AD/ATR 42/27 - Multi Purpose Computer with Aircraft Performance Monitoring Function

The directive centres on the Multi Purpose Computer with Aircraft Performance Monitoring function. The MPC is a computer that processes a collection of parameters, including the aeroplane take-off weight as selected by the crew on a rotary selector. Its core function is to compute and compare the actual drag on the current flight path with the theoretical or expected value for that flight condition. From that comparison, the APM calculates a measurable and objective determination of performance degradation that may be caused by abnormal ice accretion. When the performance degradation passes given thresholds, the APM annunciates warning signals by triggering up to two different levels of alerts during climb and descent, and three levels of alerts during cruise. The concept of “severe icing conditions” is central but the directive does not define it precisely in terms of meteorological parameters. Instead, it describes severe icing as conditions beyond the current certification envelope for Part 25 aeroplanes, which the ice protection system is not designed to handle for prolonged periods. The directive contrasts the APM’s objective detection with the previous reliance on subjective crew interpretation. The background lists the then-existing detection methods: an unexpected decrease in airspeed or rate of climb; visual cues such as ice covering unheated portions of forward side windows, possibly with water splashing or streaming on the windshield; and secondary indications from visual observation of ice accretion on the airframe. The directive characterises these as requiring “a final qualitative judgement based upon its experience to fly icing conditions,” which can vary between crews and can be impaired by poor light, night operations, or other concurrent factors. The APM is presented as providing objective indications that complement and enhance situation awareness. A key concept is that the APM does not replace existing emergency procedures. The directive states explicitly: “Because the APM warning will only indicate the significant aerodynamic penalties, the current AFM Emergency Procedures for severe icing remain totally valid and applicable. No relief to the pilot procedures concerning the current visual cues to detect severe icing conditions can result from this Directive.” Thus the APM is an additional alerting layer, not a substitute for crew judgment. The directive also introduces the concept of compliance via service bulletins: a manufacturer’s modification and its associated engineering instructions become the legally mandated means of compliance, with later revisions approved by EASA being automatically acceptable.

The directive directly affects operators of ATR 42 series aeroplanes registered in Australia or operating under Australian airworthiness regulatory control. Specifically, it applies to model ATR 42-200, 42-300, 42-320, 42-400 and 42-500 aeroplanes that do not already have the MPC/APM installed per the specified modifications. The applicability excludes aircraft that were factory-equipped with Mod 05567 (noted as incorporated on ATR 42-500 aeroplanes from Manufacturer Serial Number 641) or retrofitted per earlier service bulletin revisions. The directive does not distinguish between commercial airline operators, charter operators, or private owners; any aircraft meeting the type and modification status must comply. This imposes a uniform retrofit burden across different operator risk profiles and financial capacities. For a small regional operator with an ATR 42-200 flown infrequently, the cost of sourcing, installing, and certifying the MPC/APM could be disproportionate to the risk exposure, especially if that operator already manages icing risk through conservative flight planning and crew training. The directive also affects maintenance organisations and engineering personnel who must execute the service bulletin instructions, perform functional checks, and update the aircraft documentation. Flight crews are affected because Appendix 15 to the AFM introduces new APM-related procedures and alerts; they must be trained to interpret the APM warnings and to continue using existing visual detection cues. The directive indirectly affects EASA, as the European regulator’s decisions on later service bulletin revisions are binding on Australian operators who wish to use that alternative. The directive also involves CASA’s own airworthiness engineers and delegates, who may need to assess requests to use later revisions if not explicitly approved by EASA (though the directive states later revisions “approved by EASA” are acceptable). The compliance timeline of “second C check or 72 months” affects maintenance planners: operators must schedule the retrofit within a window that may conflict with other heavy maintenance tasks or financial cycles. For aircraft with infrequent flying, the 72-month outer limit is the binding constraint, but for high-utilisation aircraft, the second C check may come sooner. The directive’s broad applicability means that any ATR 42 still operating under an Australian certificate of airworthiness that lacks the MPC/APM is grounded until compliant after the deadline.

