AD/B737/350 - Auxiliary Fuel Tanks

Airworthiness Directive (AD) - An AD is a mandatory notification issued by the national aviation authority (here, CASA under the Civil Aviation Act 1988 and associated regulations) requiring action to correct an unsafe condition in an aircraft type, engine, propeller, or appliance. Compliance with ADs is a legal condition of maintaining the aircraft’s Certificate of Airworthiness. An AD overrides any contrary provisions in the aircraft’s flight manual or supplemental type certificates. This particular AD is identified by the serial number AD/B737/350 and dated 1/2009.
Unsafe condition - The AD refers to a review by the manufacturer (Boeing) that identified unsafe conditions for which the manufacturer has not provided corrective actions. The specific theory of risk is the potential for ignition sources inside fuel tanks, which when combined with flammable fuel vapours could cause an explosion. This is a classical fuel tank flammability safety concern, similar to the issues that led to the TWA Flight 800 investigation. The unsafe condition is not described in detail in the source, but the AD’s existence creates a presumption that the auxiliary fuel tank systems as installed under the listed STCs present an unacceptable level of risk.
Supplemental Type Certificate (STC) - An STC is a type certificate issued when an applicant has shown that an alteration to an aircraft (here, installation of auxiliary fuel tanks) complies with applicable airworthiness requirements. The AD applies only to aeroplanes fitted with auxiliary tank STCs listed in Table 1. Notably, the AD does not apply to all 737 auxiliary fuel tank installations; it targets specific STCs from specific design approval holders (e.g., PATS Aircraft, LLC). STCs listed include SA83NE, SA725NE, SA1078NE, SA1265EA, SA500NE, SA542NE, SA553NE, SA714NE, ST00040NY, ST01337NY, ST00936NY‑D, ST01650NY‑D, ST01384NY, and ST01384NY‑D. The AD excludes certain variant configurations of these STCs (e.g., SA725NE-D, Configuration 7; ST00936NY-D, Configuration 3). This creates a complex applicability matrix: an operator must know not only which STC is installed, but also which specific configuration, to determine whether the AD applies.
Deactivation - The AD uses the term “deactivation” rather than “removal”. Deactivation means rendering the auxiliary fuel tank inoperative and securing it so that it no longer performs its intended function, but the tank may remain physically installed on the aeroplane. The deactivation criteria in Appendix A require permanent draining and purging of fuel vapours, disconnection and stowage of electrical, pneumatic, and fuel/vent plumbing, pulling and collaring of circuit breakers, and revisions to the aeroplane flight manual (AFM), maintenance manual, weight and balance documentation, and instructions for continued airworthiness. A placard in the flight deck must indicate the deactivation. The AD states that after deactivation, the aeroplane’s fuel quantity indication system (FQIS) and fuel distribution systems must not be adversely affected.
CASA approval - The deactivation procedure must be approved by CASA before it can be implemented. The operator must submit the proposed procedure to the Manager, Systems and New Technologies, Airworthiness Engineering Group (fax (02) 6217 1914). Appendix A provides criteria that “should” be included, but the operator may submit additional steps perceived necessary. The approval process introduces regulatory uncertainty: if CASA does not review and approve in time, the operator may not meet the 16 December 2009 deadline. Note 1 suggests that proposed procedures should be submitted “as soon as possible”.
Two‑phase compliance - This AD has a reporting phase (within 45 days) and a substantive action phase (before 16 December 2009). The report serves a data‑gathering function for CASA, presumably to track which aeroplanes are affected and how intensively the tanks were used. The report is not a prerequisite to the deactivation; the AD does not say deactivation cannot start until the report is filed, but both requirements are independent and must be met by their respective deadlines.

