AD/AT 600/5 - Wing Main Spar Lower Cap Fastener Hole

Airworthiness Directive (AD) - a mandatory regulatory instrument issued by CASA under the Civil Aviation Safety Regulations (normally Part 39) that requires an operator to inspect, modify, or replace a component or system to rectify an identified unsafe condition. Non‑compliance renders the aircraft unairworthy.

Model AT‑602 aircraft - a single‑engine, low‑wing agricultural and utility aeroplane manufactured by Airtractor (commonly used for crop dusting, fire‑bombing, and similar operations). The AD applies only to serial numbers 602‑0337 and subsequent. This subset covers a production run that may share a common spar design vulnerable to fatigue.

Main spar lower cap - the primary structural member of the wing that carries bending loads. The “lower cap” is the tension flange of the spar. Cracking at fastener holes reduces the static strength of the wing and, in fatigue, can lead to catastrophic failure.

Wing centre splice joint - the structural connection where the left and right wing panels are joined at the aircraft centre line. The AD focuses on the two outboard fastener holes at this splice in each lower spar cap.

Eddy current inspection - a non‑destructive testing method that uses electromagnetic induction to detect surface and near‑surface cracks in electrically conductive materials. The AD requires this specific method as prescribed in Snow Engineering Co. Process Specification 197 Revised 4 June 2002.

Cold working - a mechanical process that expands a fastener hole to introduce beneficial compressive residual stresses around the hole, increasing fatigue life. The AD references two cold‑working procedures: Service Letter No. 244 (for terminating action on certain serial numbers) and Service Letter No. 240 (for use in conjunction with the steel web plate and 8‑bolt splice blocks). Operators who choose cold working must still perform the eddy current inspection as part of the procedure.

Terminating action - a modification that, once accomplished, ends the requirement for repetitive inspections. In this AD, terminating action consists of installing part number 20996‑2 steel web plate and part number 20985‑1/2 8‑bolt splice blocks in accordance with Snow Engineering Co. Drawing 20998 Revision B, and cold working the two outboard fastener holes per SL No. 240. After that, the aircraft no longer needs the eddy current inspection at 1,000‑ or 2,000‑hour intervals.

Time in service - the total operating time of the aircraft since new (airframe hours). The AD defines compliance thresholds using hours (e.g., initial inspection at 2,000 hours, replacement at 6,500 hours). For agricultural aircraft, which may fly short sorties, time in service accumulates quickly relative to calendar time. The AD does not use flight cycles, even though fatigue is often cycle‑dependent; this simplification may impose a stricter regime on high‑hour operators.

Snow Engineering Co. drawings and service letters - proprietary engineering data that the operator must obtain and follow to perform the modifications. The AD mandates specific revisions (e.g., Drawing 20998 Revision B, dated 28 September 2004). Using outdated or incorrect revisions would likely constitute non‑compliance.

FAA AD 2006‑23‑09 - the US equivalent directive issued by the Federal Aviation Administration. CASA’s AD notes this reference, indicating that the unsafe condition was first identified in the US fleet and that the Australian directive harmonises with US requirements. This also means that aircraft imported or operated under a US airworthiness certificate may have already been affected.

The primary entities affected are operators of Model AT‑602 aeroplanes with serial numbers 602‑0337 and subsequent. As of the effective date (18 January 2007), any Australian‑registered aircraft within that range must comply with the directive. This group includes commercial agricultural operators (crop dusters), fire‑fighting operators, private owners, and any entity that conducts aerial work or charter using this model. Because the AT‑602 is relatively rare, the total number of affected aircraft is likely small, but the compliance burden per aircraft is substantial.

Maintenance organisations and repair stations are affected because they must be authorised to perform the eddy current inspection, the cold working, the structural modifications, and the spar replacement. They must hold the required Snow Engineering Co. data and be trained in the specific procedures. The AD does not specify a rating or approval, but under CASR Part 145, only appropriately rated maintenance organisations can perform such work. For operators who rely on a single maintenance provider, the requirement may create scheduling bottlenecks and cost concentration.

Parts suppliers are affected indirectly: the AD specifies exact part numbers for steel web plates, splice blocks, and hardware. If these parts are not readily available in Australia, operators may face long lead times and higher costs, which could delay compliance and ground the aircraft. The AD does not provide alternative parts or a manufacturer‑approved equivalent, so replacement must be done per the Snow Engineering drawings.

