AC 120-97B

U .S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Incorporation of Fuel Tank System Instructions for Continued Airworthiness into Operator Maintenance and/or Inspection Programs Date: 9/29/26 AC No: 120-97B Initiated by: AFS-300 Change:

On May 7, 2001, the Transport Airplane Fuel Tank System Design Review, Flammability Reduction and Maintenance and Inspection Requirements Final Rule was published in the Federal Register (FR). It has since been referred to as the 2001 Fuel Tank Safety (FTS) rule. It adopted amendments to Title 14 of the Code of Federal Regulations (14 CFR) part 25, Special Federal Aviation Regulation (SFAR) 88, and operating requirements related to SFAR 88. The operating requirements included requirements to implement instructions for continued airworthiness (ICA) that design approval holders (DAH) developed in compliance with SFAR 88. On November 8, 2007, the Federal Aviation Administration (FAA) published the Enhanced Airworthiness Program for Airplane Systems (EAPAS)/FTS Final Rule. The purpose of the rule is to help ensure the continued safety of transport category airplanes by improving the design, installation, and maintenance of electrical wiring systems. The EAPAS/FTS Rule amended the operating requirements to implement FTS actions developed in accordance with SFAR 88. Integrating the incorporation of the fuel tank system and electrical wiring interconnection system (EWIS) requirements helps to ensure compatibility and to eliminate duplication. Additionally, the EAPAS/FTS Rule redesignates (replaces) 14 CFR §§ 91.410(b),

121.370(b), 125.248(b), and 129.32(b) of the FTS Rule. The new sections are §§ 91.1507,

121.1113, 125.507 and 129.113. These new rules also clarify language with reference to the approval process of the operator’s program. This advisory circular (AC) only addresses the fuel tank system safety requirements and describes acceptable means of compliance (AMC) accordingly. AC 120-102, Incorporation of Electrical Wiring Interconnection Systems Instructions for Continued Airworthiness into an Operator’s Maintenance Program, addresses EWIS requirements in the EAPAS/FTS Rule.

Tim othy R. Adams for Hugh J. Thomas Executive Director, Flight Standards Service

ii

CONTENTS Paragraph Page

Chapter 1. General1-1

1.1 Purpose of This Advisory Circular (AC)1-1

1.2 Audience1-1

1.3 Where You Can Find This AC1-1

1.4 What This AC Cancels1-1

1.5 Background1-2

1.6 Related CFRs1-2

1.7 Related ACs (current editions)1-3

1.8 FAA Fuel Tank System Policy Statement1-3

1.9 Industry Reference1-4

1.10 The EAPAS/FTS Rule1-4

1.11 AC Feedback Form1-5

Chapter 2. Fuel Tank System Requirements2-1

2.1 Requirements2-1

2.2 Unsafe Condition2-1

2.3 No Unsafe Condition2-1

2.4 Field-Approved Auxiliary Fuel Tanks2-3

2.5 STC Auxiliary Fuel Tanks2-3

2.6 Applicable FTS STCs2-3

2.7 History of Fuel Tank System Maintenance Practices2-3

Chapter 3. Design Maintenance and Inspection3-1

3.1 Airplane Fuel Tank System3-1

3.2 DAH Fuel Tank Alterations3-1

3.3 Changes to Operators’ Fuel Tank System Tasks, Intervals,

and Instructions/Procedures3-1

3.4 Fu

el Tank System Airworthiness Limitations3-2

3.5 AD Requirements3-5

Table 3-1. Airworthiness Limitation Table Example 1—ALI3-6

Table 3-2. Airworthiness Limitation Table Example 2—CDCCL3-6

3.6 Operator Compliance with Airworthiness Limitations3-7

iii

3.7 Operator-Developed Shop Instructions3-8

3.8 Exceptional Short-Term Extension of Fuel Tank System

Airworthiness Limitations3-8

3.9 Changes in ICA Resulting from Alterations3-9

3.10 Fuel Tank System Ignition Prevention Training3-9

3.11 Operator’s Maintenance Program Approval Under 14 CFR §§ 121.1113

and 129.1133-10

3.12 Operator’s Inspection Program Approval Under 14 CFR § 125.5073-10

3.13 Operator Inspection Program Approval Under 14 CFR § 91.15073-10

3.14 Operators that are Authorized OPSPEC/MSPEC/LOA D0973-11

Table 3-3. Sample D097 Table 1–Aging Aircraft Maintenance Programs3-11

Appendix A. AcronymsA-1

Appendix B. List of Applicable Supplemental Type Certificates (STC)B-1

Appendix C. DefinitionsC-1

Appendix D. Aircraft Certification Service (AIR) Offices by Airplane ManufacturerD-1

