AD/A320/174 Amdt 2 - Air Data/Inertial Reference Unit

The central legal concept is an Airworthiness Directive, a mandatory regulatory instrument issued under the Civil Aviation Act 1988 and associated regulations to correct an unsafe condition. The directive binds all operators and owners of the affected aircraft types. The product in question is the Air Data/Inertial Reference Unit, a component that supplies critical flight parameters such as attitude, heading, airspeed and altitude to the flight instruments and autopilot systems. The ADIRU is installed in an avionics shelf, and the physical contact between the unit and the shelf structure caused intermittent failures reported in service. Another key concept is the Master Minimum Equipment List, a regulatory document that permits dispatch of an aircraft with certain inoperative equipment under defined conditions. The MMEL Temporary Revisions referenced in the directive provide an acceptable means of compliance for the dispatch restrictions. For aircraft with conventional standard instruments, compliance with MMEL Temporary Revision No 01-34/02Z issue 04 is acceptable. For aircraft fitted with an Integrated Standby Instruments System (Airbus modification 27620 or SB A320-34-1261), compliance with MMEL TR No 01-34/07Z issue 03 is acceptable. The directive also introduces the concept of terminating action, a modification or replacement that eliminates the need for continued operational restrictions. Once the shelf supports are modified and the three specified ADIRUs are installed, the dispatch limitations cease to apply permanently. The background section explains the evolution of the problem: initial reports of parameter loss, the earlier AD/A320/138 mandating shelf support modification, the insufficiency of that measure, and the decision to require both shelf modification and ADIRU replacement as a package. The directive incorporates by reference two Airbus service bulletins and their later EASA-approved revisions, meaning the technical content of those documents becomes legally binding. Operators must use the approved revision in effect at the time of compliance. The amendment history shows that AD/A320/138 has been cancelled, so operators are no longer required to track that separate directive. Finally, the directive distinguishes between aircraft that are subject to all three requirements and those that are excluded because they already meet the terminating condition, that is, aircraft equipped with three ADIRUs part number 465020-0303-0316 and with the shelf modified per Airbus modification 30650 or 30872 or SB A320-25-1248 dated 16 February 2001.

The directive applies to operators, owners and maintenance organisations responsible for any Airbus A318, A319, A320 or A321 series aeroplane, all serial numbers, that is equipped with at least one Northrop-Grumman ADIRU bearing one of the part numbers listed in Table 1. The affected part numbers are: 465020-0303-0307, 0308, 0309, 0312, 0314, 0315 and 0316. Importantly, the directive explicitly excludes aircraft that already have three ADIRUs part number 465020-0303-0316 installed (the same part number required by Requirement 3) and that have also modified the shelf in accordance with Airbus modification 30650 or 30872 or SB A320-25-1248 dated 16 February 2001. Those aircraft are considered already compliant and are not subject to any of the requirements. The exclusion is narrow: the aircraft must meet both conditions. Operators of newer production aircraft or those that have already incorporated later service bulletins may need to check whether the shelf modification they have performed matches the specified modification/SB date. The directive also affects organisations that maintain or modify these aircraft types, as they must have the capability to perform the shelf support modification per SB A320-25-1248 Revision 1 and the ADIRU replacement per SB A320-34-1350. The AD does not specify which party bears the cost, but typically the operator must fund the modification and replacement out of operational budgets or insurance claims. For lessors and financiers, the directive creates a compliance obligation that may affect the value or lease terms of affected aircraft if not satisfied. Aviation regulators in other jurisdictions may accept the Australian directive or may have issued their own equivalent, meaning operators with international fleets may need to comply with multiple similar requirements. The directive also affects delegates of CASA, such as the person who signed the instrument, as well as CASA enforcement staff who will monitor compliance. Finally, the directive has an indirect effect on passengers and cargo shippers to the extent that non-complying aircraft may be grounded or subject to dispatch restrictions, leading to delays or cancellations. The economic burden falls on operators who must purchase the new ADIRUs, perform the shelf modification and schedule the associated downtime. For operators that have already complied with AD/A320/138, only Requirement 1 and Requirement 3 may be new obligations; however, the shelf modification required by AD/A320/138 may not have been exactly the same as that required here, so careful verification is necessary.