Operators - The primary duty holders are the operators of the listed Boeing 737 series aeroplanes. In aviation regulation, the “operator” is the person or organisation responsible for the continuing airworthiness of the aircraft and for operating it under the Civil Aviation Safety Regulations. This AD directly imposes obligations on the operator: to submit the report, to develop or commission a deactivation procedure, to obtain CASA approval, and to physically deactivate the tanks. The operator may delegate the engineering work to a maintenance organisation, but the legal responsibility remains with the operator.
Maintenance organisations - Any approved maintenance organisation (AMO) engaged to perform the deactivation work will need to be familiar with the AD requirements and the approved deactivation procedure. The physical tasks (draining, purging, disconnecting, capping, securing, circuit breaker collaring, leak checking, pressure testing) are typical maintenance activities, but done in the context of a permanent modification that must not affect ongoing airworthiness. The AMO must also ensure that the post‑deactivation configuration does not introduce new risks (e.g., unsecured plumbing, electrical hazards, structural deformation during decompression from vent lines not left open).
Design approval holders - The companies holding the STCs (PATS Aircraft, LLC, and others) are indirectly affected. The AD results from manufacturer‑identified unsafe conditions for which the manufacturer has not provided corrective actions; this suggests that the STC holders may have been unable or unwilling to issue their own corrective action. The AD effectively overturns the approval of those STCs for the purpose of continued operation, though the STCs themselves remain valid for other purposes. The AD does not require the STC holders to fund the deactivation; costs fall on operators.
CASA - The Systems and New Technologies branch of CASA’s Airworthiness Engineering Group has the administrative burden of receiving and reviewing deactivation procedures. The 45‑day reporting requirement also imposes a data collection function on CASA. The AD is a specific CASA regulatory act, limiting the authority’s own discretion insofar as it has committed to approving deactivation procedures that meet the criteria.
Insurers and financiers - Aeroplanes subject to this AD will have reduced value and operational capability post‑deactivation. Insurers may adjust premiums or require evidence of compliance. Lessors and financiers with security interests in the aircraft will need to ensure that lessees comply, as non‑compliance could ground the asset.
Passengers and crew - The AD ultimately aims to protect passengers and crew from the risk of fuel tank explosions. While the AD imposes costs and operational restrictions, the intended beneficiary is flight safety.
Geographic scope - This is an Australian AD. It applies to aeroplanes registered in Australia or operated under an Australian air operator’s certificate. Operators of Australian‑registered 737s fitted with the listed STCs are caught, regardless of where the aircraft normally flies. The AD does not apply to aeroplanes registered in other countries unless they operate under Australian regulatory oversight, though the FAA equivalent AD (2008‑22‑01) would apply to US‑registered aircraft.

Duty to report (Requirement 1) - Within 45 days after 15 January 2009, the operator must send a report to the Manager, Systems and New Technologies, Airworthiness Engineering Group, via fax (02) 6217 1914. The report must contain the aeroplane registration, the auxiliary tank STC number installed, and the usage frequency (total flights per year, and flights per year where the auxiliary tank was used). There is no prescribed form; a letter or fax with the required information suffices. Failure to report by the deadline is a breach of the AD, which may constitute an offence under the Civil Aviation Act.
Duty to deactivate (Requirement 2) - Before 16 December 2009, the operator must deactivate the auxiliary fuel tanks in accordance with a CASA‑approved procedure. The duty is absolute; there is no exemption for hardship, cost, or operational need. The operator cannot simply remove or modify the tanks without following an approved procedure.
Right to propose deactivation procedure - The operator has the right (and practical necessity) to develop a deactivation procedure that satisfies the criteria in Appendix A and submit it to CASA for approval. The AD does not specify who owns the procedure; the operator may use an existing CASA‑approved procedure (if any), develop one in‑house, or commission the STC holder or a third party to draft one. The AD does not give the operator a right to a particular timeline for CASA approval, but the deactivation deadline is fixed. If CASA delays, the operator may be unable to comply. The operator should factor this risk into planning.
Duty to secure and ensure no adverse effect - The AD requires that any auxiliary tank component that remains on the aeroplane after deactivation must be secured and must have no effect on the continued operational safety and airworthiness of the aeroplane. This is an ongoing duty: if a deactivated component later becomes loose or causes a hazard, that would be a continuing airworthiness failure.
Duty not to create additional ICA - The AD states that deactivation must not result in the need for additional instructions for continued airworthiness. This means the deactivation procedure itself should not introduce new recurring maintenance tasks (e.g., inspection of the disconnected tank). However, step 10 of Appendix A requires the operator to “revise the instructions for continued airworthiness, as required, after deactivation.” There is a potential tension: if the deactivation creates a new item that needs regular checking (such as condition of the capped vent line), that might be a new ICA. The operator must ensure any changes to ICA are purely reflective of the deactivated configuration rather than adding tasks.
Right to insert AD into AFM - The operator may accomplish the required AFM revision by inserting a copy of this AD into the AFM (per step 7 of Appendix A). This is a temporary method, but acceptable under the AD. The placard in the flight deck is a permanent requirement.
Duty to verify FQIS and fuel distribution - After deactivation, before returning the aeroplane to service, the operator must accomplish leak checks, pressure checks, and verification that the FQIS and fuel distribution systems have not been adversely affected (step 9). This is a critical safety check; if the deactivation inadvertently damages the OEM fuel system, the aeroplane is not airworthy.