1-1 CHAPTER 1. GENERAL

1.1 Purpose of This Advisory Circular (AC). This AC provides information regarding compliance with the fuel tank system maintenance and inspection program requirements in Title 14 of the Code of Federal Regulations (14 CFR) for parts 91 subpart K (part 91K), 121, 125, and § 129.14 foreign air carriers hereafter referred to as operators.

Instructions for continued airworthiness (ICA) (see Appendix C, Definitions) include fuel tank system inspection tasks, intervals, methods, instructions/procedures, airworthiness limitations, and fuel tank system critical design configuration control limitations (CDCCL), which are a specific type of airworthiness limitation.

Note: This is a guidance document. Its content is not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty.

Conformity with the guidance document is voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.

1.2 Audience. This AC applies to operators of transport category turbine-powered airplanes with a type certificate (TC) issued after January 1, 1958, that, as a result of original type certification or later increase in capacity, have a maximum type-certificated passenger capacity of 30 or more, or a maximum payload capacity of 7,500 pounds or more, and transport category airplanes that include 14 CFR § 25.981, Amendment 25-102 or later, in the original certification basis of the airplane or in the certification basis of modifications to the airplane.

1.3 Where You Can Find This AC. You can find this AC on the Federal Aviation Administration’s (FAA) website at https://www.faa.gov/regulations_policies/advisory_cir culars and the Dynamic Regulatory System (DRS) at https://drs.faa.gov.

1.4 What This AC Cancels. AC 120-97A, Incorporation of Fuel Tank System Instructions for Continued Airworthiness into Operator Maintenance or Inspection Programs, dated April 11, 2017, is canceled.

1.4.1 Avoiding Compliance. In the past, some designers and operators avoided applying requirements for airplanes over a specific capacity by getting a design change approval for a slightly lower capacity. Referencing the capacity resulting from original certification removes this means of avoiding compliance. Also, an airplane design could be originally certified with a capacity slightly lower than the minimum specified in the rule, but through later design changes, the capacity could be increased above this minimum. The reference to later increase in capacity in the rule ensures that, if this occurs, the airplane would have to meet the requirements of the rule.

1.4.2 Aircraft Certification Service Office. The office having oversight responsibility for the relevant TC or Supplemental Type Certificate (STC), as determined by the Administrator. (See Appendix D, Aircraft Certification Service (AIR) Offices by Airplane Manufacturer.)

1-2

1.4.3 Required ICA. Operators are required to incorporate ICA into their maintenance or inspection program that are required by Special Federal Aviation Regulations (SFAR) 88 or 14 CFR §§ 25.1529, 25.1729, 26.11, and part 25 appendix H, and that have been approved by the responsible Aircraft Certification Service office. Operators are not required to use the design approval holder’s (DAH) fuel tank ICA. While we think it is likely that operators will use the DAH ICA, we acknowledge that they are able develop their own or to contract with third parties to develop them. Operator developed ICA must be approved by the responsible Aircraft Certification Service office.

1.5 Background. Since 1959, there have been at least 18 fuel tank explosions on transport category airplanes. Most notably, on July 17, 1996, a 25-year-old Boeing 747-100 series airplane, operating as Trans World Airlines (TWA) Flight 800, was involved in an in-flight breakup after takeoff from John F. Kennedy International Airport in New York, resulting in 230 fatalities. The National Transportation Safety Board (NTSB) determined the probable cause of the TWA 800 accident was an explosion of the center wing fuel tank (CWT) due to ignition of the flammable fuel vapor and air mixture in the tank. The source of ignition energy for the explosion could not be determined conclusively, though the NTSB determined that it was most likely a combination of failures involving the wiring entering the fuel tank, such as the fuel quantity indicating system (FQIS). FQIS or other wires entering the fuel tank (such as fuel level sensor systems (FLSS), float switch, etc.) are designed to carry “intrinsically safe” levels of electrical energy inside fuel tanks.

