AD/B737/125 Amdt 3 - Centre Wing Fuel Tank Float Switch Wiring

This AD is structured around risk-reduction through inspection, measurement, repair/replacement, and operational mitigation. The main technical concepts in the instrument are:

• Applicability and scope: The instrument applies only to certain Boeing 737 series models (737-200, -200C, -300, -400, -500) with centre wing tanks activated and having L/Ns 1-3108 inclusive (Applicability). The AD excludes aeroplanes equipped with centre wing tank volumetric top-off systems or those with AC-powered centre tank float switches (Applicability note). This confines the regulatory reach to a defined fleet subset.

• Detection of electrical degradation: The AD mandates a detailed visual inspection of float switch wiring and associated conduit for signs of chafing, arcing, insulation wear, exposed conductor, or fuel contamination (Requirement 1.a). The AD defines “detailed inspection” in the text: intensified visual examination with supplementary lighting and inspection aids, potentially including surface cleaning and elaborate access procedures (Requirement 1.a Note 2).

• Insulation resistance measurement: The AD introduces an electrical acceptance criterion expressed as a resistance threshold: 200 megohms. If the measured resistance between the float switch wires and the float switch housing is less than 200 MΩ, replacement and sleeving are required (Requirement 3). If resistance is ≥ 200 MΩ, cleaning, sleeving and reinstall are authorised (Requirement 4). The threshold provides a binary decision rule for remedial action.

• Physical remediation and design mitigation: The AD requires installation of double Teflon sleeving over float switch wiring, replacement of float switch and wiring with new or improved parts, replacement of conduit sections where arcing or leakage occurred, and a requirement for installing a conduit liner system in both centre and wing fuel tank conduit assemblies (Requirements 2-4, 6-9, 13.a-13.c). These are hardware-level mitigations designed to reduce chafing and potential ignition risks.

• Operational mitigations and interim arrangements: Where replacement parts are not yet available, the AD permits deactivation of the centre tank float switch via cutting and stowing/splicing of wires at the front spar splices with specific markings and placards, and prescribes manual fuelling procedures together with one‑time fuelling-crew training per Boeing Telex M-7200-98-04486 (Requirement 1.b, Requirement 10). Dispatch with the centre tank float switch deactivated in accordance with ASB 737-28A1132 or Revision 1 is allowed until replacement float switches/wiring are available (Requirement 11). Operators must check fuelling-panel quantity indicators and replace inoperative indicators before fuelling (Requirement 10.b.i).

• Repetitive inspection regime and terminating action: Where conduit replacement is not yet achieved, a regime of repetitive detailed inspections of the conduit for fuel leaks is required at intervals not exceeding 1,500 hours time-in-service (Requirement 7). Full replacement of float switches and installation of a conduit liner system across centre and wing tanks (Requirement 13) is specified as the terminating action for the AD and must be done within the stated compliance window (Compliance section).

• Interplay with manufacturer service documents and regulator precedence: The AD references Boeing Alert Service Bulletins ASB 737-28A1132 and ASB 737-28A1141 (and revisions), and Boeing Telex M-7200-98-04486. It states that where there are differences between Boeing ASB and the Directive, the Directive prevails; likewise, where differences exist between the MMEL and the Directive, the Directive prevails (several Requirements). The AD therefore binds operators to CASA’s specified requirements even where manufacturer or MMEL language differs.

• Technical acceptability of prior work: Note 5 provides objective conditions under which replacements and installations performed before Amendment 2 are acceptable: torquing B-nuts to values in ASB 737-28A1141 Revision 3; installation and secure fastening of the float switch bonding strap as specified in ASB 737-28A1141 Revision 3; and installation of lock wire in specified bolt heads per ASB 737-28A1141 Revision 3.

• Prohibition on specific part: The AD explicitly prohibits installation of float switch part number F8300-146 on any aeroplane (Requirement 15), establishing a negative compliance obligation.

These are the operational and technical levers the Directive uses: inspection to detect degradation, electrical measurement to triage remedial actions, hardware upgrades to remove potential ignition sources, and operational/procedural controls while hardware replacements are pending. The AD also uses compliance deadlines and terminating actions to drive permanent fleet modification.

The Directive names the directly affected parties and, by implication, identifies a chain of downstream stakeholders who will face costs, operational constraints, or obligations.

