AC 120-104

U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Establishing and Implementing Limit of Validity to Prevent Widespread Fatigue Damage Date: 1/10/11 AC No: 120-104 Updated: 7/31/26 Change: (Editorial) Initiated by: AIR-600 and AFS-300

1. PURPOSE. This advisory circular (AC) offers guidance on compliance with Title 14 of the Code of Federal Regulations (14 CFR) 26.21, 26.23, 121.1115, and 129.115. It tells design approval holders (DAH) of transport category airplanes how to establish a limit of validity of the engineering data that supports the structural maintenance program (hereafter referred to as LOV) for those airplanes. It also tells design approval holders how to address maintenance actions that have been determined necessary to support an LOV. It tells operators of those airplanes how to incorporate the LOV into their Continuous Airworthiness Maintenance Programs. Finally, this AC provides guidance to anyone wishing to extend an LOV. Guidance for establishing an LOV for airplanes whose type certificate (TC) was applied for after January 14, 2011, is contained in AC 25.571-1 (as revised). Guidance for extending an LOV approved under § 25.571, § 26.21, or § 26.23 can be found here. The actions described in this AC are meant to prevent widespread fatigue damage (WFD) in the transport airplane fleet up to the LOV.

2. APPLICABILITY.

a. The regulatory basis for this AC is defined in Appendix 1. This guidance is for DAHs— holders of TCs, supplemental type certificates (STC), and amended type certificates (ATC), as defined in Appendix 2—as well as applicants for those certificates. It is also intended for airplane manufacturers, operators, foreign civil aviation authorities, and Federal Aviation Administration (FAA) transport category airplane type certification engineers and their designees, as well as FAA Flight Standards inspectors.

b. This guidance applies to metallic structure on:

(1) Turbine-powered transport category airplanes, existing at the effective date of the rule (January 14, 2011), that are operated under part 121 or 129 and have a TC issued after January 1, 1958, and a maximum takeoff gross weight (MTGW) greater than 75,000 pounds as approved by the original TC, ATC, or STC.

(2) Transport category airplanes that have had the MTGW reduced from greater than 75,000 pounds to 75,000 pounds or less if the application for that change was made after January 14, 2011.

ii

(3) All transport category airplanes with a current LOV, when that LOV is being extended. This would include LOVs approved under § 25.571, § 26.21, or § 26.23.

c. Like all AC material, this AC is not, in itself, mandatory, and does not constitute a regulation. It describes an acceptable means, but not the only means, for showing compliance with the applicable regulations. The FAA will consider other means of showing compliance that an applicant may elect to present. We derived these guidelines from extensive FAA and industry experience in showing compliance with the relevant regulations. On the other hand, if we become aware of circumstances that convince us that following this AC would not result in compliance with the applicable regulations, we will not be bound by the terms of this AC. We may require additional substantiation or design changes as a basis for finding compliance.

d. This material does not change, create any additional, authorize changes in, or permit deviations from, regulatory requirements. The regulatory basis for this AC is contained in Appendix 1.

e. The term “must” in this AC is used only in the sense of ensuring applicability of this particular means of compliance (MOC) when the acceptable MOC described herein is used.

3. WHY DO YOU NEED AN LOV? If you properly maintain an airplane, theoretically you could operate it indefinitely. But structural maintenance tasks for an airplane are not constant with time. Tasks are typically added to the maintenance program as the airplane ages. It is reasonable to expect, then, that the current structural maintenance tasks may not, at some future point, be sufficient for continued operation. Maintenance tasks for a particular airplane can only be determined based on what is known about that airplane model at any given time—from analyses, tests, service experience, and teardown inspections. To ensure that an airplane is free from WFD up to this point, a DAH would evaluate WFD-susceptible structure and establish an LOV.

4. DOCUMENT ORGANIZATION. This AC has four chapters. Chapter 1 provides an overview for establishing an LOV. Chapter 2 addresses compliance information for DAHs.

Chapter 3 addresses extended LOV. Chapter 4 gives compliance information for operators.