Without an additional failure, a short of FQIS or other wires entering the tank would not have sufficient energy to create an ignition source. The NTSB determined the most likely ignition source was an existing FQIS failure inside the CWT and a FQIS wiring failure outside the CWT. The internal failure created the condition for an ignition source if high electrical energy was applied to the FQIS wiring inside the CWT. The external failure applied excessive electrical energy on the FQIS or other wires that enter the CWT.

1.6 Related CFRs. The following 14 CFR parts and sections apply:

• Part 21, Special Federal Aviation Regulation No. 88—Fuel Tank System Fault Tolerance Evaluation Requirements.

• Part 25, Airworthiness Standards: Transport Category Airplanes.

• Section 25.981, Fuel Tank Explosion Prevention.

• Section 25.1529, Instructions for Continued Airworthiness.

• Part 25 Appendix H, Instructions for Continued Airworthiness.

• Part 26, Continued Airworthiness and Safety Improvements for Transport Category Airplanes.

• Section 43.13, Performance Rules (General).

• Section 43.16, Airworthiness Limitations.

• Section 91.403(c), General.

• Section 91.1507, Fuel Tank System Inspection Program.

1- 3

• Section 121.1113, Fuel Tank System Maintenance Program.

• Section 125.507, Fuel Tank System Inspection Program.

• Section 129.113, Fuel Tank System Maintenance Program.

1.7 Related ACs (current editions). The ACs listed below provide information that may support the method of compliance established by this AC. An electronic copy of the following ACs can be downloaded at https://drs.faa.gov/browse/AC/doctypeDetails. DRS is a set of searchable databases that contain regulatory, guidance, and aviation product information. DRS contains certain CFRs and SFARs from 14 CFR in their current version as well as historical versions. A paper copy may be ordered from the U.S.

Department of Transportation (DOT), Subsequent Distribution Office, M30, Ardmore East Business Center, 3341 75th Avenue, Landover, MD 20785.

• AC 25-8, Auxiliary Fuel System Installations.

• AC 25-19, Certification Maintenance Requirements.

• AC 25.981-1, Fuel Tank Ignition Source Prevention Guidelines.

• AC 25.981-2, Fuel Tank Flammability Reduction Means.

• AC 120-16, Air Carrier Maintenance Programs.

• AC 120-102, Incorporation of Electrical Wiring Interconnection Systems Instructions for Continued Airworthiness into an Operator’s Maintenance Program.

• AC 121-22, Maintenance Review Boards, Maintenance Type Boards, and Original Equipment Manufacturer/Type Certificate Holder Recommended Maintenance Procedures.

1.8 FAA Fuel Tank System Policy Statement. The FAA’s Aircraft Certification Service office, in coordination with Flight Standards (FS), other National Aviation Authorities (NAA), and various industry representatives, developed FAA policy memorandum ANM112-05-001, Policy Statement on Process for Developing SFAR 88-related Instructions for Maintenance and Inspection of Fuel Tank Systems, dated October 6, 2004. This policy statement provided standardized guidance to the DAH for developing maintenance instructions as a method of compliance with SFAR 88.

1.8.1 Processes for Developing ICA. The policy identified two categories of potential ignition source-related ICA and provided two processes for developing those ICA:

1.8.1.1 A safety critical action that is needed to address unsafe conditions is an airworthiness limitation item (ALI). ALIs are maintenance and inspection instructions that must be performed at the listed intervals and are adopted using the Airworthiness Directive (AD) process. That process, used for SFAR 88, is described in part 5 of the policy statement.

1.8.1.2 The remaining actions (that do not address unsafe conditions but are necessary to maintain the continued airworthiness of the ignition source prevention

1-4 features of the design) were to be evaluated using a process based on the principles of Maintenance Steering Group - 3rd Task Force (MSG-3). That process is described in part 6 of the policy statement.

