ESTABLISHING AND IMPLEMENTING LIMIT OF VALIDITY TO PREVENT WIDESPREAD FATIGUE DAMAGE

204. ESTABLISHING AN LOV 8 205. MAINTENANCE ACTIONS AND SERVICE INFORMATION 11 206. REQIREMENT FOR AIRWORTHINESS LIMITATIONS SECTION 12 207. SUPPORTING DATA 12 208. FAA OVERSIGHT OFFICE APPROVAL 13 209. MAKING LOV DATA AVAILABLE 13 210. DESIGN APPROVAL HOLDER REPORTING 13 CHAPTER 3—§ 26.23 EXTENDED LOVS 15 300. GENERAL INFORMATION ABOUT CHAPTER 3 15 301. APPLICATION FOR EXTENDED LOV 15 302. AIRPLANE STRUCTURAL CONFIGURATION FOR AN EXTENDED LOV 16 303. ESTABLISHING AN EXTENDED LOV AND SUPPORTING DATA 16 304. AIRWORTHINESS LIMITATIONS SECTION 16 305. FAA APPROVALS FOR EXTENDED LOVS 16 CHAPTER 4—COMPLIANCE INFORMATION FOR OPERATORS 17 400. GENERAL INFORMATION FOR OPERATORS 17 401. AIRWORTHINESS LIMITATIONS SECTION 17 402. AIRPLANES WITHOUT LOVS 18 403. ADDING AN AIRPLANE TO AN OPERATOR' S FLEET AFTER THE COMPLIANCE DATE HAS PASSED 18 404. EXTENDED LOVS 18 405. PMI APPROVAL 19 406. OPERATOR REPORTING 19 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 01/10/11 AC 120-104 2 APPENDIX 6—WIDESPREAD FATIGUE DAMAGE EVALUATION APPENDIX 7—EXAMPLE OF HOW TO ESTABLISH AN LOV APPENDIX 8—SAMPLE COMPLIANCE PLAN 01/10/11 AC 120-104 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 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 widespread fatigue damage 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 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). Or it may be found in multiple elements, such as adjacent frames or stringers (multiple element damage). Multiple site damage and multiple element damage 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 design approval holder 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. 01/10/11 AC 120-104 4 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 25.571-1D. 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. 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 damage- tolerance-based inspections for fatigue critical structure. For further guidance on the elements involved in ensuring continuing structural integrity for airplanes see AC 91-56B and AC 120-93. 104. MAINTENANCE ACTIONS AND AIRWORTHINESS DIRECTIVES. If you are a design approval holder 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 airworthiness directive. For the rest, airworthiness directives will need to be issued. 01/10/11 AC 120-104 5 105. DESIGN APPROVAL HOLDER 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. 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 01/10/11 AC 120-104 6 CHAPTER 2—§ 26.21 COMPLIANCE INFORMATION FOR DESIGN APPROVAL HOLDERS 200. GENERAL INFORMATION ABOUT CHAPTER 2. Table 1, in the previous chapter, listed required tasks for design approval holders and operators. This chapter gives details for the design approval holder tasks. (Although this chapter uses the term “design approval holder,” some applicants for design approvals must complete the same tasks.) If you are a design approval holder 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, amended type certificate, or supplemental type certificate that is pending approval as of the effective date of § 26.21 or those airplanes with an amended type certificate or supplemental type certificate 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 of the Instructions for Continued Airworthiness. 201. COMPLIANCE PLAN. One requirement of § 26.21 is that affected persons provide compliance plans to the FAA Oversight Office1 (as defined in § 26.3) for review and approval. The compliance plan serves to give the aircraft certification office or Transport Airplane Directorate engineer a clear idea of what the design approval holder is going to do to comply with the WFD rule, including processes and dates. The compliance plan is meant to ensure that the design approval holder 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.” Compliance plans must contain elements that describe compliance status and any proposed differences in means of compliance from those described in this AC. Any deviation from the means of compliance 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: 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 group2 schedule (if applicable). 1 The FAA Oversight Office is the aircraft certification office or office of the Transport Airplane Directorate with oversight responsibility for the relevant type certificate, supplemental type certificate, or manufacturer, as determined by the Administrator. 2 A structures task group is a group formed of manufacturers and operators to discuss maintenance procedures for specific models. 