Finally, the appendices contain useful information such as related regulations and documents (Appendix 1), definitions (Appendix 2), acronyms (Appendix 3), background to the rule (Appendix 4), examples of structure susceptible to WFD (Appendix 5), details of a WFD evaluation (Appendix 6), an example of how to establish an LOV (Appendix 7), and a sample compliance plan (Appendix 8).

D orenda D. Baker John M. Allen Director, Aircraft Certification Service Director, Flight Standards Service

TABLE OF CONTENTS

1. PURPOSEi

2. APPLICABILITYi

3. WHY DO YOU NEED AN LOV?ii

4. DOCUMENT ORGANIZATIONii

CHAPTER 1—OVERVIEW OF THE PROCESS FOR ESTABLISHING A LIMIT OF VALIDITY3

100. GENERAL INFORMATION ABOUT CHAPTER 13

101. WIDESPREAD FATIGUE DAMAGE3

102. LIMIT OF VALIDITY3

103. CONTINUED AIRWORTHINESS FOR AGING AIRPLANES4

104. MAINTENANCE ACTIONS AND ADs4

105. DAH AND OPERATOR TASKS4

CHAPTER 2—§ 26.21 COMPLIANCE INFORMATION FOR DAHs6

200. GENERAL INFORMATION ABOUT CHAPTER 26

201. COMPLIANCE PLAN6

202. RECOMMENDED ADDITIONAL COMPLIANCE PLAN ITEMS7

203. AIRPLANE STRUCTURAL CONFIGURATION8

204. ESTABLISHING AN LOV8

205. MAINTENANCE ACTIONS AND SERVICE INFORMATION11

206. REQUIREMENT FOR ALS12

207. SUPPORTING DATA12

208. RESPONSIBLE AIRCRAFT CERTIFICATION SERVICE OFFICE APPROVAL13

209. MAKING LOV DATA AVAILABLE13

210. DAH REPORTING13

CHAPTER 3—§ 26.23 EXTENDED LOVs14

300. GENERAL INFORMATION ABOUT CHAPTER 314

301. APPLICATION FOR EXTENDED LOV14

302. AIRPLANE STRUCTURAL CONFIGURATION FOR AN EXTENDED LOV15

303. ESTABLISHING AN EXTENDED LOV AND SUPPORTING DATA15

304. AIRWORTHINESS LIMITATIONS SECTION (ALS)15

305. FAA APPROVALS FOR EXTENDED LOVs15

CHAPTER 4—COMPLIANCE INFORMATION FOR OPERATORS16

400. GENERAL INFORMATION FOR OPERATORS16

401. AIRWORTHINESS LIMITATIONS SECTION (ALS)16

402. AIRPLANES WITHOUT LOVs17

403. ADDING AN AIRPLANE TO AN OPERATOR’S FLEET AFTER THE

COMPLIANCE DATE HAS PASSED17

404. EXTENDED LOVs17

405. PMI APPROVAL18

406. OPERATOR REPORTING18

2 APPENDIX 1—RELATED REGULATIONS AND DOCUMENTS APPENDIX 2—DEFINITIONS APPENDIX 3—ACRONYMS USED IN THIS AC APPENDIX 4—BACKGROUND APPENDIX 5—EXAMPLES OF STRUCTURE SUSCEPTIBLE TO WFD APPENDIX 6—WIDESPREAD FATIGUE DAMAGE EVALUATION APPENDIX 7—EXAMPLE OF HOW TO ESTABLISH AN LOV APPENDIX 8—SAMPLE COMPLIANCE PLAN

3 CHAPTER 1—OVERVIEW OF THE PROCESS FOR ESTABLISHING A

LIMIT OF VALIDITY

100. GENERAL INFORMATION ABOUT CHAPTER 1. This chapter gives an overview of widespread fatigue damage (WFD) and the various aspects of establishing a limit of validity of the engineering data that supports the structural maintenance program (LOV). It explains the relationship of the WFD rule (Amendment Nos. 25-132, 26-5, 121-351, and 129-48) to other programs that have been established to address structural integrity of airplanes as they age. It also includes design approval holder (DAH) and operator tasks and deliverables.