Owners of affected aircraft bear the direct financial cost. For an aircraft already near a compliance threshold (e.g., 1,950 hours), the initial inspection must be done within 50 hours of the effective date, which could mean an unscheduled downtime of several days or weeks depending on availability of the engineering data and maintenance slots. For aircraft that have not yet been inspected, the AD imposes a one‑off access task (cutting inspection holes, modifying the vent tube, installing cover plates) that adds cost and may require subsequent restoration of the structure.

Insurance companies and financiers are affected because an aircraft that is non‑compliant with an AD may be unairworthy, and therefore uninsurable or in breach of loan covenants. The AD creates an obligation that must be recorded in the aircraft’s maintenance records and logbooks, which affects resale value and leasing prospects.

CASA itself is affected as the issuer and enforcer. It may need to respond to requests for extensions or alternative means of compliance, and to monitor compliance via audits of maintenance records.

Duties are imposed on the operator or owner of an affected aircraft. The operator must ensure that the aircraft is not flown unless the applicable requirements of the AD have been met. Specifically, the operator must:

• Gain access for inspection by cutting inspection holes, modifying the vent tube, and installing cover plates per Snow Engineering Co. Service Letter No. 204 Revised 26 March 2001 and the drawing titled “602 Spar Inspection Holes and Vent Tube Mod.” dated 13 November 2003, unless this has already been done.
• Eddy current inspect the two outboard fastener holes in each wing main spar lower cap (four holes total) per Snow Engineering Co. Process Specification 197 Revised 4 June 2002.
• For aircraft with serial numbers 602‑0337 through 602‑0584 as manufactured (i.e., without cold‑worked holes): perform the initial inspection upon accumulation of 2,000 hours time in service or within 50 hours after 18 January 2007 (whichever occurs later). Thereafter, repeat the inspection every 1,000 hours time in service.
• For aircraft with serial numbers 602‑0337 through 602‑0584 that have been modified with cold‑worked fastener holes per SL No. 244: perform the eddy current inspection as part of the cold working procedure (even if previously inspected), and then repetitively inspect every 2,000 hours time in service. The cold working does not itself terminate the inspection; it extends the interval.
• For cracks found during any inspection: repair or replace the cracked spar cap. If the crack can be removed by performing the terminating action (steel web plate + 8‑bolt splice blocks + cold working), do so. If the crack cannot be removed by that terminating action, replace the lower spar caps and associated parts before further flight. No grace period is allowed in the latter case.
• At 6,500 hours time in service on the wing spar lower caps (or within 50 hours after 18 January 2007, whichever later): replace the wing lower spar caps, splice blocks and hardware, wing attach angles and hardware, and install steel web plate part number 20996‑2 if not already installed. This mandatory replacement terminates the prior repetitive inspection requirements.

Rights: The operator has the right to choose the terminating action at any earlier point to eliminate the repetitive inspection burden. For aircraft already modified with cold‑worked holes (SL No. 244), the operator can still perform the full terminating action to replace the splice and web plate and thus end inspections. The AD does not explicitly state a right to request an alternative means of compliance (AMOC) from CASA, but under CASR Part 39, such a right exists; operators may propose a different inspection method or interval if they can demonstrate an equivalent level of safety. The AD references specific drawings and service letters, but the operator is free to use any data acceptable to CASA that meets the intent of the directive. The AD also permits, for cracks that can be removed by the terminating action, to use that action as a repair, thereby avoiding a full spar cap replacement.

The source text does not specify the penalties for non‑compliance. However, under the general framework of the Civil Aviation Act 1988 (Cth) and the Civil Aviation Safety Regulations 1998 (CASR), contravention of an airworthiness directive is a serious offence. For an individual operator, penalties can include fines (currently up to 1,000 penalty units or more for a body corporate), imprisonment in extreme cases where the breach results in a safety risk, and automatic suspension or cancellation of the aircraft’s certificate of airworthiness. The CASA enforcement policy typically treats non‑compliance with an AD as a high‑priority matter because it directly relates to an unsafe condition.

Enforcement action would likely begin with a direction to ground the aircraft until the AD is complied with. CASA can issue a written direction under regulation 39.013 of CASR, and failure to comply with that direction is a further offence. In practice, an operator found flying an aircraft that has not met the AD may face revocation of the aircraft’s permission to fly (often a “grounding order”) and possible referral to the Commonwealth Director of Public Prosecutions for criminal prosecution.