Directly affected:

• Operators of Boeing 737-200, -200C, -300, -400 and -500 series aeroplanes on which centre wing fuel tanks are activated, with line numbers 1-3108 inclusive (Applicability). These operators must carry out the inspections, measurements, repairs, replacements, manual fuelling changes and training dictated by the AD.
• Maintenance organisations and engineers performing the inspections, resistance measurements, repairs, conduit replacements, sleeving installations and replacements required by the ASBs referenced in the Directive. They must follow the Accomplishment Instructions in Boeing ASB 737-28A1132 and ASB 737-28A1141 and conform to the Directive where it prevails over the ASBs.
• Fuelling crews involved in fuelling operations for affected aeroplanes, who must receive the mandated one-time training in the specified Boeing Telex and implement manual fuelling procedures when the float switch is deactivated (Requirement 10.a-10.c).

Indirectly affected:

• Air operators’ commercial operations: If the centre tank float switch is deactivated or centre tank fuel capacity is conservatively reduced (Requirement 1.b.i and 1.b.ii require painting a ‘Caution’ showing a conservative maximum fuel capacity), aircraft range, payload and dispatch planning will be affected. Operators may need to adjust schedules, planning margins and possibly carry less cargo or fewer passengers on certain sectors.
• Logistics and supply chain: The AD contemplates replacement float switches and improved conduits; parts availability may influence operators’ ability to accomplish terminating actions. The Directive explicitly recognises that replacement parts may not be available and permits interim measures in that circumstance (Requirement 1.b and elsewhere).
• Training providers and compliance officers: One-time training per Boeing Telex M-7200-98-04486 is a required action prior to using manual fuelling procedures (Requirement 10.a). Operators must arrange this training and show compliance before manual procedures are used.
• Regulators and oversight bodies: CASA (the issuer) and any delegated authorities or persons responsible for continuing airworthiness must track amendments, compliance deadlines and inspect compliance evidence where applicable. The Directive confirms CASA’s procedural precedence over manufacturer materials and MMEL where differences occur.
• Aircraft lessors and financiers: For leased aircraft, lessors will be interested in whether modifications are permanent (the AD sets terminating action) and in any restrictions on ferry or dispatch; operators may need consent from lessors for hardware modifications or temporary deactivations that affect aircraft performance characteristics.
• Insurers: Although the AD does not reference insurance, insurers will be attentive to compliance with mandatory ADs as a condition of coverage for operations; failure to comply with mandatory directives typically has insurance implications in practice, though the Directive itself does not state penalties.

In-scope exemptions and constraints:

• The AD expressly excludes aeroplanes equipped with centre wing tank volumetric top-off systems or AC-powered centre tank float switches (Applicability). That narrows the affected fleet.
• The AD is limited to specific line numbers (1-3108 inclusive). Aircraft of the listed models with L/Ns above 3108 are outside the AD’s stated applicability.

Who pays:

• The Directive places the direct compliance cost burden on operators (inspections, replacements, training), and on maintenance organisations performing the work. The document does not allocate costs to manufacturers or the regulator. The compliance deadlines and prior-approval provisions (e.g., when replacement parts are not available) will shape who bears operational disruption costs such as reduced range, additional stops, or temporary grounding.

Who decides:

• The Directive prescribes the required methods and references (specific Boeing ASBs and telexes). CASA’s decision to issue and amend the AD determines the mandatory obligations, and CASA sets precedence over ASBs and MMEL where differences arise.

The Directive creates affirmative duties for operators and maintenance organisations and establishes specific technical requirements. It also creates limited conditional rights (permitted interim actions) subject to the AD’s constraints.

Key duties:

• Perform an initial detailed inspection of the centre tank float switch wiring by removing the fuelling float switch and following Boeing ASB 737-28A1132 (Requirement 1.a) prior to accumulating 30,000 total hours TIS or within 30 days from the effective date of the original Directive (Compliance section). The detailed inspection standard is given in the Directive (Note 2).
• If discrepancies (arcing, exposed conductor, fuel presence/scent, worn insulation) are detected in the inspection, follow the required remedial steps including conduit replacement and subsequent resistance checks or replacement as specified (Requirements 6-9).
• If no discrepancy is found, measure the resistance between the wires and float switch housing per ASB procedures (Requirement 2.a), and proceed with replacement if < 200 MΩ or cleaning/sleeving/reinstall if ≥ 200 MΩ (Requirements 3-4).
• Replace float switches, wiring and install conduit liners per ASB 737-28A1141 Revision 3 across centre and wing tanks within the timeframe set in Requirement 13 and the Compliance section (Requirement 13.a-13.c; Compliance: no later than 2 years from Amendment 1).
• For aircraft where the float switch is deactivated per Requirement 1.b, ensure fuelling crews are trained prior to using the manual fuelling procedures (Requirement 10.a), check fuelling-panel centre tank quantity indicators prior to fuelling and replace inoperative indicators before further flight if found inoperative (Requirement 10.b and 10.b.i), and use one of the two manual fuelling procedures for each fuelling occurrence (Requirement 10.c).
• Maintain the conservative centre tank fuel capacity marking on the underside of the right-hand wing near the fuelling station door and install an INOP placard on the fuelling panel where deactivation options are used (Requirement 1.b.i-ii).
• Replace any section of conduit where arcing or leaking occurred with new conduit in accordance with the ASB (Requirements 6-9, 12).
• Ensure the B-nuts on float switch cable conduit are torqued, bonding straps installed and lock wire applied as specified in ASB 737-28A1141 Revision 3 where prior replacements are relied upon to demonstrate compliance (Note 5).