101. WIDESPREAD FATIGUE DAMAGE. Structural fatigue damage is progressive. It begins as minute cracks, and those cracks grow under the action of repeated stresses. This can happen because of normal operational conditions and design attributes or because of isolated situations or incidents, such as material defects, poor fabrication quality, or corrosion pits, dings, or scratches. Fatigue damage can occur locally, in small areas or structural design details, or globally. Global fatigue damage is general degradation of large areas of structure with similar structural details and stress levels. Global damage may occur in a large structural element such as a single rivet line of a lap splice joining two large skin panels (multiple site damage (MSD)), or it may be found in multiple elements, such as adjacent frames or stringers (multiple element damage (MED)). MSD and MED cracks are typically too small initially to be reliably detected with normal inspection methods. Without intervention, these cracks will grow, and eventually compromise the structural integrity of the airplane, in a condition known as widespread fatigue damage. Widespread fatigue damage, hereafter referred to as WFD, is increasingly likely as the airplane ages, and is certain if the airplane is operated long enough without any intervention.

102. LIMIT OF VALIDITY. The LOV is the period of time (in flight cycles, flight hours, or both), up to which it has been demonstrated that WFD is unlikely to occur in an airplane’s structure by virtue of its inherent design characteristics and any required maintenance actions.

An airplane may not operate beyond the LOV, unless an extended LOV is approved. To support establishment of the LOV, the DAH must demonstrate by test evidence and analysis at a minimum and, if available, service experience or service experience and teardown inspection results of high-time airplanes, that WFD will not occur in that airplane up to the LOV. An LOV applies to an airplane structural configuration common to a fleet, as described in § 26.21. Any reference in this AC to the LOV for an airplane refers to the LOV for all the airplanes with that structural configuration. When establishing an LOV under § 26.21, or an extended LOV under § 26.23, you are not required to evaluate repairs and design changes for WFD, except for modifications and replacements mandated by airworthiness directives (AD).

For airplanes with an LOV, the Changed Product Rule, 14 CFR 21.101, would require applicants for significant design changes to include the latest amendment of § 25.571 in the design change certification basis. Guidance for evaluating repairs and design changes according to the latest requirements of § 25.571 is included in AC 25.571-1 (as revised).

103. CONTINUED AIRWORTHINESS FOR AGING AIRPLANES. The requirement to establish an LOV is the last element of a series of initiatives meant to ensure the continued airworthiness of aging airplane structure. The following programs for existing airplanes, or their equivalent for new airplanes, have already been established—

• The Supplemental Structural Inspection Program (SSIP). This AD-mandated program made damage-tolerance-based inspections on certain airplane models a requirement.

• The Mandatory Modification Program. This AD-mandated program was a review of service bulletins in order to decide whether inspections are sufficient or whether terminating actions should be made mandatory.

• The Repair Assessment Program. This is an operational rule that mandated evaluations of certain repairs to determine whether inspections or terminating actions are necessary.

• The Corrosion Prevention and Control Program. This AD-mandated program made inspections for corrosion on certain airplane models a requirement.

• The Aging Airplane Safety Rule (which addresses repairs, alterations, and modifications). This is an operational rule that mandates development of damagetolerance-based inspections for fatigue critical structure.

For further guidance on the elements involved in ensuring continuing structural integrity for airplanes, see AC 91-56 and AC 120-93 (as revised).

104. MAINTENANCE ACTIONS AND ADs. If you are a DAH complying with § 26.21, you are not required to identify and develop maintenance actions if you can show that such actions are not necessary to prevent WFD before the airplane reaches the LOV. If you establish an LOV that does rely on maintenance actions, you must identify them. Service information, which is typically issued in service bulletins, includes a description of the maintenance actions and the procedures for accomplishing them. Some of the maintenance actions required for the LOV may have already been issued in a service bulletin and mandated by AD. For the rest, ADs will need to be issued.