The AD does not specify a penalty for non‑compliance. However, under the Australian regulatory framework for airworthiness, failure to comply with an AD is a serious matter. The Civil Aviation Act 1988 and the Civil Aviation Safety Regulations (CASR) make it an offence to operate an aircraft that does not meet its approved type design or is not in a condition for safe operation. An AD is a mandatory directive; operating contrary to it could lead to enforcement action including infringement notices (fines), suspension or cancellation of the Air Operator’s Certificate or Certificate of Airworthiness, and in serious cases, criminal prosecution.
Specifically, CASR Part 21 (Certification of aircraft) and Part 39 (Airworthiness directives) impose obligations. Part 39 provides that an AD is legally binding and must be complied with within the prescribed compliance time. A person who contravenes an AD may be liable for a civil penalty of up to a certain amount. Additionally, the Civil Aviation Safety Regulations 1998, regulation 91.025 (or equivalent in earlier versions) requires the operator to ensure the aircraft is airworthy before each flight; an unaddressed AD would render the aircraft unairworthy.
The practical enforcement mechanism is that the aircraft’s Certificate of Airworthiness is conditional on compliance with all applicable ADs. If a CASA inspector or authorised person finds that the AD has not been complied with, they can ground the aircraft by withdrawing its certificate or issuing a direction not to fly. Civil liability may also arise: if an operator fails to comply and an accident occurs, the operator could face negligence claims, and insurance coverage may be voided for non‑compliance with regulatory requirements.
The AD itself does not create a right to an extension or waiver. The delegate of CASA who signed the AD (James Coyne) has the authority to approve the deactivation procedure, but nothing in the source suggests the delegate can extend the compliance date. In practice, if an operator cannot meet the 16 December 2009 deadline, it would need to apply to CASA for an alternative means of compliance or a deviation under CASR Part 39, but that is speculative.
The reporting requirement (Requirement 1) also carries enforcement risk. If an operator fails to submit the report within 45 days, that is a distinct breach. CASA could take enforcement action on that alone, even if the deactivation is completed on time.
Given the safety rationale (risk of fuel tank explosion), CASA is unlikely to treat non‑compliance leniently. The AD originates from a manufacturer‑identified unsafe condition with no corrective action from the STC holder; this is a mandatory safety directive with high priority.