01/10/11 AC 120-104 7 b. A proposed means of compliance 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 FAA Oversight Office not less than 60 days before the applicable compliance date. d. Plans for distribution of the approved LOV. If you are the design approval holder, the process for completing all of the above will require ongoing communication with personnel from the FAA Oversight Office. You have the option of developing a single LOV to apply to all the models on the type certificate data sheet, a separate LOV for each model, or an LOV for different groups of airplanes within the type certificate. 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 design approval holder personnel who will be the contacts for the FAA Oversight Office. Conversely, although not part of the communication and coordination plan, the FAA Oversight Office should make sure the design approval holder 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 design approval holder asks for delegated authority for app66roving compliance with these regulations on behalf of the FAA, then that 01/10/11 AC 120-104 8 delegation plan must first be approved by the FAA. Except for the WFD evaluation methodology, 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 type certificate 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 type certificate, 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 type certificate, 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, the LOV may be approximately equal to the design service goal with few, if any, required maintenance actions. For airplane models with high-time airplanes 01/10/11 AC 120-104 9 approaching or exceeding the design service goal, the LOV may be significantly greater than the design service goal 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 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, design approval holders 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 multiple site damage or multiple element damage 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 multiple site damage, multiple element damage, 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 2 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 01/10/11 AC 120-104 10 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 multiple site damage/multiple element damage 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 SMP – 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 WFD Evaluation for Each Susceptible Structure Done Y Yeess N Noo SMP and/or ISP Service Experience Laboratory Tests Engineering Data Analyses Maintenance Actions? WFD Behavior Candidate LOV 01/10/11 AC 120-104 11 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—  Fleet demographics.  Maintenance considerations.  Operator input.  Economics. 205. MAINTENANCE ACTIONS AND SERVICE INFORMATION. Based on the evaluation described above, the design approval holder will establish an LOV. The LOV may depend on maintenance actions and, if so, you must identify them. Maintenance actions could be inspections, modifications, or replacements. a. For each required inspection, you should include—  The structure to be inspected.  The method of inspection.  The inspection start point (the point in time at which to begin inspections).  The repeat interval. b. For required modifications or replacements, you should include—  The structure to be modified or replaced.  The method of modification or replacement.  The structural modification point (the point in time to begin the modification). The service information for these maintenance actions must include all the information above plus the procedures for accomplishing them. See Appendix 6 for further information on inspection start point and modification start point. c. The maintenance actions required to support the LOV, if any, fall into one or more of the following categories— 01/10/11 AC 120-104 12 (1) Mandatory Maintenance Actions Supporting the LOV. Design approval holders may decide to use only existing type design data and maintenance actions for which service information has been mandated by an AD. Modifications and replacements mandated by AD would be identified with the airplane structural configuration to be evaluated, and thus would be already included in the LOV. If an inspection previously mandated by AD to address WFD will be used to support the LOV, it should be identified and included with the LOV. (2) Maintenance Actions Identified and Already Issued. Design approval holders may choose to use previously published service information to establish the LOV. A list of such maintenance actions must be submitted to the FAA Oversight Office. The FAA will then issue ADs to ensure that operators perform these maintenance actions to support the LOV. (3) Maintenance Actions Identified, but Not Issued. Design approval holders may propose to support an LOV farther out in time by using new maintenance actions— maintenance actions for which service information has not yet been published. In this case, the design approval holder must submit to the FAA Oversight Office a list identifying each of those maintenance actions and a binding schedule for when they will be made available. This schedule should allow for the time required by the FAA AD rulemaking process, as well as for the time required for operators to incorporate those maintenance actions into their maintenance programs and carry out those maintenance tasks. This is to ensure that high-time airplanes do not pass the threshold for beginning inspections or modifications before those inspections or modifications have been made mandatory. 