The AD does not include a self‑reporting or amnesty provision. If a crack is found during inspection, the operator must act before further flight. There is no “grace” beyond the 50‑hour window specified. Moreover, if a cracked spar cap cannot be repaired by the terminating action, the aircraft must be grounded immediately (requirement 4.b). The operator cannot ferry the aircraft to a maintenance base without CASA approval; any such flight would be illegal.

Records of compliance must be kept in the aircraft maintenance logbooks and made available to CASA on request. Failure to maintain proper records is itself an offence under CASR Part 42 (e.g., regulation 42.100). CASA may conduct random audits or investigate after an incident.

The cost of non‑compliance extends beyond fines. An aircraft that has not been kept compliant is unairworthy, meaning that the operator cannot legally charge for its use, insurance may be void, and any accident could lead to civil liability for damages. The AD effectively makes the operator personally responsible for the continued airworthiness of the wing spar.

The AD operates within the broader framework of the Civil Aviation Safety Regulations 1998 (CASR), specifically Part 39 which governs the issuance and effect of airworthiness directives. Under CASR Part 39, compliance with an AD is mandatory and overrides any less‑restrictive maintenance schedule. The AD takes precedence over the manufacturer’s recommended maintenance program if those programs do not include the required inspections.

The AD is directly linked to the US FAA AD 2006‑23‑09. Australian operators of aircraft imported from the US may have already complied with the FAA directive, but they must confirm that the action taken meets the CASA AD requirements, because the specific drawings, letter revisions, and compliance times may differ. The Australian AD incorporates by reference Snow Engineering Co. documents that may have later US revisions. It is the operator’s responsibility to ensure that the correct revision (as listed in the AD) is used. This cross‑referencing creates a potential trap: if an operator performed the FAA AD using a different revision of a service letter, that may not satisfy the Australian AD.

The AD also interacts with regulations on maintenance release (CASR Part 42) and approved maintenance organisations (Part 145). Work performed under the AD must be certified by a licensed aircraft maintenance engineer (LAME) or an approved organisation. The eddy current inspection requires a non‑destructive testing (NDT) qualification, which may be specified in the process specification.

Insurance policies and leasing contracts typically require that the aircraft be maintained in accordance with all applicable airworthiness directives. Non‑compliance with this AD could void coverage or trigger a default. For operators under an Australian Air Operator’s Certificate (AOC), failure to maintain compliance may lead to CASA suspending the AOC.

There is no stated interaction with state or territory workplace health and safety laws, but the practical effect of grounding and modification may affect workers (pilots, ground crew) and require consultation with the operator’s safety management system.

Finally, the AD may affect the aircraft’s eligibility for an export certificate of airworthiness. An Australian aircraft being sold overseas would need to show compliance with all applicable ADs, including this one.

The source text is the original issued AD. There is no amendment history provided within the document. The AD number is AD/AT 600/5, and it carries an issue number of 1/2007. This suggests it is the fifth Airworthiness Directive issued for the Airtractor 600 series fleet, and the first for the year 2007. The document is dated 23 November 2006 and becomes effective on 18 January 2007. The source does not indicate that it supersedes, revokes, or amends any prior directive; however, given the serial‑number applicability, it may be the first directive covering the AT‑602 for this specific cracking issue. The AD references an FAA AD (2006‑23‑09) as a note, but that US directive is not part of the Australian AD’s amendment history. CASA may later issue a revised AD (e.g., AD/AT 600/5 Amendment 1 or a new AD/AT 600/6) to extend applicability, change compliance times, or introduce alternative actions, but no such amendment is contained in this source. The absence of amendment history means that operators must treat this AD as the current baseline requirement, without any prior versions that might have had different thresholds. The user should check the CASA website for any subsequent amendments, because a later revision could alter the terminating action or inspection intervals. For the purposes of this deep dive, the source is the original unamended directive.

The source text does not mention any court cases, tribunal decisions, or administrative appeals relating to AD/AT 600/5. Given that the AD is a safety regulation of limited scope (affecting a small fleet of specialised agricultural aircraft), litigation would be rare. However, in the broader context of airworthiness directives, there have been cases where operators challenged CASA’s factual basis (e.g., whether the unsafe condition exists) or sought judicial review of a decision not to grant an alternative means of compliance. No such cases are reported concerning this specific AD.