1.8.2 Categories of SFAR 88 ICA. These two categories of SFAR 88 ICA have been referred to as “Unsafe Condition” and “No Unsafe Condition” related ICA. The “Unsafe Condition” ICA are incorporated in the applicable airplane Airworthiness Limitations Section (ALS) mandated by AD. The “No Unsafe Condition” ICA are identified in the DAH maintenance documents under Air Transport Association of America (ATA) iSpec 2200, ATA 28 for the airplane.

1.8.3 Design Review. The policy could also be useful to an operator in performing a design review of any field-approved auxiliary fuel tank installations (see paragraph 2.4).

1.8.4 Electronic Copy. An electronic copy of the policy letter can be downloaded at https://drs.faa.gov/browse/AIR_POLICY/doctypeDetails.

1.9 Industry Reference. The ATA MSG-3, Operator/Manufacturer Scheduled Maintenance Program Development, and later revisions.

1.10 The EAPAS/FTS Rule. The EAPAS/FTS Rule (72 Federal Register (FR) 63364 effective December 10, 2007) includes revisions to the operational requirements that were originally adopted by the 2001 FTS Rule (66 FR 23085, effective June 6, 2001).

This AC describes a method of compliance with these revised FTS operational requirements.

1.10.1 The 2001 FTS Rule. On June 6, 2001, the Transport Airplane Fuel Tank System Design Review, Flammability Reduction and Maintenance and Inspection Requirements Rule became effective. It is referred to as the 2001 FTS Rule. It amended 14 CFR part 21 and created SFAR 88. SFAR 88 required DAH to conduct a safety review of the applicable airplanes identified in the rule to determine that their design meets the FTS ignition requirements described in 14 CFR §§ 25.901 and 25.981(a) and (d). If their design was determined to not meet those requirements, they were required to develop design changes for those non-compliant designs that were determined to meet the unsafe condition criteria using FAA policy memorandum 2003-112-15, SFAR 88—Mandatory Action Decision Criteria. The FAA is issuing ADs mandating those design changes.

1.10.2 Design Changes. Operators must comply with the DAH and Aircraft Certification Service office approved design changes in accordance with the respective ADs. In some instances, an interim repetitive maintenance or inspection task is required on specific airplanes allowing the operator additional time to accomplish the design change.

1.10.3 Compliance Dates. These fuel tank system design change ADs, and any associated repetitive maintenance and inspection tasks, have varying compliance dates. The operator must track these dates using its currently established AD tracking system.

1.10.4 ICA for Preventing an Ignition Source. SFAR 88 also requires the DAH to develop specific ICA needed to maintain design features that prevent the existence or

1-5 development of an ignition source within the fuel tank, which could be accomplished using policy memorandum ANM112-05-001. The operator must incorporate Aircraft Certification Service office-approved ICA into their maintenance or inspection program by the dates specified in the operational rule or AD. The actions specified in the ICA must be accomplished in accordance with the times specified in the ICA.

1.11 AC Feedback Form. For your convenience, the AC Feedback Form is the last page of this AC. Note any deficiencies found, clarifications needed, or suggested improvements regarding the contents of this AC on the Feedback Form.

2- 1 CHAPTER 2. FUEL TANK SYSTEM REQUIREMENTS

2.1 Requirements.

2.1.1 Fuel Tank System Program Requirements. This AC provides information to support operators’ compliance with the fuel tank system maintenance and inspection program requirements contained in the EAPAS/FTS Rule. The operational rules contained in 14 CFR §§ 91.1507, 121.1113, 125.507, and 129.113 require operators to incorporate Aircraft Certification Service office-approved ICA into their maintenance or inspection program.

2.1.2 Types of ICA. In accordance with policy memorandum ANM112-05-001, DAH identified two categories of maintenance and inspection instructions. For the purposes of this guidance, we have simplified the descriptions of these types of ICA and have provided two classifications: “unsafe condition” and “no unsafe condition” as defined below:

2.1.2.1 Unsafe Condition. Those that are directly related to an unsafe condition and require mandatory action.

2.1.2.2 No Unsafe Condition. Those that do not have a direct adverse effect on operational safety, but for which developing maintenance inspections, certain standard practices, or procedural warnings can reduce the potential for an ignit ion source.