FAA AD 2008‑22‑01 - The Australian AD explicitly references the US FAA AD 2008‑22‑01 as a related document (Note 2). That FAA AD likely has equivalent requirements. For operators with fleets operating under both US and Australian regulatory oversight, the deactivation must satisfy both authorities. The Australian AD requires CASA approval of the deactivation procedure; the FAA AD may require FAA approval. Operators must ensure the proposed procedure meets both sets of criteria, which may be similar but not identical.
Civil Aviation Act 1988 and Civil Aviation Safety Regulations (CASR) - This AD is made under the authority of the Civil Aviation Safety Authority, principally through Part 39 of CASR (Airworthiness Directives) and the enabling provisions in the Act. The AD itself is not a legislative instrument but an administrative directive. It must be read together with the continuing airworthiness obligations in Part 42 of CASR (if applicable to the operator) and Part 21 regarding modifications and repairs.
Supplemental Type Certificates (STCs) - The AD overrides the normal operational approval of the listed STCs. An STC is an approved alteration; normally an operator may rely on it indefinitely. This AD removes that operational approval for the auxiliary fuel tank function. The STC itself is not revoked; its airworthiness basis remains, but the AD makes it illegal to use the system it covers. The deactivation may require a new minor modification record to show the configuration change, but the AD does not require a new STC or modification for the deactivation itself.
Airworthiness standards - The deactivation criteria in Appendix A are based on AC 25‑8 Auxiliary Fuel System Installations (referenced in step 4). That advisory circular provides methods acceptable to the FAA for auxiliary fuel system installations; here it is used as a reference for deactivation procedures. The AD also refers to weight and balance documentation standards and flight manual requirements under CASR.
International obligations - Australia is a signatory to the Convention on International Civil Aviation (Chicago Convention). Compliance with ADs is part of maintaining an aircraft’s continuing airworthiness under Annex 8 to the Convention. If an Australian‑registered aircraft fails to comply, other states may deny it entry or detain it.
Insurance - While not a statutory interaction, insurance policies typically include a warranty that the aircraft is operated in compliance with all applicable ADs. Non‑compliance could void coverage. Operators must be able to demonstrate compliance to underwriters.
Lease agreements - Operators who lease aircraft from foreign lessors must check whether the lease requires compliance with all manufacturer and authority directives. If the auxiliary tank deactivation reduces value or payload, the lessor may need to consent, or the operator may need to provide evidence of compliance.

The source provided is the original issuance of AD/B737/350. It is dated 27 November 2008, bears the signature of James Coyne as Delegate of CASA, and becomes effective on 15 January 2009. There is no amendment history shown in the source. The AD number “1/2009” likely indicates it is the first or only amendment in 2009 for this particular AD number, but the AD itself does not record any previous versions.
Within the AD, there is a reference to FAA AD 2008‑22‑01 Amdt 39‑15696 dated 9 October 2008. That is the US equivalent and is not an amendment of the Australian AD.
The appendix is included as part of the same AD, titled “AD/B737/350Appendix A” and labelled “Auxiliary Fuel Tanks 1/2009”. It is not a separate amendment; it is part of the original directive.
In practice, an AD may be amended by CASA if compliance deadlines need to be extended, if the list of affected STCs changes, or if alternative means of compliance are developed. However, nothing in the source indicates that such amendments have occurred. This deep dive is based solely on the source, which presents the AD as issued. There is no reference to superseded ADs or earlier versions. Operators should check the current CASA AD register to see if AD/B737/350 has been revised or superseded since 2009. For the purpose of this analysis, the AD stands as originally issued.

The source provides no litigation history relating to this AD. No cases are cited, and no information is given about appeals, judicial review, or administrative challenges.
In general, airworthiness directives are rarely challenged in court because they typically address genuine safety risks and are issued under broad delegated powers. An operator challenging an AD would need to argue that CASA acted beyond its statutory authority, that the AD is irrational or procedurally flawed, or that the operator was not given proper notice. Given the safety justification (potential fuel tank explosions), a court would likely give CASA wide deference.
If there were litigation, it might have concerned the applicability of the AD to a particular STC configuration, the reasonableness of the compliance deadline, or the approval process for deactivation procedures. However, no such litigation is recorded in the source.
The AD references FAA AD 2008‑22‑01. In the United States, FAA ADs can be challenged in the US Court of Appeals, but again no cases are known from the source.
For this deep dive, we cannot invent litigation. The absence of litigation history in the source suggests that the AD was implemented without judicial contest. That may reflect the limited commercial consequences (operators accepted the decision) or the cost of challenge outweighing the benefit. Alternatively, any litigation may have occurred after the source document date and is not captured. The instruction to the analyst is to “only name cases the source names” - the source names none. Therefore, the litigation history section must note there is none from the source.

