AD/FU24/53 Amdt 2 - Flap Control Torque Tube - Inspection

The central concepts in this AD are recurring inspection, terminating action, fatigue cracking, flight cycle dependency, usage categorisation, and approved modification. A recurring inspection is a mandatory periodic check that must be performed at a specified interval (in this case, 100 or 220 flight hours) until a terminating action is accomplished. A terminating action is a modification or repair that eliminates the root cause of the defect, thereby ending the need for further inspections. Here the terminating action is installation of the strengthened torque tube lugs per SuperAir Engineering Order S46. Fatigue cracking refers to the progressive structural damage that occurs when a component is subjected to repeated loading below its ultimate tensile strength. The flap control torque tube is susceptible to fatigue cracking at the outer attachment lugs because agricultural operations involve very frequent flap deployments, imposing many load cycles. The AD distinguishes between three usage categories: agricultural only, non-agricultural only, and mixed. This distinction is based on evidence that the number of flight cycles correlates with the risk of fatigue failure. Flight cycles here essentially equal flap deployments, which occur on every take-off and approach. Agricultural aircraft often perform many short flights per hour, thus accumulating cycles far faster. The inspection itself is to be performed in accordance with NZ Aerospace Service Bulletin ASB/FU/033 (issued 30 May 1977). That bulletin sets out the inspection procedure, which is not reproduced in the AD text. The term “approved modification” refers to a design change that has been approved under CAR 35 (Civil Aviation Regulations 1988) or its successor. The AD explicitly states that only modifications that have demonstrated in-service reliability can be considered terminating actions, and the only one to do so is the SuperAir S46. This means that even if another modification has CAR 35 approval, it may not automatically satisfy the AD unless CASA agrees after reviewing in-service data. The concept of “sufficient in-service reliability” is not quantified; it is a discretionary call by CASA. The AD also refers to the New Zealand Airworthiness Directive DCA/FU24/164, indicating that this is an adopted or aligned directive (the aircraft is of New Zealand design). The AD is issued by a delegate of the Civil Aviation Safety Authority (CASA) under the Civil Aviation Act 1988. The background notes that the original issue of this AD became effective in 1977, meaning the flap torque tube issue has been a known risk for decades.

This AD directly affects every owner, operator, and maintenance organisation responsible for any FU24 series aeroplane. The FU24 is a single-engine agricultural aircraft originally designed by New Zealand Aerospace (later Pac Aerospace) and operated primarily for crop dusting, topdressing, and similar agricultural tasks, but also used for other utility operations like parachute dropping or passenger transport in some configurations. The AD applies to “all models” of the FU24 series, so regardless of variant or modification state, every aircraft on the Australian register (or being operated by an Australian operator, even if registered elsewhere) is captured. The primary burden falls on owners and operators, who must ensure compliance with the inspection intervals based on their aircraft’s usage. If the aircraft is used solely for agricultural operations, the owner must schedule an inspection every 100 flight hours. If used solely for non-agricultural operations, the interval extends to 220 hours. However, if the aircraft is used in any mixed capacity (including even occasional agricultural work), the inspection interval defaults back to 100 hours. This imposes a record-keeping burden: the operator must be able to demonstrate the usage profile to CASA or an authorised person. Maintenance organisations (Licensed Aircraft Maintenance Engineers, approved maintenance organisations) must be familiar with the NZ Aerospace SB and the inspection method. They also need to be able to install the SuperAir S46 modification if the owner chooses that path. The modification involves welding reinforcement strips to the existing lugs, which requires a welding approval and possibly a design approval from CASA (though the modification itself is already approved under CAR 35). CASA delegates (like the signatory David Villiers) are responsible for issuing such ADs and for considering requests to accept other modifications as terminating actions. The source notes that other CAR 35 approved modifications may be considered after consultation with CASA. This gives CASA discretionary power to determine that another modification has sufficient in-service reliability. That affects third-party modifier businesses: any company that has developed an alternative flap torque tube reinforcement must go through a consultation process with CASA and likely must provide data demonstrating reliability. SuperAir (the manufacturer of the S46 modification) benefits from a de facto monopoly as the only modification with proven reliability; competitors face a higher compliance burden to achieve the same regulatory recognition. Finally, the New Zealand Civil Aviation Authority is indirectly affected through the reference to NZ AD DCA/FU24/164; cross-jurisdictional recognition may apply.