Conditional permissions/rights:

• The AD allows operators to choose among several options for addressing the issue for particular aircraft, for example: removing the float switch and inspecting (1.a) or deactivating the float switch in specific ways (1.b options i-iii). This gives operators some choice over whether to pursue immediate hardware replacement or interim operational mitigations.
• If replacement float switches and wiring are not available, operators may accomplish the cutting/stowing deactivation options and follow the operational mitigations (Requirement 1.b; Requirement 11 permits dispatch with centre tank float switch deactivated until replacement parts are available).
• The AD allows prior work performed under earlier ASB revisions or in accordance with ASB 737-28A1141 Revisions 1 or 2 to be accepted provided specific conditions in Note 5 are met. This authorises recognition of some pre-existing work as compliant if the stated installation criteria are satisfied.

Constraints and mandatory prioritisation:

• Where there are differences between the Boeing ASB and this Directive, the Directive prevails; likewise it prevails over the MMEL where differences exist (explicit statements in the AD). This removes operators’ discretion to follow ASBs or MMEL contrary to the AD’s text.
• Requirement 15 is mandatory: a float switch having part number F8300-146 must not be installed on any aeroplane as of the effective date of Amendment 1. This is an absolute prohibition, not a conditional preference.

Technical thresholds and enforcement triggers:

• The 200 megohm resistance threshold in Requirements 2-4 functions as a binary duty trigger: below 200 MΩ, replacement is mandatory; at or above 200 MΩ, cleaning, sleeving and reinstallation are authorised. This provides a measurable compliance standard for technicians.
• Specific timelines in the Compliance section trigger mandatory action windows (e.g., many corrective actions are required prior to further flight; Requirement 13 must be done within two years of Amendment 1). The AD’s Compliance section makes those timing duties explicit.

Documentation and method duties:

• The AD requires carrying out actions “in accordance with” specific Boeing ASBs and their revisions, except where the AD expressly provides otherwise (Requirement 1.a, 1.b, 2, 13, etc.). This binds how work is to be performed via the referenced manufacturer procedures, albeit subject to the Directive’s precedence language.

What the Directive does not explicitly provide:

• The text does not include an express provision detailing recordkeeping obligations, enforcement sanctions or fines in the instrument itself. The AD makes actions mandatory but defers to CASA's regulatory framework for enforcement and oversight; the Directive does specify effectivity and compliance dates but does not itself state criminal or civil penalties.

The Directive itself does not set out specific statutory penalties in its text. It prescribes mandatory technical and procedural actions with specified compliance times and,by its nature as an Airworthiness Directive,creates binding continuing airworthiness obligations on operators of the affected aircraft. The text records effective dates and compliance windows: the Amendment 3 instrument becomes effective on 14 April 2005 (header) and the Compliance section gives precise timing for each Requirement (e.g., initial inspection prior to accumulation of 30,000 hours TIS or within 30 days from the effective date of the original Directive; many corrective actions are “prior to further flight”) and Requirement 13’s two-year compliance window from the effective date of Amendment 1 (Compliance section).