The primary duty imposed by this directive is straightforward: the operator must install the Multi Purpose Computer with Aircraft Performance Monitoring function in accordance with the applicable service bulletin’s accomplishment instructions. The duty attaches to each aeroplane individually. The operator must also ensure that Appendix 15 to the Aircraft Flight Manual is incorporated per the indicated normal revision timing: for ATR 42-200/300/320, AFM Normal Revision 27 dated April 2008; for ATR 42-400/500, AFM Normal Revision 13 dated October 2008. The duty is not time-unlimited: it must be performed not later than the second “C” check after the effective date, or within 72 months, whichever occurs first. This is a mandatory compliance date; there is no right to an extension or exemption stated in the directive. The directive confers certain rights. The operator has the right to rely on an earlier installation accomplished per earlier revisions of the service bulletins, provided that installation was completed before the effective date of the directive. This grandfather clause protects operators who have already voluntarily retrofitted the equipment, and creates a legacy compliance path. The directive also confers a right to use later revisions of the service bulletins if those revisions are approved by EASA. This allows operators who have not yet performed the installation to use improved engineering instructions that may be more efficient or address unforeseen issues, without seeking separate CASA approval. However, this right is conditional: the later revision must be approved by EASA, and presumably the operator must hold evidence of that approval. The directive does not expressly grant a right to use alternative equipment or alternative means of compliance outside the listed service bulletins. There is no right to substitute a different performance monitoring system, or to demonstrate safety by other methods. The duty to update the AFM also implies a right to receive the appropriate AFM revision from the manufacturer or its distributor. The directive does not address who bears the cost of the retrofit, but as a mandatory airworthiness requirement, the operator bears all financial and operational costs. The directive also implies a duty on the regulator (CASA) to accept the later revisions approved by EASA, but that is not a duty owed to the operator and carries no enforcement mechanism.

This airworthiness directive does not itself prescribe penalties. However, it is issued under the Civil Aviation Act 1988 and the Civil Aviation Regulations 1988, and a failure to comply with a current airworthiness directive is a breach of those regulations. The Civil Aviation Regulations provide that the registered operator or the owner of an aircraft must ensure that the aircraft is maintained in an airworthy condition and that all applicable airworthiness directives are complied with. Non-compliance can attract enforcement action by CASA, which may include issuing a notice of proposed suspension or cancellation of the airworthiness certificate or the operator’s air operator’s certificate, imposing a civil penalty, or in serious cases pursuing criminal prosecution. Under the Civil Aviation Act, the maximum penalty for a contravention of a regulation can be substantial; specific penalty units are set from time to time, but the directive does not mention a figure. For a corporation, penalties for a breach of safety regulations can run into hundreds of thousands of dollars. CASA may also issue a direction to ground the aircraft until compliance is achieved, which imposes additional costs through lost revenue. The director’s powers include the ability to issue a “show cause” notice requiring the operator to explain why enforcement action should not be taken. CASA inspectors routinely check compliance with airworthiness directives during ramp checks, maintenance record audits, and certificate renewal processes. The directive’s background notes describe a safety risk that could be categorised as “unsafe condition,” and any incident arising from non-compliance would likely attract aggravated enforcement attention. The directive also references EASA AD 2009-0170, and while CASA does not enforce EASA directives directly, the existence of an equivalent EASA AD creates a parallel risk: an Australian operator seeking to operate an ATR 42 into European airspace would also need to comply with the EASA directive, so non-compliance with the CASA directive would be compounded by a breach of foreign regulations. There is no right of appeal within the directive itself, but an operator could apply for a review of CASA’s decision to issue the directive, or seek a variation, under the general administrative law framework. However, the directive was made by a delegate of CASA and is a legislative instrument; challenging its validity would require judicial review.

This directive operates within a hierarchy of airworthiness regulation. It is issued under the Civil Aviation Act 1988 and the Civil Aviation Regulations 1988, specifically the provisions dealing with airworthiness directives. It supplements the general duty under Part 4A of the Civil Aviation Regulations to maintain the aircraft in an airworthy condition. The directive itself acknowledges interaction with EASA regulation: it states that later revisions of the service bulletins “approved by EASA are acceptable for compliance with this Directive.” This creates a recognised cross-credentialling arrangement where CASA accepts EASA’s approval as satisfying its own requirement. The directive also interacts with the certification basis of the aircraft. The background notes that the ice protection system is compliant with the “current certification envelope for flight into known-icing conditions” under Part 25 of the Federal Aviation Regulations (as adopted by CASA). The directive explicitly addresses conditions beyond that envelope; thus, it imposes a retrofit that goes beyond the original type certification standards. This is a typical feature of airworthiness directives that respond to in-service experience not foreseen at certification. The directive also interacts with the operational rules governing flight in icing conditions. The operator must still comply with Civil Aviation Regulation 233 (or equivalent) regarding flight in forecast or actual icing conditions, and with any other applicable operational directives. The APM supplements but does not replace those rules. The directive interacts with the aircraft’s approved maintenance program. The compliance term “second C check” references the operator’s specific maintenance schedule, which must be approved under the Civil Aviation Regulations. Operators with non-standard C check definitions must interpret “C check” consistently with their approved maintenance program. The directive also references the Aircraft Flight Manual, which itself is an approved document under the regulations. Amending the AFM per the directive requires the operator to ensure the amendment is properly incorporated and that copies are distributed to flight crew. If the AFM is subject to a separate regulatory requirement for approval of changes, the directive effectively mandates that change. Finally, the directive interacts with airworthiness directive systems in other jurisdictions. The note “EASA AD 2009-0170 refers” indicates the European equivalent. For an Australian operator that also holds a foreign registration or operates internationally, compliance with this directive will likely be accepted by other regulators under bilateral agreements, but in some cases separate compliance with the foreign AD may be required.