Note: See paragraph 1.9 for reference to ANM112-05-001.

2.2 Unsafe Condition. The DAH are required to develop design changes and ICA to address features of the fuel tank system design that are determined to meet the SFAR 88 unsafe condition criteria, and mandated by AD. These AD-mandated actions could include design changes, operational procedures, or ALIs containing repetitive maintenance and inspection instructions, or all three. FTS airworthiness limitations include CDCCL, inspections, or other procedures that are necessary to prevent development of ignition sources within the fuel tank system. CDCCLs are a type of fuel tank system airworthiness limitation introduced by 14 CFR § 25.981(d) (see paragraph 3.4 for a detailed description of CDCCLs).

2.3 No Unsafe Condition. The DAH were also required to develop ICA for all safety-related Failure Effect Category (FEC) 5 and 8 tasks developed from analysis of Maintenance Significant Items (MSI) that include those fuel tank system design features. These tasks address those features of the fuel tank system design that are determined to be a no unsafe condition but are necessary to maintain the continued airworthiness of the ignition source prevention features of the fuel tank system. The DAH, through the Maintenance Review Board (MRB) process, subjects these MSIs to maintenance program development logic using the latest version of MSG-3. Inspection tasks, intervals, and task instructions/procedures are developed as a result of this process. They are Aircraft Certification Service office-approved. They will normally be found in the TC holder’s Maintenance Review Board Report (MRBR) or a source document required by 14 CFR

2- 2 part 25 appendix H, § H25.5(b). AC 120-102, Incorporation of Electrical Wiring Interconnection Systems Instructions for Continued Airworthiness into an Operator’s Maintenance Program, provides information on the “source document.” The STC ICA will be contained in the FAA-approved STC document.

Note: For administrative purposes, some DAH have put both the “no unsafe condition” and the “unsafe condition” category ICA in their ALS mandated by an AD. An operator’s AD tracking system must ensure it does not make a distinction between categories such that one is tracked differently from the other. Both are mandated by an AD and require the same level of visibility and attention.

Changes to either category must be approved by the responsible Aircraft Certification Service office as an alternative method of compliance (AMOC) to the AD.

2.3.1 Operators Without MSG-3 Programs. The FAA recognizes that some operators may not have MSG-3 maintenance programs. Those operators must still incorporate the FAA-approved fuel tank system ICA into their maintenance or inspection program.

2.3.2 Inspection Tasks, Intervals, and Task Instructions/Procedures. The inspection tasks, intervals, and task instructions/procedures are selected to ensure that fuel tank system protective features are properly maintained while still allowing flexibility for the operators to schedule maintenance tasks within their maintenance or inspection programs.

The DAH will make these inspection tasks, intervals, and task instructions/procedures available to the operators for incorporation into their maintenance or inspection programs in the following manner.

2.3.3 FTS ICA or FTS/Electrical Wiring Interconnection System (EWIS) ICA. The fuel tank systems ICA are identified as FTS ICA, or if the FTS ICA is also part of a EWIS ICA it may be identified as FTS/EWIS ICA. Both are Aircraft Certification Service office-approved. AC 120-102 provides additional information on the FTS/EWIS ICA.

The DAH will sometimes identify a separate cross-referencing document where these fuel tank systems ICA are located. While the FAA expects that the DAH fuel tank system ICA tasks will be located in ATA, Chapter 28, Fuel Systems, some may also be located in other ATA chapters such as Chapter 20, Standard Practices.

Note: The procedures to carry out the inspections such as access, equipment removal, etc., should be contained in the DAH Aircraft Maintenance Manual (AMM) as well as be included in their applicable job/work cards.

2.3.4 FTS ICA. In accordance with the operational rules, operators must include fuel tank system ICA in their maintenance or inspection program. Operators should have a tracking system within their maintenance or inspection program so that they do not lose their identity as an FTS ICA. Operator changes to FTS ICA must be in accordance with the guidance contained in this AC. The operator should submit the data that supports any changes to approved ICA through the principal inspector (PI), who may add comments and then forward them to the responsible Aircraft Certification Service office for approval.