Enforcement mechanisms and likely consequences (as can be derived from the structure of the Directive and aviation regulatory practice, but not explicitly stated in the instrument):

• The AD imposes mandatory technical obligations and specific compliance deadlines. CASA-issued ADs typically form part of mandatory continuing airworthiness requirements; non‑compliance with an AD usually exposes operators to regulatory actions under the applicable aviation safety legislation and oversight regime. The Directive itself, however, does not specify which administrative, civil or criminal sanctions will follow non-compliance; the reader must look to enabling legislation and CASA enforcement policy for concrete penalty regimes.
• The AD explicitly states that where there are differences between its text and Boeing ASBs or MMEL, the Directive prevails. That means an inspector or regulator will expect compliance with the AD regardless of manufacturer or MMEL wording. Operationally, this precedence reduces ambiguity for inspectors and is a compliance enforcement lever.
• The AD uses common airworthiness enforcement levers: many corrective requirements are stated as “prior to further flight”, creating a direct operational enforcement trigger,if an aircraft is found non‑compliant, it would not be permitted to fly in normal regulatory practice until compliance is restored. The Directive itself does not spell out the mechanism for denying dispatch; however, “prior to further flight” obligations are standard in ADs to effectively ground affected aircraft for non-compliance unless interim permitted actions are in place.

Interaction with foreign authority actions:

• The AD references FAA AD 2004-15-04 and an FAA Alternative Method of Compliance (AMOC) letter (Note 6). That indicates coordination with FAA action; an FAA AD and an FAA AMOC are relevant to compliance and approvals in other jurisdictions. CASA’s AD takes precedence in Australia, but the presence of FAA material signals that other regulators have parallel or related requirements and that alternate compliance paths may be administered by the FAA (and possibly through bilateral arrangements), subject to CASA’s own acceptance frameworks. The Directive mentions an AMOC letter but does not itself endorse or adopt alternate methods; it makes an explicit allowance for alternate methods via references to ASB Revision 3 and the AMOC in the FAA materials.

Proof and audit:

• The AD requires specific technical measurements (e.g., resistance measurement at 200 MΩ threshold), documentation of inspections and replacement actions performed “in accordance with” designated Boeing ASBs. In practice, that will produce maintenance records that inspectors can audit; the Directive itself does not detail recordkeeping formats or retention periods; operators should follow standard continuing airworthiness recordkeeping requirements under CASA practice.

In short, the AD creates mandatory technical duties with enforceable timing triggers and defines acceptable methods and references, but its text does not specify particular penalties. Enforcement and sanction details are governed by the broader CASA legislative and regulatory framework, which is not included in the Directive text provided.

The Directive situates itself as a CASA-issued regulatory instrument that interfaces with manufacturer service documents, the MMEL, and foreign authority actions, but the text does not recite or reproduce upstream legislation establishing CASA’s power. From the Directive’s terms the following concrete interactions emerge:

• Precedence over manufacturer service bulletins: The Directive repeatedly states that where there are differences between Boeing ASB documentation and the Directive, the Directive prevails (Requirement language). Practically, this means obligations are not optional manufacturer recommendations; CASA’s AD imposes mandatory modifications and inspection regimes, and operators must follow the AD even if Boeing ASBs suggest different steps or timelines. The Directive nevertheless references specific ASBs and their revisions as the required method of accomplishment for many tasks, thereby folding manufacturer technical procedures into the regulatory obligation while maintaining CASA’s precedence where conflicts arise.

• Interaction with the MMEL: The Directive explicitly states that where there are differences between the MMEL and this Directive, the Directive prevails (Requirement 11). The MMEL typically governs dispatchability criteria for inoperative equipment; the AD overrides any MMEL provisions that would be inconsistent with its requirements, for example where the AD allows dispatch with the centre tank float switch deactivated only in accordance with specified ASB provisions and with mandated marking/placarding. The Directive therefore constrains the MMEL’s usual operational flexibility where public safety concerns are implicated.

• Reference to FAA materials and an AMOC: Note 6 references FAA AD 2004-15-04 and FAA AMOC letter 140S-04-194 dated 7 December 2004. This evidences a cross-jurisdictional technical alignment and suggests alternate means of compliance may exist under FAA processes, but the CASA AD prescribes its own accepted methods and timelines. The Directive does not itself adopt FAA AMOC terms; it notes the FAA materials as related references. Operators must comply with the CASA AD in Australia; any reliance on FAA AMOC would require CASA acceptance or bilateral recognition, which is not provided by this Directive text.

• Relation to Boeing ASBs and revisions: The AD requires work to be done in accordance with specific Boeing ASB numbers and revisions (e.g., ASB 737-28A1132 dated 2 December 1998 or Revisions 1 and 2, and ASB 737-28A1141 Revision 3 dated 9 December 2005). It limits acceptable ASB revisions after the effective date: “After the effective date of this Directive only Revision 2 may be used” (Requirement 1.a Note 2) for ASB 737-28A1132, and Note 4 states that after the effective date only a new, improved float switch and wiring may be installed. The Directive therefore both references and constrains manufacturer documentation as part of the regulatory regime.