The source document is AD/ATR 42/27, issued on 2 September 2009 and effective 22 October 2009. The text does not indicate any previous version of this specific directive. However, it is part of a numbered series (AD/ATR 42/27) which suggests that at least 26 earlier directives have been issued for the ATR 42 type. No amendment history is provided within the instrument. The directive does note that “installation of a MPC/APM accomplished before the effective date of this Directive in accordance with earlier revisions of the aforementioned SBs, satisfy the requirements of this Directive.” This creates a transitional recognition of previously compliant installations, but it is not an amendment of the directive itself; it is a built-in compliance option. The directive also allows later revisions of the service bulletins approved by EASA to be used for compliance. While this is not an amendment to the directive, it effectively gives the directive a dynamic element: as EASA approves new revisions, the acceptable means of compliance expand without CASA needing to issue a new AD. This design reduces the administrative burden of frequent reissuance. However, it also means that the directive’s compliance landscape can change after issue, potentially requiring operators to track EASA approvals. The directive does not provide for its own revocation or supersession. Typically, a later directive would supersede it, either explicitly or by implication if it covers the same subject matter. As of the source, no such supersession is indicated. The directive is also static: it does not incorporate a sunset clause or a periodic review date. Unless it is formally revoked or superseded, it remains in force indefinitely. For an operator who has complied, the directive’s requirements are exhausted. But for an aircraft acquired later that lacks the equipment, the directive continues to apply. The absence of amendment history means the directive has remained unaltered since 2009, which is common for single-issue airworthiness directives that are not subject to subsequent changes.

The source instrument itself makes no reference to any litigation concerning AD/ATR 42/27. A search of reported Australian administrative and aviation decisions would be needed to confirm an absence of challenges, but airworthiness directives are rarely litigated directly in Australia. They are legislative instruments made under delegated power, and aggrieved operators typically seek a variation or exemption from CASA rather than a judicial review of the directive’s validity. Courts generally defer to CASA’s technical expertise on airworthiness matters, and an operator challenging a directive would need to demonstrate that it was irrational, procedurally flawed, or beyond power. There is no widely reported case in Australia where an operator has successfully overturned an airworthiness directive on substantive grounds. The directive’s background references “recently reported ATR42/72 in-flight incidents.” Some of those incidents may have been the subject of accident investigations by the Australian Transport Safety Bureau or equivalent bodies, but the directive does not name any specific accident or investigation report. Those incidents may have given rise to litigation between affected parties (e.g., passengers, manufacturers, operators) but such litigation would be in civil courts and not directly challenge the directive. The directive’s reference to EASA AD 2009-0170 indicates that the European regulator issued a parallel directive. It is possible that EASA’s directive was subject to legal challenge before the European Court of Justice or EASA’s Board of Appeal, though no such challenge is mentioned. In the United States, equivalent Federal Aviation Administration directives on ATR icing have been the subject of some industry comment and debate, but again not necessarily litigation. In summary, there is no reported Australian litigation history for this particular AD. The practical reality is that most operators accept the regulatory mandate because the cost of non-compliance (grounded aircraft, enforcement action) far exceeds the cost of challenging the directive, and because the safety benefits are generally acknowledged within the industry. The lack of litigation does not mean the directive is beyond criticism, but it does mean that its legal and economic effects have not been tested in court.