The key duty is to inspect the flap control torque tube in accordance with NZ Aerospace Service Bulletin ASB/FU/033 at the specified interval determined by the aircraft’s usage category. The duty is periodic and ongoing unless the terminating modification is installed. The compliance section of the AD sets out the three cases. For aircraft engaged only in agricultural operations: inspect every 100 flight hours. For aircraft engaged only in non-agricultural operations: inspect every 220 flight hours. For aircraft engaged in mixed usage (any agricultural and any non-agricultural operations): inspect every 100 flight hours. The duty is to determine the correct category and maintain records of usage to justify the interval. The duty to inspect is strict: the AD does not provide a grace period if the interval is inadvertently exceeded, nor does it allow the owner to adopt a longer interval based on the owner’s own risk assessment. The only way to end the inspection duty is to accomplish the terminating action: installation of SuperAir Engineering Order S46 (or another modification if CASA agrees). The duty to perform the terminating action is not mandatory; it is a right (an option) to terminate inspections. If the owner chooses to modify, the AD requires that the modification be done in accordance with the approved SuperAir order, which was approved on 17 April 1980 by the Department of Aviation (now CASA). The note states that “other CAR 35 approved modifications may be considered acceptable for the purposes of terminating the inspection requirements of this AD, however, the only modification that has demonstrated in service reliability is the approved SuperAir Engineering Order S46.” This creates a right to propose an alternative modification for acceptance, but the decision is at CASA’s discretion, and the burden of proof (showing sufficient in-service reliability) falls on the applicant. There is no right to a hearing or automatic acceptance. The operator also has the right to challenge the AD itself through administrative review (though that is a general right under the Civil Aviation Act, not specified in the AD). The AD does not explicitly state a duty to report findings from the inspection, but generally under Part 4 of the Civil Aviation Safety Regulations, defects must be reported to CASA if they affect airworthiness. However, that is outside this AD. The AD imposes no duty on the manufacturer (NZ Aerospace or SuperAir) beyond the original issuance of the service bulletin and the modification approval.

The AD itself does not prescribe specific penalties, but it is issued under the Civil Aviation Act 1988 and Civil Aviation Safety Regulations 1998 (CASR). Non-compliance with an airworthiness directive is a serious breach of the continuing airworthiness obligations. Under regulation 42.100 of CASR, the holder of an airworthiness certificate (or the operator) must ensure that the aircraft meets the applicable airworthiness requirements, which include compliance with ADs. Failure to comply can result in enforcement action by CASA, including issuance of a direction to ground the aircraft, suspension or cancellation of the airworthiness certificate, or a civil penalty (infringement notice). The maximum penalty for an individual who operates an aircraft not in compliance with an AD can be up to 50 penalty units (currently about $11,000) per offence, and for a body corporate up to 250 penalty units (around $55,000). Additionally, if non-compliance leads to an accident or incident, criminal liability could arise under the Civil Aviation Act section 20A for reckless or negligent operation endangering life. There is also the risk of liability in tort if an unmodified flap torque tube fails and causes damage. The AD does not create any new enforcement mechanism; it relies on the existing regulatory framework. The CASA delegate signed the amendment on 29 March 2006, giving it legal effect from 11 May 2006. Any aircraft that had its torque tube inspection overdue on that date would be immediately non-compliant. There is no transition period for the new intervals; the AD requires that the inspection be done each 100 or 220 hours “from the effective date” based on usage. For an aircraft that was previously on a 100-hour interval and now qualifies for 220-hour interval under Amdt 1, the compliance time is the applicable interval. Record-keeping is essential because if an inspector or CASA investigates, the operator must prove the aircraft’s usage history to justify the longer interval. If the operator claims 220-hour interval but later discovers some agricultural flights occurred, the operator may be liable for past non-compliance. The AD’s note that mixed usage defaults to 100 hours is a trap: even one agricultural flight in an otherwise non-agricultural history could trigger the stricter interval. Enforcement discretion could include verbal warnings, but the AD is mandatory and strict liability offences apply.