• Implicit interaction with civil aviation safety law: While the Directive does not recite the enabling statute, CASA’s issuance of an AD presupposes statutory authority to mandate airworthiness actions. The Directive’s mandatory language, compliance deadlines and effective dates operate within that broader legal regime. The Directive does not itself enumerate sanctions; enforcement would be exercised under CASA’s general statutory powers and enforcement mechanisms.

• Interaction with operational safety management: By requiring training (Boeing Telex M-7200-98-04486) and pre-fuelling checks on quantity indicators, the AD interacts with operators’ flight and ground operations procedures, safety management systems and human factors processes. It imposes obligations on operations and ground maintenance functions, which typically sit within the scope of regulatory requirements for procedures and training.

• Recognition of earlier compliance work under conditions: Note 5 lists exact installation criteria that, if met, cause earlier replacements and installations to be considered acceptable for compliance with the AD. This creates an interaction between historical maintenance records and the AD compliance assessment: operators with prior work must demonstrate that the specified installation steps were completed (torque values, bonding strap installation, lock wiring).

In summary, the Directive operates as an instrument that integrates manufacturer technical material into a mandatory regulatory framework, supersedes conflicting manufacturer or MMEL provisions within Australia, references related FAA actions, and relies on CASA’s broader statutory enforcement apparatus (not detailed in the Directive). The AD’s practical interactions with other laws and documents arise from these precedence and referencing mechanisms rather than from any standalone statutory penalty framework included in the text.

The Directive records a clear amendment track in its Background and Compliance sections and in the header information. The chronological record in the document is as follows:

• Original issue: The original issue of the Directive became effective on 18 March 1999 (Background). The initial Directive set out the primary inspection and interim action requirements in response to reports of chafing and arcing in centre tank float switch wiring.

• Amendment 1: Amendment 1 of the Directive became effective on 20 January 2005 (Background). The text states that Amendment 1 introduced additional Requirements 12, 13, 14 and 15 and provided terminating action for the Directive. Specifically:

  • Requirement 12 addresses aeroplanes on which Requirement 6.b (the detailed visual inspection for fuel leaks of electrical conduit) had been done prior to the effective date of the Directive but where Requirement 8 replacement had not been completed; Requirement 12 requires replacement of any conduit sections where arcing/leaking occurred with new conduit in accordance with ASB 737-28A1132 Revision 2.
  • Requirement 13 sets out actions for all aeroplanes to replace existing centre fuel tank float switches with new improved float switches and install a conduit liner system, and to replace wing fuel tank float switches and conduit assemblies with new improved assemblies including a liner system, all in accordance with ASB 737-28A1141 Revision 3. It also specifies that such replacements and conduit replacements constitute terminating action for the AD.
  • Requirement 14 requires Model 737-200C series aeroplanes L/Ns 1 through 3108 to accomplish Requirements 1 to 9 inclusive, with a carve-out if Requirement 13 actions have been completed within the compliance time.
  • Requirement 15 prohibits installation of float switch part number F8300-146 on any aeroplane.

• Amendment 2: Amendment 2 became effective on 9 February 2005 (Background). The document states Amendment 2 contained some minor formatting and editorial changes, and introduced an alternate method of compliance. The AD text includes Note 5 which addresses acceptance of prior replacements performed before Amendment 2, subject to specific installation criteria; this note operates as an interaction point with earlier work.

• Amendment 3: Amendment 3 corrected a typographical error in Requirement 13.a and the Amendment 3 instrument (this current text) became effective on 14 April 2005 (header). The top of the document states “This Amendment becomes effective on 14 April 2005.” Amendment 3 also appears in the header where it lists “AD/B737/125 Amdt 3” and dates “4/2005.”

Other historical details:

• The Compliance section enumerates many time-based obligations that are tied back to the original Directive effective date and Amendment 1’s effective date (for example, Requirement 12’s replacement deadline prior to the accumulation of 1,500 flight hours or 6 months after the effective date of Amendment 1, whichever occurs first; and Requirement 13’s replacement deadline of no later than 2 years from the effective date of Amendment 1).
• Note 5 provides conditional acceptance criteria for replacements performed prior to Amendment 2, indicating CASA’s intent to recognise earlier compliance work where it meets specified technical installation standards.

The Amendment history shows a progression from initial inspection and interim measures (original AD) to specification of long-term hardware changes and terminating actions (Amendment 1), adjustment for alternate compliance and formatting (Amendment 2), and a minor typographical correction (Amendment 3). The AD records cross-references to FAA materials (FAA AD and AMOC letter) as relevant to the broader international regulatory context (Note 6).