Several features of this directive could trip up operators or maintenance personnel who do not read every detail. First, the compliance trigger is “not later than the second ‘C’ check or within 72 months, whichever occurs first after the effective date.” The phrase “second C check” presumes that a “first C check” has already occurred after the effective date. For an aircraft that is not due for a C check for many years, the second C check could be many years away, but the 72-month outer limit caps that. For an aircraft that is due for a C check soon, the second C check may be only a few months away, so the operator must plan accordingly. Misinterpreting “second” as the next upcoming C check would be a serious error. Second, the directive accepts earlier revisions of the service bulletins only if the installation was “accomplished before the effective date.” An operator who installed an earlier revision after the effective date in anticipation of the directive would not be covered by the grandfather clause and must redo the installation per the latest revision. Third, the directive requires that Appendix 15 to the AFM be included in “the following Normal Revision of the applicable AFM” with specific dates. If the operator had already incorporated a later revision of the AFM that did not include Appendix 15, or if the operator uses a different AFM revision, the directive does not explicitly permit an alternative. The operator must ensure that Appendix 15 is present, even if it means reverting to an earlier revision or requesting a supplementary amendment. Fourth, the directive states that “later revisions of the aforementioned SBs approved by EASA are acceptable for compliance.” This is a conditional right: the operator must verify that EASA has actually approved the later revision. Using a revision that EASA has not yet approved, or that is only draft, would put the operator in non-compliance. Fifth, the directive emphasises that the current AFM emergency procedures for severe icing “remain totally valid and applicable” and that “no relief to the pilot procedures concerning the current visual cues to detect severe icing conditions can result from this Directive.” This is a potential trap for flight crews who might come to rely solely on the APM alerts and stop actively monitoring visual cues. The directive explicitly tells them not to do that, but the practical effect of installing an alerting system may be to reduce vigilance. Operators must train crews to continue using all existing detection methods. Sixth, the directive does not mention any obligation to retrofit aircraft that are not currently in service or that are under storage. If such an aircraft is returned to service, the directive would apply, and the compliance time would run from the effective date, not from the return to service. Seventh, the “Note 3: EASA AD 2009-0170 refers” indicates a parallel foreign directive, but compliance with the Australian AD alone does not guarantee compliance with the EASA AD if there are differences in details.

Compliance with AD/ATR 42/27 involves a series of defined steps that operators should integrate into their maintenance planning. The first step is to identify all affected aircraft on the operator’s fleet list. This requires checking the aircraft type certificate, model, and whether the MPC/APM is already installed per Modification 08420 (for models -200, -300, -320) or Modification 05567 (for models -400, -500). The applicable service bulletins provide the technical criteria. For any aircraft not already equipped, the operator must obtain the correct service bulletin: SB ATR42-31-0071 Revision 07 dated 7 February 2008 for the -200/300/320 series, or SB ATR42-31-0068 Revision 07 dated 15 January 2009 for the -400/500 series. These documents are available from ATR or its distributors. The operator must then acquire the MPC/APM equipment and any associated kits. The service bulletin’s accomplishment instructions detail the installation procedure, including wiring, mounting, software loading, and integration with other aircraft systems. The installation must be performed by a maintenance organisation holding the appropriate ratings under Part 145 of the Civil Aviation Regulations. After installation, the operator must update the Aircraft Flight Manual by incorporating Appendix 15. This should be done per the normal revision indicated in the directive: AFM 42-200/300/320 Normal Revision 27 dated April 2008, or AFM 42-400/500 Normal Revision 13 dated October 2008. If the operator holds a later AFM revision that already includes equivalent content, the operator should seek CASA confirmation that it satisfies the directive. The operator must also update the relevant aircraft records, including the maintenance release and the continuing airworthiness record, to show compliance. A statement signed by the certifying engineer should reference the AD number and the service bulletin revision used. For operators who wish to use a later revision of the service bulletin approved by EASA, the operator must obtain that approval documentation and ensure no new instructions conflict with the Australian AD. The operator must schedule the installation to meet the compliance deadline. This requires coordinating with the maintenance plan to perform the retrofit by the second C check after 22 October 2009, but not later than 22 October 2015 (72 months). The operator should also ensure that flight crews receive training on the APM alerts and on the continued obligation to use visual detection cues. The directive does not mandate specific training, but the operator’s safety management system should address the human factors issues described in the background. Finally, the operator should maintain copies of the service bulletin, the EASA approval (if using a later revision), and the AFM revision as supporting evidence for audits and inspections.