The primary duty is to achieve compliance with the three requirements by the specified deadlines. For Requirement 1, the duty is ongoing from the effective date: whenever an aircraft is dispatched with one ADIRU inoperative, the standby compass and standby attitude indicator must be operative. The operator may use the stated MMEL Temporary Revisions as a means to document that compliance. For Requirement 2, the duty is to modify the ADIRU shelf supports within six months of 16 March 2012, unless the aircraft already meets the terminating condition. For Requirement 3, the duty is to have installed three ADIRUs part number 465020-0303-0316 by 31 May 2010. Because that date has passed, any aircraft that has not yet done so is already in breach, and the operator must immediately arrange for the replacement to remedy the non-compliance. A key right accrues from full compliance with both Requirements 2 and 3: the dispatch restrictions in Requirement 1 are terminated. This means that once the shelf modification and ADIRU replacement are completed, the operator may dispatch the aircraft with one ADIRU inoperative without the standby compass and standby attitude indicator conditions, subject to normal MMEL provisions. The operator also has the right to use an alternative means of compliance if approved by EASA, as the directive references “later EASA approved revision” for both service bulletins. The directive does not expressly authorise alternative means of compliance not referenced in the text, but under general airworthiness regulation, operators may apply for an alternative method of compliance through CASA. The operator’s right to fly the aircraft is constrained by the directive; failure to comply may result in a grounding order. The directive also confers a right to rely on the work performed under the cancelled AD/A320/138, provided that the shelf modification performed under that AD matches the requirements of Requirement 2. The background notes that AD/A320/138 mandated modification of ADIRU shelf supports, and this amendment reiterates that requirement. Operators who complied with AD/A320/138 should verify whether the modification they performed meets the SB A320-25-1248 Revision 1 standard (or later approved revision). There is also an implicit duty to maintain accurate records of compliance, as the directive is likely to be audited by CASA or by foreign regulators. Operators who lease or sublease aircraft must ensure that the lessee is aware of the compliance status, as the duty falls on the operator at the time of flight.

The Airworthiness Directive itself does not specify penalties, but it is issued under the regulatory framework of the Civil Aviation Act 1988 and the Civil Aviation Safety Regulations 1998. Non-compliance with a mandatory Airworthiness Directive is a serious breach that can attract civil penalties, including fines of up to several hundred thousand dollars for operators and potentially imprisonment for individuals in cases of reckless endangerment. CASA may also take administrative enforcement action, such as suspending or cancelling an Air Operator’s Certificate, grounding aircraft, or imposing conditions on operations. The directive becomes effective on 16 March 2012, meaning that any dispatch with one ADIRU inoperative from that date without the standby instruments operative would be an immediate violation. For Requirement 2, the six-month compliance period ends on 16 September 2012; after that date, any aircraft that has not completed the shelf modification is non-compliant. Requirement 3 had a compliance deadline of 31 May 2010, which is before the effective date of this amendment. Therefore, any aircraft that was subject to the original AD/A320/174 or its first amendment and did not meet the 31 May 2010 deadline has been in continuous breach since that date. The amendment does not grant an extension; it merely restates the requirement. Operators who are only now discovering the AD through this amendment are effectively already non-compliant for Requirement 3 and should take immediate corrective action. CASA has the power to issue a direction to ground the aircraft, and the operator may be liable for any costs arising from a grounding or from an accident caused by the non-compliance. Enforcement action may also be taken against maintenance organisations that certified work that did not meet the requirements. The directive does not provide for any grace period after the effective date for Requirement 1; it applies immediately. The background indicates that the European Aviation Safety Agency advised that the ADIRU replacement together with shelf modification provides terminating action. An operator that does not comply faces not only domestic enforcement but also potential refusal of entry or operation rights in other countries that recognise Australian ADs. The enforcement regime is therefore substantial, and the practical cost of non-compliance includes aircraft downtime, legal fees, regulatory scrutiny and reputational damage.

This Airworthiness Directive forms part of a hierarchy of safety regulation under the Civil Aviation Act 1988. It is a specific instrument that takes precedence over more general operational rules where there is a conflict. The directive explicitly cancels AD/A320/138 and merges its requirements into this instrument, thereby eliminating any duplication or confusion between the two. It also references two Airbus service bulletins (SB A320-25-1248 and SB A320-34-1350) and allows for later EASA-approved revisions. Those service bulletins are not themselves law but become legally binding when incorporated by reference; the exact version approved by EASA at the time of compliance governs. The directive interacts with the Master Minimum Equipment List. The MMEL Temporary Revisions cited (01-34/02Z issue 04 and 01-34/07Z issue 03) provide an acceptable means of compliance for Requirement 1. Operators who follow those TRs will be deemed compliant with the dispatch restrictions. If an operator does not use those TRs, it must still satisfy the core requirement that the standby compass and standby attitude indicator are operative when dispatching with one ADIRU inoperative. The directive also has implications for manufacturers and design approval holders. Airbus must have approved the service bulletins and modifications under its own design organisation approval. The directive likely refers to the type certificate data sheet and the approved maintenance instructions. Because the AD is issued by CASA, it may be recognised under bilateral safety agreements with other states, such as the United States FAA or EASA, meaning operators flying into those jurisdictions must comply with the Australian AD or have an equivalent foreign AD. The directive does not directly interact with contract law, but a lease or purchase agreement may require the lessor or seller to deliver the aircraft in compliance with all applicable ADs. An operator that fails to comply may be in breach of contract. The directive also interacts with insurance: an insurer may deny coverage for an accident caused by non-compliance with a mandatory AD. The cancellation of AD/A320/138 means that operators need no longer track that separate instrument, but the physical work done under that AD remains relevant for Requirement 2. There is also an interaction with the time limits for corrective action: the directive allows six months for the shelf modification but prescribes a past date for the ADIRU replacement. This creates potential conflict with other regulatory requirements that require ongoing airworthiness. The note that AD/A320/138 has been cancelled is significant: any operator that had a compliance schedule under that AD can now consolidate all work under this AD. Finally, the directive cross-references EASA 2007-0217, which is a similar EASA directive; operators with aircraft registered in both jurisdictions may need to ensure they meet both requirements, though they are likely identical in substance.