This AD interacts with several layers of Australian aviation legislation and referenced standards. The primary authorising legislation is the Civil Aviation Act 1988, under which CASA issues airworthiness directives. The AD is a legislative instrument (a disallowable instrument) but is often treated as a binding direction. It operates in conjunction with the Civil Aviation Safety Regulations 1998 (CASR), particularly Part 42 (Continuing Airworthiness) and Part 145 (Approved Maintenance Organisations). Under CASR, compliance with ADs is a condition of maintaining an aircraft’s airworthiness certificate. The AD also references CAR 35, which is the old Civil Aviation Regulations (1988 version) governing the approval of design changes and modifications. CAR 35 has largely been superseded by Part 21 of CASR, but the reference survives because the SuperAir modification was approved under CAR 35. The AD implicitly recognises that other modifications approved under CAR 35 (or their CASR Part 21 equivalent) could be acceptable, but only after consultation with CASA. This creates a gatekeeper role for CASA. The AD also references New Zealand AD DCA/FU24/164, which is the original New Zealand directive that this Australian AD likely mirrors. This cross-reference matters because the FU24 is a New Zealand design, and many spare parts and modifications originate from NZ. An Australian operator performing the modification in New Zealand would need to ensure it meets the Australian AD requirements, not just the NZ ones. The AD does not explicitly mention the Civil Aviation (Standards) Regulations or the Airworthiness Advisory Circulars, but those may contain guidance on interpreting “sufficient in-service reliability.” The AD also interacts with the bilateral agreements between Australia and New Zealand on airworthiness (the Mutual Recognition Agreement for aeronautical products). That agreement may affect the automatic acceptance of NZ-approved modifications, but the AD specifically requires consultation with CASA for any modification other than S46, suggesting that mere NZ approval is not enough. Further, the AD affects the contractual relationships between owners and maintenance providers: a maintenance contract must reference compliance with this AD. Insurance policies may also require compliance; non-compliance could void coverage. Finally, the AD interacts with state and territory occupational health and safety laws in the context of welding and modifications, but those are not aviation-specific.

The original AD/FU24/53 was issued with an effective date of 1 August 1977. It required inspection of the flap control torque tube at 100-hour intervals, with no distinction based on usage. That original AD was based on the then-current knowledge of fatigue cracking in the torque tube. Amendment 1 became effective on 20 March 2003. Its purpose was to distinguish between aircraft engaged in agricultural operations and those engaged in non-agricultural operations, based on evidence that fatigue is a function of the number of flight cycles (flap deployments) and that agricultural aircraft accumulate cycles at an order of magnitude higher rate. Under Amendment 1, aircraft engaged only in agricultural operations remained at the 100-hour interval. Aircraft engaged only in non-agricultural operations were allowed to inspect at 220-hour intervals. The inspection procedure itself did not change. Mixed usage aircraft were not specifically addressed in the source for Amendment 1, but the current amendment (Amdt 2) implies that mixed usage defaults to 100 hours. Amendment 2, the subject of this analysis, became effective on 11 May 2006. It introduces a terminating action: the SuperAir Engineering Order S46 modification. The modification had already been approved on 17 April 1980 by the Department of Aviation (CASA’s predecessor) but was not previously listed as a terminating action in the AD. Over 25 years of service experience (from 1980 to 2006) demonstrated that no cracking occurred in modified aircraft. The amendment thus gives owners a clear path to end the recurring inspections. The amendment also clarifies the mixed usage case. There is no indication in the source of any subsequent amendments. The AD number remains “AD/FU24/53 Amdt 2”, meaning it is the second amendment. The background section of the amendment explains the rationale: the modification eliminates susceptibility to fatigue cracking by reinforcing the lugs. The amendment was issued by David Villiers, a delegate of CASA, on 29 March 2006, with a 43-day lead time before effectiveness. This multi-decade history illustrates a classic pattern in ADs: initial mandatory inspections, followed by refinement of intervals based on operational data, and finally introduction of a terminating modification once long-term reliability data is available. The 23-year gap between the modification’s approval and its inclusion as a terminating action suggests that the AD process can be slow, and that operators who installed the modification early may have continued inspections unnecessarily until 2006.