The Directive text contains no litigation history, case references, judicial decisions or pending legal challenges. It references FAA AD 2004-15-04 and an FAA AMOC letter, but does not cite any court proceedings or interpretive rulings that address the AD’s validity or application.

Because the provided document does not include any litigation material, there are no cases named, no judicial interpretations of the directive’s provisions, and no recorded disputes over compliance within the text. Any litigation or enforcement proceedings arising from non‑compliance with the AD would be administered under the broader CASA enforcement framework and the general aviation safety legislation; those materials are outside the content of this Directive and therefore not discussed in the instrument.

Practically, operators seeking to challenge an AD’s requirements, or to obtain an alternative method of compliance or relief, would typically engage CASA’s administrative processes or seek judicial review under general administrative law channels. The AD itself does not describe an internal CASA appeal or review mechanism within its text.

The Directive contains several technical, procedural and drafting specifics that are easy to miss but critical for correct compliance. Below are concrete “gotchas” drawn directly from the AD text that practitioners and compliance teams should watch for:

• Narrow applicability window: The AD applies only to models 737-200, -200C, -300, -400 and -500 series aeroplanes having centre wing tanks activated and with line numbers 1-3108 inclusive (Applicability). Aircraft outside these line numbers or with AC-powered float switches or volumetric top-off systems are excluded. Misidentifying an aircraft’s line number or centre tank configuration could lead to unnecessary work or missed obligations.

• ASB revision constraints: The AD requires compliance in accordance with Boeing ASB 737-28A1132 dated 2 December 1998 or Revision 1 dated 15 January 1999 or Revision 2 dated 17 June 1999 for the initial inspection, but includes the explicit constraint “After the effective date of this Directive only Revision 2 may be used” (Requirement 1.a Note 2). This means technicians must use the specific ASB revision called up by the AD for the task. Work done to the wrong ASB revision may be non‑compliant.

• Resistance threshold unit clarity and measurement: The Directive sets a threshold of 200 megohms for insulation resistance checks (Requirement 2.a, Requirement 3 and Requirement 4). The text uses “200 megohms” in some places and a truncated “200” without unit in others; the substantive requirement is the 200 megohm threshold. Technicians must ensure correct measurement units and instrumentation calibrated to measure megaohms, and must record the measured value to apply the correct remediation path.

• Interim deactivation requires operational controls and marking: If the float switch is deactivated by cutting/stowing the wires, the AD requires painting a ‘Caution’ showing a conservative maximum fuel capacity for the centre tank on the underside of the right-hand wing near the fuelling station door and installing an ‘INOP’ placard on the fuelling panel (Requirement 1.b.i). Operators must ensure these physical markings are applied correctly and are visible during fuelling operations.

• Training is mandatory and one-time before manual fuelling: Requirement 10.a mandates a one-time training of fuelling crews per Boeing Telex M-7200-98-04486 dated 1 December 1998, to be accomplished prior to utilising the manual fuelling procedures. Failure to complete this training before applying manual procedures would be non‑compliant.

• Quantity-indicator operability requirement before fuelling: Requirement 10.b requires that prior to fuelling the aeroplane, a check to verify that the fuelling panel centre tank quantity indicator is operative must be done. If inoperative, it must be replaced prior to further flight (Requirement 10.b.i). Operators must not only do manual fuelling training but also ensure instrumentation used in fuelling is serviceable.

• Dispatch while deactivated is only conditionally allowed: Dispatch with the centre tank float switch deactivated in accordance with ASB 737-28A1132 or Revision 1 is allowed until replacement parts are available (Requirement 11). However, where differences exist between MMEL and the Directive, the Directive prevails. That removes an operator’s freedom to rely on an MMEL permissibility that conflicts with the AD.

• Repetitive inspection interval and terminating action: The AD requires re-inspection at intervals not to exceed 1,500 hours TIS (Requirement 7) until the replacement specified by Requirement 8 is accomplished. Replacement per Requirement 13 (installation of new improved float switches and conduit liner systems) is terminating action. Operators must track 1,500-hour intervals carefully to avoid lapse into non-compliance when conduit replacement is delayed.

• Prohibition on specific part number: Requirement 15 bans installation of float switch part number F8300-146. This absolute prohibition is easy to miss in a long AD but is fatal to compliance if inadvertently installed.