The instrument is titled “AD/A320/174 Amdt 2”, indicating it is the second amendment to a directive originally issued as AD/A320/174. The source provides the full text of Amendment 2 only and does not reproduce the original directive or Amendment 1. The effective date of Amendment 2 is 16 March 2012. The background reveals that the original AD/A320/174 introduced revised dispatch limitations for the MMEL after an earlier measure, AD/A320/138, proved insufficient. The original AD was then itself found to be insufficient, so Amendment 2 was issued to require replacement of the three ADIRUs in addition to the shelf modification, providing terminating action for the dispatch limitations. Additionally, Amendment 2 cancels AD/A320/138 and incorporates its requirements directly into AD/A320/174. This consolidation means that operators who had only complied with AD/A320/138 (shelf modification) are now required to also comply with the ADIRU replacement and the dispatch restrictions unless they meet the terminating condition. The amendment history is therefore important for understanding what obligations have changed. Before Amendment 2, an operator subject to AD/A320/174 Amdt 1 would have been required to adhere to dispatch restrictions and complete a shelf modification, but not necessarily to replace the ADIRUs. Now, the ADIRU replacement is mandatory and has a past deadline. The amendment also updates the service bulletin references: Requirement 2 refers to SB A320-25-1248 Revision 1 dated 16 April 2003 or later EASA approved revision, and Requirement 3 refers to SB A320-34-1350 or later EASA approved revision. The note that AD/A320/138 has been cancelled is a key amendment; that directive is no longer in force, and any actions taken under it are still valid for establishing the basis of shelf modification compliance. The amendment was signed by David Villiers as Delegate of CASA on 14 March 2012. For legal research, one would need to trace whether there were any prior amendments (Amdt 1) and whether there have been any subsequent amendments (Amdt 3 etc.) not shown here. The directive as presented is self-contained, but an operator must be aware of any later EASA-approved revisions to the service bulletins. The background indicates a progression of regulatory response: initial shelf modification under AD/A320/138, then dispatch restrictions under original AD/A320/174, and finally the full terminating action under Amdt 2. This history is useful for understanding the regulatory philosophy and the iterative nature of safety fixes.

The source text contains no reference to any litigation history concerning AD/A320/174 Amdt 2, AD/A320/174 generally, or the related AD/A320/138. Airworthiness directives are typically not litigated heavily because they are grounded in immediate safety findings and the regulator has broad powers. Operators generally comply rather than challenge, given the severe consequences of operating an unsafe aircraft and the limited grounds for judicial review. There may have been disputes over the scope of the directive or the reasonableness of compliance deadlines, but no such case is mentioned. In Australian law, an aggrieved person could seek judicial review of a CASA decision under the Administrative Decisions (Judicial Review) Act 1977, but the grounds would be limited to procedural error, unreasonableness or jurisdictional error. Given that the directive is based on reports of parameter loss and engineering investigation, a court would likely defer to CASA's safety determination. The fact that the directive has been through two amendments suggests that CASA has refined its approach in response to technical findings, which may reduce the likelihood of a successful challenge. The absence of cited litigation does not prove no litigation exists, but a thorough source-grounded analysis must note that the directive itself does not mention any court cases. A researcher could check Westlaw or other databases for decisions citing AD/A320/174, but that is beyond the source. Practitioners should be aware that the compliance deadlines are mandatory and that the regulator’s enforcement discretion is broad. The directive’s language is prescriptive and leaves little room for interpretation, further limiting the scope for litigation. The only potential legal friction might arise from the retrospective nature of Requirement 3, which required compliance before the effective date of this amendment. An operator that acquired an aircraft after 31 May 2010 and before 16 March 2012 might argue that it could not have complied before owning the aircraft. However, an airworthiness directive continues to bind successive owners and operators, and the duty is to bring the aircraft into compliance as soon as possible, which would mean immediate action upon becoming aware of the directive. The regulator may exercise discretion but is not bound.