105. DAH AND OPERATOR TASKS. The following table displays an overview of the tasks necessary for establishing an LOV and incorporating it into the maintenance program. These tasks are further developed in Chapter 2 of this AC. They are necessary for establishing a point in time up to which affected airplanes will remain free of WFD. These design approval tasks will support the operator tasks necessary for compliance with §§ 121.1115 and 129.115.

5 Table 1 Required Tasks for Design Approval Holders and Operators Design Approval Holders Operators Develop and submit compliance plan [§ 26.21(d)] Obtain approved revised ALS or new ALS containing the LOV from DAH [§ 121.1115(b) or § 129.115(b)] or use the default LOV from Table 1 of § 121.1115(f) or § 129.115(f) Identify the airplane structural configuration for affected airplane model/models [§ 26.21(b)(1)] Submit revised maintenance program with revised or new ALS incorporating the LOV to the PMI [§ 121.1115(e) or § 129.115(e)] Establish the LOV and demonstrate that WFD will not occur in the airplane up to the LOV [§ 26.21(b)(1)] Stop operating the airplane if the LOV is not incorporated by compliance date [§ 121.1115(b) or § 129.115(b)] Submit list of any maintenance actions needed to support the LOV if service information has been issued but not mandated by AD [§ 26.21(b)(2)(i)] Stop operating the airplane if the airplane reaches the LOV [§ 121.1115(d) or § 129.115(d)] Submit a list of any needed maintenance actions for which service information has not been issued, and a binding schedule for when it will be available [§ 26.21(b)(2)(ii)]

Create an ALS of the ICA if one does not exist [§ 26.21(b)(3)]

Incorporate the LOV into the ALS and submit it (and supporting data) [§ 26.21(b)(4)]

AD—Airworthiness Directive ALS—Airworthiness Limitations section DAH—Design Approval Holder ICA—Instructions for Continued Airworthiness PMI—Principal Maintenance Inspector

6 CHAPTER 2—§ 26.21 COMPLIANCE INFORMATION FOR DAHs

200. GENERAL INFORMATION ABOUT CHAPTER 2. Table 1, in the previous chapter, listed required tasks for design approval holders (DAH) and operators. This chapter gives details for the DAH tasks. (Although this chapter uses the term “design approval holder (DAH),” some applicants for design approvals must complete the same tasks.) If you are a DAH who must comply with § 26.21, this chapter tells you how to establish an LOV for affected existing airplanes. It tells you how to establish an LOV for those airplanes with a type certificate (TC), amended type certificate (ATC), or supplemental type certificate (STC) that is pending approval as of the effective date of § 26.21 or those airplanes with an ATC or STC that will be approved in the future. It explains the role of maintenance actions in complying with these requirements and how they are identified, approved, and implemented. This chapter also tells you how to create or revise the Airworthiness Limitations section (ALS) of the Instructions for Continued Airworthiness (ICA).

201. COMPLIANCE PLAN. One requirement of § 26.21 is that affected persons provide compliance plans to the responsible Aircraft Certification Service office1 for review and approval. The compliance plan serves to give the responsible Aircraft Certification Service office or the Policy and Standards Division, Airframe (P&S) Section engineer a clear idea of what the DAH is going to do to comply with the WFD rule, including processes and dates. The compliance plan is meant to ensure that the DAH and the FAA have a common understanding and agreement about what is necessary to achieve compliance. Specific details of the acceptable contents of a compliance plan, and procedures for submission to the FAA, are described in AC 26-1, “Part 26, Continued Airworthiness and Safety Improvements” (as revised).

Compliance plans must contain elements that describe compliance status and any proposed differences in means of compliance (MOC) from those described in this AC. Any deviation from the MOC described in this AC may require an issue paper. Integral to the compliance plan will be the inclusion of procedures to allow the FAA to monitor progress toward compliance. A compliance plan must include:

1 The responsible Aircraft Certification Service office is the office with oversight responsibility for the relevant type certificate (TC), supplemental type certificate (STC), or manufacturer, as determined by the Administrator.

a. A project schedule identifying all major milestones for meeting the compliance dates.