Exclusion traps - The applicability table includes exclusions for certain STC configurations: SA725NE‑D Configuration 7 is excluded for most models; ST00936NY‑D Configuration 3 is excluded for 737‑700. An operator might incorrectly assume their aircraft is affected when it is not, or vice versa. The AD does not define what constitutes “Configuration 7” or “Configuration 3”. The operator may need to trace back to the STC paperwork to determine the precise configuration. If the STC holder has changed configurations over time, ambiguity could arise.
Deactivation must not create additional ICA - The AD states (in the requirement text) that deactivation must not result in the need for additional instructions for continued airworthiness. However, Appendix A step 10 says “Revise the instructions for continued airworthiness, as required, after deactivation.” If the deactivation creates a need for periodic inspections of the capped vent lines or the stowed electrical connections, that would new ICA tasks, potentially contradicting the requirement. The operator must ensure any revision is purely administrative (reflecting removal of auxiliary tank maintenance) and does not add new tasks. If the deactivation introduces a hazard (e.g., a vent line that could degrade), the operator may need alternative engineering judgment to avoid creating a new ICA.
The report usage data - The operator must provide “total number of flights per year and total number of flights per year for which the auxiliary tank is used.” This requires historical data. If the operator does not keep precise records of which flights used auxiliary fuel, this data may be difficult to obtain accurately. The AD does not specify whether estimates are acceptable or what year “per year” refers to (e.g., the calendar year 2008 or the preceding 12 months). This ambiguity could lead to non‑compliance if CASA later rejects the report.
CASA approval timeline - The deactivation procedure must be approved by CASA. The AD does not set a deadline for CASA to respond. The operator must submit as soon as possible, but if CASA takes months to review, the operator may be forced to deactivate after 16 December 2009 (too late) or to implement an unapproved procedure (also non‑compliance). The risk of regulatory delay is a serious gotcha.
Auxiliary tank remains - The AD does not require removal of the auxiliary tank shell. The tank remains installed but deactivated. This may impose a permanent weight penalty and reduce payload. The tank may also be a dead weight that affects centre of gravity. The operator may need to revise the weight and balance document (step 6) to reflect the empty tank’s weight and its moment. Failure to do so could affect flight performance calculations.
Plumbing disconnection - Step 4 requires disconnecting fuel feed and vent plumbing interfaces with OEM tanks, capping at the aeroplane tank side, and securing the disconnected auxiliary fuel tank vent lines leaving them open to prevent structural deformation during decompression. If the operator closes both ends, differential pressure could deform the tank. The AD specifies leaving them open, but the operator must ensure the open vent does not allow ingress of foreign objects or water. This detail is easy to miss.
Circuit breakers collared - Pulling and collaring circuit breakers (step 5) is a standard engineering practice, but if later maintenance personnel mistakenly remove the collar and reset the breaker, the auxiliary tank could become energized again. The operator must ensure the collaring is as permanent as possible (e.g., using a metal lock) and that all future maintenance documentation reflects the deactivation.
No exemption for recently installed tanks - The AD does not differentiate between tanks installed long ago and those installed recently. The compliance dates are the same, regardless of installation date. An operator who installed a tank after the AD’s effective date may be required to deactivate it almost immediately.
No funding or compensation - The AD places all costs on operators. There is no provision for government assistance, insurance claim against the STC holder, or manufacturer warranty. This is a pure compliance cost.