Several subtle aspects of this directive can trap the unwary operator or compliance manager. The most significant trap is the compliance date for Requirement 3: “Before 31 May 2010.” This date is nearly two years before the effective date of Amendment 2 (16 March 2012). Any operator who receives this directive for the first time on or after 16 March 2012 and who has not already replaced the ADIRUs is already in breach. The directive does not provide a grace period for Requirement 3; it merely states that the requirement exists and must have been met by that past date. This means that unless the aircraft was already compliant, the operator is immediately subject to enforcement. A related trap is that the exclusion for aircraft that already have three ADIRUs P/N 465020-0303-0316 and the modified shelf does not extend to aircraft that have only one of those conditions. The exclusion requires both the correct ADIRU part numbers and the specific shelf modification based on modification 30650, 30872 or SB A320-25-1248 dated 16 February 2001. An aircraft that has the correct ADIRUs but a different shelf modification, or the correct shelf modification but older ADIRUs, is fully subject to the directive. Another trap lies in the MMEL compliance for Requirement 1. The acceptable means of compliance differ depending on whether the aircraft has conventional standard instruments or an ISIS. Using the wrong MMEL Temporary Revision may result in non-compliance. Additionally, those MMEL TRs are dated; the directive does not state what happens if a later revision is issued. The safe approach is to use the exact revision referenced or an EASA-approved later revision. The shelf modification required by Requirement 2 must be performed in accordance with SB A320-25-1248 Revision 1 dated 16 April 2003 or a later EASA-approved revision. An operator that performed the modification under an earlier revision (e.g., the original issue dated 16 February 2001) may not be compliant unless that earlier revision is specifically covered by the “later EASA approved revision” language. The directive says “later EASA approved revision”, which implies that only revisions issued after the original that are approved by EASA count. The background note that AD/A320/138 mandated the modification of shelf supports; if that AD required an earlier version of the SB, compliance with that AD may not satisfy the requirement of this amendment unless the work was done to Revision 1 level. The operator must verify the exact revision used. Another trap is the phrase “no further action is required by this Amendment” once Requirements 2 and 3 are completed. This does not mean that other ongoing ADs are satisfied; it only means no further action under this specific AD. Operators must continue to comply with all other applicable directives. Finally, the effective date of the amendment is 16 March 2012, but the dispatch restrictions in Requirement 1 apply from that date. An operator that dispatches an aircraft with one ADIRU inoperative on 16 March 2012 without the standby instruments operative will be in breach immediately, even if the aircraft will undergo the shelf modification two months later.

Compliance requires a systematic approach covering each requirement and the record keeping. For Requirement 1 (dispatch restrictions), the operator should ensure that all flight crew and dispatch staff are aware that from 16 March 2012, any dispatch with one ADIRU inoperative is permitted only if the standby compass and standby attitude indicator are operative. The operator may adopt the relevant MMEL Temporary Revision as an acceptable means of compliance. For aircraft with conventional instruments, use MMEL TR 01-34/02Z issue 04; for aircraft with ISIS, use MMEL TR 01-34/07Z issue 03. The operator should have a copy of the applicable TR in the flight operations manual or MMEL. The dispatch restriction remains in place until the terminating action is completed. For Requirement 2 (shelf modification), the operator must schedule the work within six months of 16 March 2012, i.e., by 16 September 2012. The modification must comply with SB A320-25-1248 Revision 1 dated 16 April 2003 or any later EASA-approved revision. The operator should contact Airbus or an approved maintenance organisation to determine the current EASA-approved revision. The work includes modifying the ADIRU shelf supports to prevent contact with the ADIRU. The operator should ensure that the modification is certified by a licensed aircraft maintenance engineer and entered into the aircraft logbook. If the aircraft already had the shelf modification performed under AD/A320/138, the operator must verify whether that modification meets the SB revision required; if not, the modification must be re-done to the correct standard. For Requirement 3 (ADIRU replacement), the operator must install three Northrop-Grumman ADIRUs part number 465020-0303-0316 in accordance with SB A320-34-1350 or any later EASA-approved revision. The compliance deadline was 31 May 2010, so the operator must check the aircraft records to confirm whether this replacement was already performed. If not, the operator must arrange the replacement immediately, as the aircraft is currently non-compliant. The operator should use the latest EASA-approved revision of the SB to ensure the installation is acceptable. The ADIRU replacement and the shelf modification may be performed concurrently to minimise downtime. Once both Requirements 2 and 3 are completed, the dispatch restrictions in Requirement 1 cease; the operator should update the MMEL accordingly and remove any temporary limitations. The operator must retain evidence of compliance, including copies of the service bulletins used, work orders, logbook entries and certificates of release to service. For leased aircraft, the operator should coordinate with the lessor to ensure that the modifications are approved and that costs are allocated per the lease agreement. Finally, the operator should review the applicability again after compliance to confirm the aircraft now falls within the exclusion clause and is not subject to any further action under this AD.