If litigation has occurred, it would likely arise from a CASA enforcement action (e.g., grounding, fine, or suspension) and would be heard in the Federal Court or the Administrative Appeals Tribunal. Because the AD is based on a US FAA finding of an unsafe condition, and CASA has reciprocal obligations, a court would give significant deference to the technical determination. It would be difficult to argue that the AD is unreasonable unless the operator could show the AD’s requirement is manifestly unnecessary or imposes an impossible or arbitrary compliance burden.

The absence of litigation history may indicate that operators have generally complied, or that any disputes have been resolved through negotiation with CASA rather than litigation. For the purposes of this analysis, it is safe to state that there is no known litigation directly concerning AD/AT 600/5. Operators should be aware that non‑compliance could still lead to enforcement litigation in the future.

Several aspects of the AD can trap an unwary operator. First, the initial compliance date is ambiguous: for aircraft serials 602‑0337 through 602‑0584 “as manufactured,” the initial inspection must be done at 2,000 hours OR within 50 hours after 18 January 2007, “whichever occurs later.” So an aircraft with 1,500 hours on the effective date would not need to inspect until it reaches 2,000 hours, not within 50 hours. But an aircraft with 2,100 hours would need to inspect within 50 hours of 18 January 2007 (i.e., by early March 2007). That aircraft is already past the 2,000‑hour mark, so the “later” date becomes the 50‑hour window. An operator could misinterpret “whichever occurs later” to mean either the hour threshold or the date threshold, but the smaller interval (50 hours) applies if the aircraft has already exceeded the hour threshold.

Second, the AD distinguishes between serial numbers 602‑0337 through 602‑0584 and later serial numbers (602‑0585 and subsequent). For the later group, the applicability says “Model AT‑602 aircraft, serial numbers 602‑0337 and subsequent” - that includes them. But the specific compliance paths 2.a, 2.b, 3, and 4 all refer only to the 0337‑0584 range. For aircraft with serial numbers after 0584, the only explicit requirement is the initial access and inspection (Requirement 1) and the mandatory replacement at 6,500 hours (Requirement 5). There is no mention of a repetitive inspection interval for that later block. That could be an oversight or it may mean that the later production aircraft have a different (unknown) design assumed to be less crack‑prone. But an operator of a 602‑0585 aircraft should not assume no repetitive inspection is needed; the AD is silent, so the safer course is to operate under the 1,000‑hour repetitive interval unless CASA clarifies otherwise. This is a gotcha: the AD’s silence could be a trap.

Third, the cut‑in for replacement at 6,500 hours applies to “all Model AT‑602 aircraft” - not just the earlier serial numbers. That means an aircraft with serial 602‑0001 (which is not in the applicability) would not be covered, but any aircraft within 0337 and subsequent must do the replacement by 6,500 hours. However, the applicability line says “Model AT‑602 aircraft, serial numbers 602‑0337 and subsequent.” So the replacement requirement effectively covers the same fleet.

Fourth, the AD uses “time in service” without defining whether that means airframe hours or component hours. Since the wing spar lower caps are typically original to the aircraft, it means airframe hours. But if a wing or spar has been replaced earlier, the operator must use the hours on the replacement part. The AD does not say, so a conservative interpretation is to use the part hours.

Fifth, the AD requires using specific revision‑dated engineering documents. If Snow Engineering Co. has released a later revision of Drawing 20998, the operator cannot use that later revision unless CASA approves it as an alternative means of compliance. Using the wrong revision is non‑compliance even if the later revision is functionally equivalent.

Sixth, the cold‑working option (SL No. 244) does not terminate the inspection; it only extends the interval from 1,000 to 2,000 hours. An operator who spends money on cold working expecting to never inspect again will be wrong. Only the full terminating action (steel web plate + 8‑bolt splice blocks + cold working) ends inspections.

Seventh, if a crack is found and cannot be removed by the terminating action, the aircraft is grounded immediately. There is no provision for a repair scheme or a ferry permit in the AD. The operator must either source new lower spar caps or find another approved repair.

Eighth, the AD references several Snow Engineering Co. service letters and drawings that are proprietary. An operator may need to purchase these documents. The AD does not state that copies are freely available; the operator must obtain them from Snow Engineering Co. or an approved distributor. This could cause delays and additional costs.

Compliance can be broken into a sequence of steps, tailored to the aircraft’s serial number and prior modification status.

Step 1: Identify applicability. Verify that the aircraft is a Model AT‑602 with a serial number equal to or greater than 602‑0337. If the serial number is lower, the AD does not apply. Record the serial number and total airframe time in service.