The source text does not mention any litigation concerning this specific AD. There are no recorded challenges to AD/FU24/53 in the courts or administrative tribunals mentioned in the document. However, airworthiness directives in Australia have been the subject of judicial review under the Administrative Decisions (Judicial Review) Act 1977 (ADJR Act) and merits review in the Administrative Appeals Tribunal (AAT) under the Civil Aviation Act. To date, there are no publicly reported decisions that deal with this particular AD. The lack of litigation may reflect the narrow technical scope of the AD; it is prescriptive and grounded in empirical evidence of fatigue cracking, making it difficult to argue the directive is unreasonable or disproportionate. Additionally, the amendment introduced a terminating action that owners would welcome, reducing the incentive to challenge. If litigation were to arise, it would likely involve a dispute over whether a particular approved modification qualifies as “sufficient in-service reliability” to end the inspection requirement. The AD gives CASA discretion to accept other modifications, but that discretion could be challenged on grounds of unreasonableness if CASA refuses without adequate basis. Alternatively, an owner who mistakenly used the 220-hour interval when they had mixed usage might dispute enforcement action, arguing that de minimis agricultural flights should not trigger the 100-hour interval. The AD’s wording “engaged only in operations other than agricultural” and “engaged only in agricultural operations” versus “mixed usage operations (i.e., a combination of agricultural and non-agricultural operations)” suggests a strict reading. A court would likely uphold the strict intervals. The source’s Note 2 stating that “other approved modifications may be considered as terminating modifications after consultation with CASA and only after demonstrating sufficient in service reliability” leaves room for administrative challenge if CASA acts arbitrarily. The alternative would be an appeal to the AAT for merits review, where the owner could present evidence that their chosen modification is as reliable as SuperAir’s. No such case is recorded. In general, ADs have been challenged rarely in Australia, likely due to the cost and the fact that most are based on mandatory reporting of defects. The lack of litigation history does not mean the AD is immune from future challenge, but it provides stability for those relying on the current requirements.

Several less obvious aspects of this AD can trap the unwary operator, maintainer or owner. First, the inspection interval for mixed usage defaults to 100 flight hours even if the aircraft flies 99 non-agricultural hours and one agricultural hour. The AD defines mixed usage as “a combination of agricultural and non-agricultural operations” with no de minimis exception. An operator who occasionally tows a banner or drops a parachutist from an otherwise dedicated agricultural aircraft must still comply at 100 hours. Conversely, an operator who occasionally fertilises a field from a passenger-hauling aircraft triggers the 100-hour limit. This strict reading can impose disproportionate cost on low-frequency mixed use. Second, the terminating modification (SuperAir S46) is not mandatory. An owner who never installs it must inspect forever at the applicable interval. The AD does not force the modification, but the recurring inspection cost and down-time eventually make modification economically sensible. Third, although the AD says other CAR 35 approved modifications may be accepted as terminating, the only one with proven reliability is S46. An owner who uses a different modification (even one approved under CAR 35 or Part 21) cannot stop inspections without first consulting CASA and proving in-service reliability. This creates uncertainty and cost for alternative designs. The owner could spend money on a modification that CASA later refuses to accept. Fourth, the inspection procedure is contained in New Zealand Aerospace Service Bulletin ASB/FU/033 issued 30 May 1977. That bulletin may be difficult to obtain; it is not reproduced in the AD. An owner must locate the original document or rely on an approved maintenance organisation that has it. Failure to obtain the correct procedure (e.g., using a later revision that CASA has not approved) could mean non-compliance even if the inspection was performed. Fifth, the compliance time is flight hours, not calendar time. An aircraft that sits idle for years could be compliant even if the last inspection was decades ago, as long as the hours have not been exceeded. But once it flies again, the hours count. This can be tricky for owners who park and then reactivate aircraft. Sixth, the AD applies to “all models” - that includes NTCA (experimental) or limited category aircraft? The AD does not exempt non-type certificated aircraft. If an FU24 is operated under an experimental certificate, the AD may still apply because it is a condition of the experimental certificate that the aircraft complies with ADs applicable to the type design. Operators of modified or amateur-built variants should verify. Seventh, the AD’s original effective date was 1977; many aircraft may have had the modification installed long ago. But if records are lost, proving the terminating action is complete becomes difficult. CASA may require re-inspection unless the modification can be confirmed. Finally, the note about “other CAR 35 approved modifications” implies that if the modification is not CAR 35 approved (e.g., approved under a Supplemental Type Certificate issued by CASA under Part 21), it may not even be eligible for consultation. The wording is narrow.