This schedule should include, but is not limited to—

(1) A schedule for submission of an LOV.

(2) Establishment of a Structures Task Group (STG)2 schedule (if applicable).

2 A Structures Task Group (STG) is a group formed of manufacturers and operators to discuss maintenance procedures for specific models.

b. A proposed MOC with the requirements, such as methods and procedures for—

(1) Identifying the airplane structural configuration to be evaluated.

(2) Identifying WFD-susceptible structure. (See Appendix 5 for further guidance.)

(3) Identifying the source of engineering data that will be used to perform the required evaluations.

(4) Evaluating structure for WFD. (See Appendix 6 for further guidance.)

(5) Establishing an LOV. (See Appendix 7 for further guidance.)

(6) Identifying and developing maintenance actions, if any.

c. A plan for submitting a draft of all required compliance items for review by the responsible Aircraft Certification Service office not less than 60 days before the applicable compliance date.

d. Plans for distribution of the approved LOV.

If you are the DAH, the process for completing all of the above will require ongoing communication with personnel from the responsible Aircraft Certification Service office. You have the option of developing a single LOV to apply to all the models on the TC data sheet, a separate LOV for each model, or an LOV for different groups of airplanes within the TC. If you have not yet decided on the airplane model grouping at the time the compliance plan is submitted, or if you have not yet identified WFD-susceptible structures, we would expect you to communicate that information as soon as possible. The compliance plan should include a schedule for when that information will be made available. We also expect that the compliance plan would specify milestone dates for transmitting any information not yet known. Appendix 8 of this AC includes a sample compliance plan.

202. RECOMMENDED ADDITIONAL COMPLIANCE PLAN ITEMS. In addition to the required elements, we recommend you include the following elements in your compliance plan:

a. A Proposed Communication and Coordination Plan. This plan should identify the DAH personnel who will be the contacts for the responsible Aircraft Certification Service office. Conversely, although not part of the communication and coordination plan, the responsible Aircraft Certification Service office should make sure the DAH knows who to contact within the FAA. The plan should also include the frequency of and mechanism for status updates and the sharing of other information.

b. A Proposed Delegation Plan. If a DAH asks for delegated authority for approving compliance with these regulations on behalf of the FAA, then that delegation plan must first be approved by the FAA. Except for the WFD evaluation methodology,

8 we may delegate to our designees the review and approval of supporting data (see paragraph 207 of this AC) used for the establishment of an LOV or any maintenance actions. However, just as we do not delegate approval of certification plans, we will not delegate review and approval of this rule’s compliance plan, binding schedule, and LOV to designees.

203. AIRPLANE STRUCTURAL CONFIGURATION. You must identify the airplane structural configuration for each airplane model you are evaluating for WFD. Under § 26.21(c)(3) or (4), if you amend the TC in the future, this requirement would also apply to any configuration change. For existing and pending airplanes, the airplane structural configuration must consist of all model variations and derivatives approved under the TC, and include any structural modifications or replacements mandated by AD as of January 14, 2011. You may then develop a single LOV to be applied to all those model variations and derivatives collectively, or you may develop separate LOVs for each model variation and derivative. Similarly, if a new model is added to the TC, the configuration of that new model must be identified and evaluated for WFD as well. The result of that evaluation may be that the LOV for the originally certificated model can also be applied to the new one. Alternatively, the WFD evaluation may result in a new LOV being established because the configuration of the new model is different enough from the originally type-certificated model that the WFD behavior will be different as well.

204. ESTABLISHING AN LOV. The process for establishing an LOV involves four steps—

(a) Identifying a “candidate LOV.”

(b) Identifying WFD-susceptible structure.

(c) Performing a WFD evaluation of all susceptible structure.

(d) Finalizing the LOV.

