aeroplanes\n\nAD/AC-SNOW/23 Vertical Fin Rear Spar 10/95\n\nApplicability: All aircraft with model and serial number listed in Ayres Corporation Service Bulletin SB-AG-38.\n\nRequirement: Visually inspect the vertical fin rear spar for cracks at the upper, centre and lower rudder hinge bearing mounts.\n\nCompliance: Initially inspect before 100 hours’ time in service after 14 September 1995. Re-inspect at intervals not to exceed 100 hours.\n\nModification in accordance with Ayres Corporation Service Bulletin SB-AG-38 or a scheme accepted by the Authority cancels the Requirement of this Directive.\n\nBackground: When installing the modification of Ayres Corporation Service Bulletin SB-AG-38 on one aircraft the vertical fin rear spar was found to be cracked in several locations.\n\nThe cracks may be aggravated by internal corrosion of the bearings restricting free movement.","sortOrder":0}],"analysis":{"flash_summary":{"complexity_score":3,"scope_assessment":{"changed":false,"description":"The directive, as drafted, addresses the specific problem reported in the Background (cracks discovered during installation of SB-AG-38 and possible aggravation by bearing corrosion) by mandating inspections and offering a modification that cancels the inspections. The instrument applies only to aircraft listed in SB-AG-38 and does not, on its face, broaden or narrow that set or extend requirements beyond the remedy described. (Applicability; Requirement; Modification; Background)"},"complexity_factors":["Narrow applicability tied to specific models and serial numbers listed in Ayres SB-AG-38 (Applicability).","Simple, single technical task required: a visual inspection at three named hinge bearing mounts (Requirement).","Time-based compliance schedule with an initial deadline referenced to a historical date and a fixed repeat interval (Compliance).","Existence of a one-time modification that cancels recurring obligations, plus allowance for an Authority-accepted alternative scheme (Modification), introducing a choice and regulator discretion.","Dependence on an external technical document (Ayres SB-AG-38) for detailed modification work introduces an external reference point implementers must consult."],"plain_english_summary":"What this instrument changes, mechanically:\n\n- It requires operators of certain Ayres Thrush (Snow) Commander aeroplanes to carry out visual inspections of the vertical fin rear spar at three specific locations: the upper, centre and lower rudder hinge bearing mounts. (Requirement)\n- The first inspection must be done before the aircraft has accumulated 100 hours time in service after 14 September 1995, and inspections must be repeated at intervals not exceeding 100 hours. (Compliance)\n- If an operator carries out the modification described in Ayres Corporation Service Bulletin SB-AG-38, or a modification scheme accepted by the Civil Aviation Safety Authority, the inspection requirement is cancelled. (Modification)\n\nWho is affected:\n\n- Owners and operators of the Ayres Thrush (Snow) Commander models and serial numbers listed in Ayres Corporation Service Bulletin SB-AG-38. (Applicability)\n- Maintenance organisations and mechanics who perform the required inspections or the SB-AG-38 modification.\n\nWhy it matters (source statements and practical implications):\n\n- The AD is prompted by a discovery of multiple cracks in the vertical fin rear spar when the SB-AG-38 modification was installed on one aircraft. The AD text also notes those cracks may be aggravated by internal corrosion of the bearings that restricts free movement. (Background)\n\nPractical effects, incentives and trade-offs (source-cited):\n\n- Who pays: operators bear the cost and labour of periodic visual inspections and, if chosen, the cost of performing the SB-AG-38 modification. (Requirement; Compliance; Modification)\n- Behaviour required: until an approved modification is installed, operators must schedule and perform repeat inspections every 100 hours to check for cracks at the three rudder-hinge bearing mounts. This creates a recurring maintenance obligation. (Requirement; Compliance)\n- Incentive created: performing the SB-AG-38 modification (or an Authority-accepted alternative) removes the recurring inspection obligation, so operators face a trade-off between recurring inspection costs and a one-time modification cost. (Modification)\n- Compliance burden and operational impact: the AD imposes periodic downtime for inspection every 100 hours; the initial compliance deadline is time-based relative to 14 September 1995, which governs when the first inspection must occur. These requirements affect scheduling and aircraft availability. (Compliance)\n- Regulatory discretion: the Authority may accept an alternative scheme in place of the SB-AG-38 modification; that acceptance is a point of regulator discretion and provides an approved pathway other than the specific bulletin. (Modification)\n- Risk addressed: the AD responds to an observed structural cracking issue and the potential role of bearing corrosion in aggravating cracks; the inspections and modification are intended as mitigations. (Background)\n\nOperational and market notes (grounded in the instrument):\n\n- The AD applies only to aircraft identified in SB-AG-38, so its direct obligations are concentrated on that set of operators. (Applicability)\n- The AD creates demand for inspection and modification work from maintenance providers and may change operators' maintenance planning and costs. (Requirement; Modification)\n\nSource sections used: Applicability; Requirement; Compliance; Modification; Background."},"kimi_summary":{"content_quality":"ok","complexity_score":2,"scope_assessment":{"changed":false,"description":"This is a discrete, targeted safety directive addressing a specific structural defect in a specific aircraft model. It has not expanded beyond its original purpose of mandating inspection for cracked tail spars."},"complexity_factors":["Single, specific aircraft type affected (Ayres Thrush/Snow Commander series)","Only 4 operative paragraphs: applicability, requirement, compliance, and modification cancellation","Zero defined terms requiring interpretation","Simple temporal trigger (100 hours after fixed date) and recurring interval","Single cross-reference to manufacturer service bulletin (SB-AG-38)","No nested conditions, exceptions, or conditional logic beyond the modification exemption"],"plain_english_summary":"**What this is:** This is an Airworthiness Directive (AD) — essentially a mandatory safety notice issued by Australia's Civil Aviation Safety Authority (CASA). It requires owners and operators of specific agricultural aircraft to inspect a particular part for dangerous cracks.\n\n**Who it affects:** Owners and operators of **Ayres Thrush (Snow) Commander Series aeroplanes** — these are crop-dusting and agricultural spraying aircraft. Only specific aircraft identified by model and serial number in a manufacturer service bulletin (SB-AG-38) are covered.\n\n**What must be done:**\n- **Inspect the vertical fin rear spar** — that's the structural backbone at the back of the tail fin where the rudder attaches\n- **Check specifically for cracks** at three hinge points (upper, centre, and lower) where the rudder connects to the fin\n- **First inspection:** Must happen before the aircraft clocks up 100 flight hours after 14 September 1995\n- **Ongoing inspections:** Every 100 flight hours thereafter\n\n**The 'get out' clause:** If you permanently modify the aircraft according to the manufacturer's Service Bulletin SB-AG-38 (or an alternative scheme approved by CASA), you can stop doing these inspections.\n\n**Why it matters:** Cracks were discovered in this part during routine modifications. The cracks can worsen if internal corrosion seizes up the bearings, preventing the rudder from moving freely. A failed tail structure could cause loss of control and crash."}},"importantCases":[],"_links":{"self":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar","history":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar/history","analysis":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar/analysis","conflicts":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar/conflicts","importantCases":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar/important-cases","documents":"/api/acts/ad-ac-snow-23-vertical-fin-rear-spar/documents"}}