2-3

2.3.5 Specific Instructions. The DAH no unsafe condition ICA will typically be in the form of inspection tasks and intervals and task instructions/procedures. They may also be specific instructions that are referenced in a task but are contained in other manuals such as the AMM, the airplane standard wiring practice manual (SWPM), or in the STC holder’s manual that contains the ICA for the STC. These DAH ICA are approved by the responsible Aircraft Certification Service office. The operator will submit to the PI for review the revisions to their maintenance and inspection program incorporating the Aircraft Certification Service office-approved DAH ICA changes. When the review is complete, the PI will issue or amend Operations Specification (OpSpec)/Management Specification (MSpec) D097, Aging Aircraft Programs.

2.4 Field-Approved Auxiliary Fuel Tanks. Per paragraph (b) of the operational rules (refer to 14 CFR §§ 91.1507, 121.1113, 125.507, and 129.113) if an operator has any of these tanks installed in their airplanes and chooses to continue operation with them installed, the operator must perform a design review and develop ICA in accordance with the design review requirements detailed in SFAR 88. The operator, being the DAH for these tanks, is in the best position to develop ICA for them. The operator should submit proposed field-approved auxiliary tank data through their PI to the Aircraft Certification Service office responsible for approving their airplane type. After Aircraft Certification Service office approval, the operator will submit to the PI for review the revisions to their maintenance and inspection program incorporating the Aircraft Certification Service office-approved ICA changes. When the review is complete, the PI will issue or amend OpSpec/MSpec D097. In the event the operator chooses to deactivate or remove a field-approved auxiliary tank, such action can only be accomplished using approved data.

2.5 STC Auxiliary Fuel Tanks. According to the operational rules, if an operator has STC auxiliary fuel tanks installed in their airplanes, they must incorporate ICA meeting SFAR 88, if any, into their maintenance or inspection program. If the FAA determines any particular STC auxiliary tank installations pose a safety risk, the FAA will issue ADs mandating such tanks be deactivated or removed in accordance with data submitted by the operators and approved in accordance with the AD.

2.6 Applicable FTS STCs. In order for an operator to incorporate all the applicable STC ICA they must first establish what STCs are installed on their airplanes. An FAA engineering approved list of applicable STCs is provided in Appendix B, List of Applicable Supplemental Type Certificates. The operators should review this list and determine if any of the applicable STCs are installed on their airplanes. Under normal circumstances a records review by the operators should be sufficient to determine which of their airplanes has any of the applicable STCs. If records are inadequate, it may be necessary to physically inspect airplanes to confirm applicable STC installation;

however, this should not entail physically checking the routing of wiring, etc. The operator should provide the PI a list of airplanes that have any of the applicable STCs installed and present their proposed STC ICA for review and approval. (See paragraphs 3.10–3.14 for the review and approval process.)

2.7 History of Fuel Tank System Maintenance Practices. Most fuel tank system maintenance involves zonal inspections to determine the condition of units or systems

2-4 with regard to continued serviceability. Corrective action is taken only when indicated by the condition of a particular unit or system. The most common type of zonal inspection for certain components of fuel tank systems is a general visual inspection (e.g., an examination of an interior or exterior area, installation, or assembly to detect obvious damage, failure or irregularity). Typically, operators conducted these general visual inspections as part of other zonal inspections of the fuel tanks. A limitation of a general visual inspection of the fuel tank system is that often the inspection does not provide sufficient information to determine continued airworthiness of internal or hidden system components. This is because certain degraded conditions or failures are difficult, or even impossible, to detect without extensive and detailed inspection or functional checks.

Examples of such degraded conditions or failures are worn wiring routed through conduit to fuel pumps, accumulated debris inside fuel pumps, corrosion of bonding wire interfaces, and broken or missing bonding straps. As a result of the FTS review, certain general visual inspections of the fuel tank system under the zonal inspection concept may become detailed inspections and may include specific pass/fail criteria.