• Conditions for accepting prior work: Note 5 contains three specific installation conditions that must be met for prior replacements and liner installations to be accepted as compliant: torque values for B-nuts per ASB 737-28A1141 Revision 3; installation/secure fastening of the bonding strap; and installation of lock wire in the bolt heads on the front spar as specified. Operators relying on earlier work must be able to evidence these three specific installation conditions.

• Variants and carve-outs for Model 737-200C: Requirement 14 has operational nuances for Model 737-200C series L/Ns 1-3108 inclusive,if Requirements 1-9 have been accomplished pre-effective date, no further action is necessary; conversely, if Requirement 13’s actions are accomplished within compliance time, Requirements 1-9 need not be done. These substitution rules must be applied correctly based on actual work already accomplished.

• Timing traps: The Compliance section contains multiple timing triggers that vary by Requirement. For example, initial inspection timing is “prior to the accumulation of 30 000 hours total time in service or within 30 days from the effective date of the original Directive (18 March 1999), whichever occurs later,” whereas many remedies require action “prior to further flight.” Requirement 12 has a unique window: “prior to the accumulation of 1500 flight hours or 6 months after the effective date of Amendment 1 to this Directive, whichever occurs first.” Operators must apply the correct timeline to each Requirement.

• Differences between ASB and Directive: The AD repeatedly states the Directive prevails over ASBs where there are differences. That means an operator cannot unilaterally rely on an ASB detail that conflicts with the AD text,even if the ASB appears more practical or clearer. Compliance plans must be drafted around the AD’s text first.

• Reference to FAA AMOC: The AD refers to FAA AD and an FAA AMOC letter (Note 6). Operators may be tempted to assume an FAA AMOC automatically provides relief in Australia; the AD does not grant that relief. Any alternate method of compliance will require CASA acceptance.

• Documentation and proof: The AD requires that actions be accomplished “in accordance with” specified ASBs and telexes. However, the AD itself does not state record-keeping details. Operators must retain sufficient maintenance evidence to show compliance with the AD and the ASBs’ Accomplishment Instructions; failing to keep traceable records risks regulatory non‑compliance even if the physical work was done.

This section sets out a practical compliance blueprint derived strictly from the AD text. Operators and maintenance organisations should map these steps to their maintenance control processes, scheduling and training systems:

1. Confirm applicability and aircraft identity

   • Verify the aeroplane model (737-200, -200C, -300, -400, -500), confirm that the centre wing tank is activated, and verify the aircraft line number is within 1-3108 inclusive. If the aircraft has a centre wing tank volumetric top-off system or AC-powered centre tank float switch, the Directive is not applicable (Applicability).

2. Schedule and conduct the initial detailed inspection

   • If applicable, ensure the initial detailed inspection under Requirement 1.a is scheduled to meet the compliance window: prior to accumulating 30,000 hours total time in service or within 30 days from the original Directive effective date (18 March 1999), whichever occurs later (Compliance).
   • Perform the inspection per Boeing ASB 737-28A1132 Revision 2 (after the Directive’s effective date only Revision 2 may be used), following the AD’s definition of “detailed inspection” (Requirement 1.a and Note 2). Remove the fuelling float switch and inspect wiring for arcing evidence, exposed copper conductor, presence/scent of fuel on wiring, or worn insulation.

3. Triage based on inspection findings

   • If discrepancies found (arcing, exposed conductor, presence/scent of fuel, worn insulation):

     • Follow Requirement 6-9 timelines and actions. Replace conduit sections where arcing occurred in accordance with the ASB (Requirement 6.a), or perform a detailed visual inspection for fuel leaks per the ASB (Requirement 6.b). If a fuel leak is identified, replace the damaged conduit section and immediately accomplish the resistance test or remediation per Requirements 8 and 2.b or 2.a (Requirement 8 and 9).
     • Repetitive fuel-leak inspections per Requirement 7 may be required until terminating action is completed.

   • If no discrepancy found:

     • Measure the resistance between wires and float switch housing per ASB (Requirement 2.a). Use instrumentation capable of reading in the megaohm range and record readings.
     • If measured resistance < 200 MΩ, replace the float switch and wiring with a new float switch and install double Teflon sleeving, or replace with the new improved assembly per ASB 737-28A1141 Revision 3 if available (Requirement 3).
     • If measured resistance ≥ 200 MΩ, blow dirt out of the conduit, install double Teflon sleeving over the wiring and reinstall the existing float switch (Requirement 4).