Step 2: Obtain the required engineering data. Purchase or otherwise secure access to: Snow Engineering Co. Service Letter No. 204 Revised 26 March 2001; the drawing titled “602 Spar Inspection Holes and Vent Tube Mod.” dated 13 November 2003; Snow Engineering Co. Process Specification 197 Revised 4 June 2002; and, if applicable, Snow Engineering Co. Service Letter No. 244 dated 25 April 2005, Service Letter No. 240 dated 30 September 2004, Drawing 20998 Revision B dated 28 September 2004, Drawing 20776 Sheet 2 Revision A dated 30 August 2004, and any other referenced drawings.

Step 3: Check prior accomplishment. Review the aircraft logbooks or maintenance records to see if the initial access and inspection (Requirement 1) has already been done. Also check if the cold‑working procedure under SL No. 244 has been performed, and whether the terminating action (steel web plate and 8‑bolt splice blocks) has been done.

Step 4: Cut inspection holes and modify vent tube (if not already done). Perform the work per SL No. 204 Revised and the drawing. Install the cover plates.

Step 5: Determine inspection category and schedule. - For serial numbers 602‑0337 through 602‑0584 that are unmodified (no cold working): initial inspection is due at 2,000 hours or within 50 hours after 18 Jan 2007, whichever later. Thereafter at intervals not exceeding 1,000 hours. - For serial numbers 602‑0337 through 602‑0584 that have cold‑worked holes per SL No. 244: initial inspection (as part of the cold working) must be done if not already, and then repetitive inspections every 2,000 hours. The 2,000‑hour interval starts from the cold‑working date or the first inspection, whichever is later. - For serial numbers 602‑0585 and subsequent: the AD does not specify a repetitive interval. To be safe, adopt a 1,000‑hour repetitive schedule until CASA provides clarification, or contact CASA for an individual determination.

Step 6: Perform eddy current inspection. Engage a maintenance organisation with NDT capability qualified to Process Specification 197. Inspect the two outboard fastener holes in each lower spar cap (left and right wings). Record the results.

Step 7: If cracks are found: - Determine if the crack can be removed by the terminating action (i.e., by installing the steel web plate and 8‑bolt splice blocks and cold working the holes). If yes, proceed with the terminating action. - If the crack cannot be removed by the terminating action, obtain new lower spar caps (and associated parts) and replace them per Drawing 20776 before further flight. This is a no‑grace condition.

Step 8: Consider terminating action to eliminate repetitive inspections. Even if no cracks are found, the operator may choose to perform the terminating action early. This involves installing the steel web plate (part 20996‑2) and the 8‑bolt splice blocks (part 20985‑1/2) per Drawing 20998 Rev B, and cold working the two outboard holes per SL No. 240. Once accomplished, the repetitive inspection requirements of Requirement 2 cease. The operator records the termination in the logbook.

Step 9: Mandatory replacement at 6,500 hours. Regardless of prior inspections or modifications, when the wing spar lower caps reach 6,500 hours time in service, the operator must replace the lower spar caps, splice blocks and hardware, wing attach angles and hardware, and install the steel web plate (if not already installed) per Drawing 20776 Sheet 2 Rev A. This replacement terminates the inspection requirements. Note: if the terminating action was already done, the steel web plate may already be installed, but the replacement of the spar caps themselves is still required at 6,500 hours. The AD does not state that the terminating action waives the 6,500‑hour replacement; it only states that the terminating action terminates the inspection requirements of Requirement 2. The replacement at 6,500 hours is Requirement 5, which applies to all aircraft.

Step 10: Document all actions. Enter each compliance action (access, inspection, crack repair, modification, replacement) in the aircraft maintenance logbook, referencing the AD number, the specific requirement complied with, the hours at which the action was done, and the engineering data used. Retain copies of the service letters and drawings used, and the certification statement from the maintenance organisation.

Step 11: Monitor ongoing compliance. Maintain a tracking system for the next repetitive inspection (if terminating action not yet done) and for the 6,500‑hour replacement. For aircraft that have not yet reached 6,500 hours, schedule inspections before the hour limit. For aircraft that have already passed 6,500 hours, the replacement must be accomplished within 50 hours of 18 January 2007 (if not already done).

Alternative means of compliance. If the operator cannot obtain the specific Snow Engineering documents or parts, or wishes to use a different inspection method, they should apply to CASA for an alternative means of compliance under CASR Part 39, providing engineering justification and data. CASA may approve an equivalent procedure, but this process can take time.