Compliance requires a systematic approach covering identification, record-keeping, inspection, and optional modification. Step one: determine the exact model and serial number of the FU24 aeroplane to confirm it falls within “all models” under the AD. Step two: obtain a copy of NZ Aerospace Service Bulletin ASB/FU/033 (30 May 1977). This document contains the inspection procedure; it is not reproduced in the AD. It may be purchased from the original manufacturer or from CASA’s library. Step three: classify the aircraft’s usage. Review the aircraft’s flight logs, pilot reports, and operational records to determine whether every flight over the past period (e.g., the preceding 100 or 220 hours) has been entirely agricultural, entirely non-agricultural, or mixed. Mixed usage includes any single flight that involved agricultural operations (e.g., spraying, spreading) plus any non-agricultural flight. There is no threshold; one agricultural flight in an otherwise non-agricultural period qualifies as mixed usage, requiring the 100-hour interval. If uncertain, default to the stricter interval. Step four: schedule the inspection at the appropriate interval based on the determined usage. For agricultural-only: every 100 flight hours from the last inspection (or from the AD effective date if no inspection yet). For non-agricultural-only: every 220 flight hours. For mixed: every 100 flight hours. Keep a log of the hours at which each inspection was carried out. Step five: perform the inspection per the service bulletin. This will involve removing access panels, inspecting the flap control torque tube (likely visually and possibly with dye penetrant or eddy current) at the outer attachment lugs. The specific method should be followed exactly; any deviation could render the inspection non-compliant. Step six: if the inspection reveals cracking, the aircraft must be grounded until repaired. Repair options include replacement with a new part or installation of the strengthening modification. Step seven: consider whether to install the terminating modification (SuperAir Engineering Order S46 Issue 1). This modification, approved 17 April 1980, involves welding reinforcement strips to the outer attachment lugs. The modification is a one-time cost but ends all future inspection requirements under this AD. The AD notes that the only modification with proven reliability is S46; however, if the operator has already installed a different CAR 35 approved modification, they must consult CASA and provide evidence of in-service reliability to have that modification accepted as terminating. Without CASA’s written acceptance, the inspection requirements continue. Step eight: after installation of an accepted terminating modification, update the aircraft’s maintenance records to show that the AD compliance is “terminated” against requirement items 1, 2, or 3 as applicable. Keep a copy of the SuperAir order and the maintenance release. Step nine: for aircraft that continue with recurring inspections, keep a running log of flight hours and usage type. Any change in usage (e.g., from agricultural-only to mixed) triggers an immediate reset of the inspection interval to the stricter one. Document the date of change and the hours flown at that point. Step ten: ensure that the maintenance organisation performing the inspection or modification is appropriately authorised (e.g., CAR 30 or CASR Part 145 maintenance organisation, or a Licensed Aircraft Maintenance Engineer with appropriate ratings). Record keeping must comply with CASR Part 42 regulations regarding continuing airworthiness records. Failure to maintain proper records could result in CASA deeming the AD non-compliant.