4. Implement interim deactivation where replacement not available

   • If replacement float switches/wiring are not available, operators may deactivate the centre tank float switch by cutting the two wires at the splices on the front spar, capping and stowing the four wire ends, painting a ‘Caution’ that shows a conservative maximum centre tank fuel capacity on the underside of the right-hand wing near the fuelling station door, and installing an ‘INOP’ placard on the fuelling panel (Requirement 1.b.i).
   • Alternatively, cut, stow and splice the two wires at the splices on the front spar, paint the caution marking and install the INOP placard (Requirement 1.b.ii).
   • As another alternative, accomplish the pressure fuelling system inspection/modification per ASB 737-28A1141 Revision 3 (Requirement 1.b.iii).

5. Train fuelling crews and implement manual fuelling procedures

   • Before using manual fuelling procedures associated with float switch deactivation, ensure fuelling crews receive one‑time training per Boeing Telex M-7200-98-04486 dated 1 December 1998 (Requirement 10.a).
   • Implement a pre-fuelling check that the fuelling panel centre tank quantity indicator is operative; if it is inoperative, replace it with a serviceable part prior to further flight (Requirement 10.b and 10.b.i).
   • Require that one of the two Boeing Telex manual fuelling procedures is used for each fuelling occurrence where manual fuelling is relied upon (Requirement 10.c).

6. Plan for conduit replacements and terminating action

   • Replace any conduit sections where arcing or leaking occurred per ASB 737-28A1132 Revision 2 where required (Requirement 12). Note that such replacement constitutes terminating action for the repetitive inspections required by Requirement 7.
   • For all aeroplanes, plan and budget to replace existing centre fuel tank float switches and install conduit liner systems, and to replace wing fuel tank float switches and conduit assemblies with improved assemblies per ASB 737-28A1141 Revision 3 (Requirement 13.a-13.b). Accomplish these replacements no later than two years from the effective date of Amendment 1 to the Directive (Compliance).
   • Where prior replacements or installations were done before Amendment 2, verify and document that B-nuts were torqued as required, the bonding strap was installed/fastened, and lock wire was installed in front spar bolt heads as specified in ASB 737-28A1141 Revision 3 to rely on Note 5 acceptance.

7. Maintain inspection cycles until terminating action

   • If the conduit in the centre fuel tank has not been replaced as specified, maintain the 1,500 hours time-in-service inspection interval for conduit/fuel-leak checks (Requirement 7) and log the intervals carefully to ensure compliance until the AD’s terminating action (Requirement 13) is completed.

8. Prohibit certain parts

   • Ensure procurement and parts control processes block the installation of float switch part number F8300-146 (Requirement 15). Remove any such parts from inventory for use in affected aeroplanes.

9. Record-keeping and evidence

   • Although the AD does not prescribe record-keeping formats, maintain detailed maintenance records showing the date, identity of the technician, ASB revision used, resistance measurement values, descriptions of any replaced components, lock-wire installation, torque values, conduit liner installation and training records for fuelling personnel. These records will be needed to demonstrate that work was performed in accordance with ASBs and/or the Directive and to show reliance on Note 5 where applicable.

10. Coordinate with regulators and document alternative compliance attempts

    • If there is a need to pursue an alternative method of compliance (AMOC), document the proposed AMOC and engage CASA early; Note 6 shows related FAA AMOC activity, but acceptance by CASA is not automatic under the AD. Work that deviates from the AD or the specified ASBs should be preceded by formal approval.

11. Logistics and operational planning

    • Anticipate potential operational consequences while float switches are deactivated or while conservative centre tank fuel capacities are in force (Requirement 1.b.i-ii). Update flight planning, dispatcher guidance, fuel planning margins and customer communications accordingly.
    • Consider parts lead times for float switches, wiring, and conduit liner systems, and build replacement scheduling to meet terminating action windows. Where parts are unavailable and interim deactivation is used, ensure training and procedural checks are in place and maintained.

12. Verify compliance with MMEL interactions

    • Because the AD prevails over the MMEL where differences exist (Requirement 11), confirm that any reliance on MMEL provisions is consistent with AD requirements. Update operational manuals and MMEL applications accordingly.

13. Internal audit and assurance

    • Perform an internal compliance audit after initial implementation to ensure inspections, resistance measurements, replacements, training and marking have been completed per the AD. Maintain evidence to show that where prior work is relied upon for compliance under Note 5, the specific torque, bonding strap, and lock wire requirements are documented.

Following these concrete steps, and mapping them into an operator’s maintenance control system and compliance checklists, will align actions with the AD’s explicit requirements. Operators should ensure maintenance personnel are working to the specified ASB revisions cited by the AD, document all measurements and remedial steps, and plan for the AD’s terminating actions within the stated compliance windows.