Appendix 7 provides an example of how to establish an LOV using the process summarized below.

a. Step 1—Candidate LOV. An LOV is applicable to an airplane model with a defined structural configuration. The LOV will be dependent on the fatigue knowledge available for the airplane structural configuration at the time the LOV is established. It may also depend on maintenance actions considered necessary to preclude WFD in susceptible areas. There is no single “valid” LOV. For existing younger airplane models with many years of operation remaining between the time on the high-time airplane of that model and its design service goal (DSG), the LOV may be approximately equal to the DSG with few, if any, required maintenance actions. For airplane models with high-time airplanes approaching or exceeding the DSG, the LOV may be significantly greater than the DSG and dependent on a large number of maintenance actions.

Conceptually, any LOV can be valid as long as it is demonstrated that the airplane model

9 will be free from WFD up to the LOV based on the airplane’s inherent fatigue characteristics and any required maintenance actions. An established LOV can always be extended in accordance with § 26.23 as discussed in Chapter 3.

Early in the WFD evaluation process, DAHs may have an LOV in mind as a likely candidate for compliance with § 26.21. This “candidate LOV” will be based on high-level technical considerations, such as the age and service experience of the fleet, and results of previously accomplished fatigue tests and analyses. It may also be based on economic considerations. Subsequently, as susceptible areas are evaluated in detail, the LOV may be adjusted up or down based on the impact of the proposed maintenance actions.

b. Step 2—Identification of WFD-Susceptible Structure. If you are developing an LOV, you must identify the structure that is susceptible to WFD. Appendix 5 provides examples and illustrations of structure where MSD or MED has been documented. This list is not meant to be inclusive of all structure that might be susceptible on any given airplane model, and it should only be used for general guidance. It should not be used to exclude any particular structure. When developing the list of structure susceptible to WFD you should:

(1) Consider the list of fatigue-critical baseline structures identified under § 26.43, the Damage Tolerance Data for Repairs and Alterations rule.

(2) Develop rationale for including and excluding structure. This should be part of your compliance data.

(3) Establish criteria that could be used for identifying what structure is susceptible to WFD based on the definitions of MSD, MED, and WFD. For example, structural details and elements that are repeated over large areas and operate at the same stress levels are obvious candidates. This should be part of your compliance data.

c. Step 3—WFD Evaluation of Susceptible Structure. Each susceptible structure must be evaluated to some degree. For some structures, you may have data such as fatigue tests, analysis, and service experience that demonstrates that WFD will not occur before an airplane reaches its LOV. In that case, you would not have to go through the process of quantifying the point in time when, without intervention, 50% of the fleet is expected to have developed WFD. That point in time is referred to as the “WFD average behavior” for that structure. For other structures, you will have to do an evaluation.

Figure 1 shows the evaluation process. Engineering data from service experience or laboratory tests or both must be available to support the evaluation. Based on the data available, a time period is determined for WFD(average behavior) and is then compared to the candidate LOV. It may be found that maintenance actions are needed before the airplane reaches the candidate LOV to preclude WFD in that airplane. When that is the case, additional work may be required to—

• Assess MSD/MED inspection reliability and practicality (e.g., detectable size of cracks and probability of detection).

• Determine when inspections should start (this is the inspection start point (ISP)).

• Determine inspection intervals.

• Determine when modifications or replacements should occur (this is the structural modification point (SMP)).

This process will determine the maintenance actions necessary for the airplane to reach the LOV. The additional work required to complete the service information is discussed in paragraph 205. The WFD evaluation process is discussed in detail in Appendix 6.

Figure 1 WFD Evaluation for Each Susceptible Structure

SM P – Structural Modification Point ISP – Inspection Start Point

d. Step 4—Finalize LOV. Once all susceptible structures have been evaluated, the final step is to determine where to establish the LOV that you will propose for compliance. Based on the WFD evaluation under Step 3, you may decide to establish the final LOV by increasing or decreasing the candidate LOV identified in Step 1. This may result in fewer maintenance actions, or more of them. See Appendix 7 for an example of establishing an LOV with the process described in this chapter. In addition to the technical considerations, the LOV may be based on several other factors, including—