Compliance involves two distinct phases: reporting (by 1 March 2009, approximately) and deactivation (by 16 December 2009). The following steps outline the practical path.

Phase 1: Reporting.
Within 45 days after 15 January 2009, the operator must compile and send the report. The report must include:

• Aeroplane registration (tail number).
• Auxiliary tank STC number installed (from the list in Table 1).
• Usage frequency: total number of flights per year (presumably for the aircraft’s fleet or per aircraft) and total number of flights per year where the auxiliary tank was used.

The operator should pull historical flight logs, fuel consumption records, and auxiliary tank usage logs. If the operator does not have precise data, it should use the best available estimate, but be prepared to justify the figure if CASA queries. The report should be faxed to (02) 6217 1914. No other contact details are given; a confirmatory call may be wise.

Phase 2: Deactivation planning.
As soon as possible after the AD’s effective date (15 January 2009), the operator should begin developing the deactivation procedure. The procedure should follow the 11 criteria in Appendix A:

1. Permanently drain and purge fuel vapours.
2. Disconnect all electrical connections (FQIS, fuel pumps, float switches, etc.) and stow them at the tank interface.
3. Disconnect all pneumatic connections, cap at the source, secure.
4. Disconnect fuel feed and vent plumbing from OEM tanks, cap at the aeroplane tank side, secure. Leave disconnected auxiliary tank vent lines open (to prevent structural deformation during decompression). One approved method for capping is in AC 25‑8.
5. Pull and collar all circuit breakers used to operate the auxiliary tank.
6. Revise weight and balance document (if needed) and obtain CASA approval for that change.
7. Amend the AFM: remove auxiliary tank operating procedures; add a limitation that the AFM Supplement for the STC is not in effect; place a placard in the flight deck stating the auxiliary tank is deactivated. The AFM revision can be done by inserting a copy of this AD.
8. Amend the aeroplane maintenance manual to remove auxiliary tank maintenance procedures.
9. After deactivation, perform leak checks and pressure checks deemed necessary. Verify that the FQIS and fuel distribution systems are not adversely affected.
10. Revise the instructions for continued airworthiness as required.
11. Include any additional steps the operator deems necessary.

The operator must submit the proposed procedure to CASA for approval. There is no prescribed format; a detailed engineering document referencing the AD and Appendix A criteria is required. The operator should include step‑by‑step instructions, the approved methods for capping and securing, and evidence that the deactivation will not create additional ICA. The submission should be sent to the Manager, Systems and New Technologies, Airworthiness Engineering Group, fax (02) 6217 1914. Keep a copy.

Phase 3: Implementing the approved procedure.
Once CASA approves the procedure (preferably well before 16 December 2009), the operator or its maintenance organisation must implement the deactivation. All steps in the approved procedure must be performed strictly. The physical work will likely be done during a scheduled maintenance visit. After completion, the maintenance release must note compliance with AD/B737/350. The placard must be installed. The AFM and maintenance manual amendments must be made. The weight and balance document must be updated (if necessary) and approved by CASA. The ICA must be revised.

Phase 4: Post‑deactivation.
The operator must record the deactivation in the aircraft’s continuing airworthiness record. The deactivation is permanent; no further action is required unless CASA issues a superseding AD. The operator should ensure that future maintenance personnel do not inadvertently reconnect or recommission the tank. The collared circuit breakers and disconnected plumbing should be physically locked or tagged.

Cost considerations - The operator should budget for engineering time (drafting the procedure), CASA approval fees (if any), maintenance labour, parts (collars, caps), documentation, and potential revenue loss from reduced fuel capacity. There may be a need for a ferry flight to a maintenance base if the tank has fuel on board; the AD does not allow a grace period for flying with a partially deactivated tank.

Verification - The operator should retain copies of the report fax confirmation, CASA approval letter for the deactivation procedure, maintenance work orders, AFM inserts, and weight and balance revisions as evidence of compliance. CASA inspectors may request these records during audits.