206. REQUIREMENT FOR AIRWORTHINESS LIMITATIONS SECTION. The Airworthiness Limitations section of the Instructions for Continued Airworthiness is required by part 25, Amendment 25-54 (September 11, 1980) and later. Airplanes certificated to Amendment 25-54 and later will have an Airworthiness Limitations section specifying those items with mandatory replacement or inspection times and related structural inspection procedures approved under § 25.571. Before 1980 those airworthiness limitations had been contained in chapter 5 of the airplane maintenance manual. Under § 26.21, a design approval holder with airplanes that do not have an Airworthiness Limitations section must create one and include the LOV. If an airplane does have an Airworthiness Limitations section, the design approval holder must revise it to include the LOV. Any new Airworthiness Limitations section and any Airworthiness Limitations section revision must be submitted to the FAA Oversight Office for review and approval. 207. SUPPORTING DATA. Supporting data for the LOV must include: a. Identification of the airplane structural configurations that were evaluated. 01/10/11 AC 120-104 13 b. Summary of the operational statistics of the fleet, including accumulated flight cycles and flight hours. c. Assumptions made about operational loading. d. Identification and description of the structure susceptible to WFD. e. Identification and description of the analytical methods used in the WFD evaluation. f. Results of the WFD evaluations. g. Guidance on reporting inspection findings (see paragraph 210 and Chapter 4 of this AC for further information). 208. FAA OVERSIGHT OFFICE APPROVAL. The items that require FAA approval are: a. The compliance plan. b. An Airworthiness Limitations section with the LOV incorporated (and supporting data). c. The list of any required maintenance actions not mandated by airworthiness directives. d. A binding schedule for publishing any required service information not yet published. The time it takes for FAA approval of compliance data will be determined by the quality of design approval holders’ submissions and the responsiveness of design approval holders to issues raised by the FAA. 209. MAKING LOV DATA AVAILABLE. Once the initial LOV is approved by the FAA Oversight Office, you must make the LOV available to operators of affected airplanes. This could be by normal business procedures, such as through the existing revision distribution processes, by website access, by e-mail, or by U.S. mail. 210. DESIGN APPROVAL HOLDER REPORTING. Reporting requirements for design approval holders remain the same. However, when reporting structural defects in compliance with existing rules, design approval holders should include the following information: a. Description (with a sketch) of the damage, including crack length, orientation, location, flight cycles/hours, and condition of structure. b. Results of follow-up inspections by operators that identify similar problems on other airplanes in the fleet. 01/10/11 AC 120-104 14 c. Findings from inspections performed during repair, modification, or replacement that identify additional similar damage sites. d. Description of adjacent repairs within the same structure. 01/10/11 AC 120-104 15 CHAPTER 3—§ 26.23 EXTENDED LOVS 300. GENERAL INFORMATION ABOUT CHAPTER 3. This chapter gives guidance for extending an LOV approved under § 25.571, § 26.21, or § 26.23. The requirements for extending an LOV are the same as those for establishing the initial LOV except for the differences noted in Table 2 below. This chapter explains the process involved, the documentation required, and the deliverables. Table 2 Differences between LOV and Extended LOV § 26.21 (Initial LOV— required) § 25.571 (Initial LOV—required) § 26.23 (Extended LOV—optional) Who Establishes the LOV? DAH Applicant Any Person (STC required if not DAH) What Airplanes Are Affected? Airplanes> 75,000 lbs MTGW All Transport Category Airplanes Airplanes with an LOV per §§ 25.571, 26.21, 26.23 What Configuration is Considered? Configuration at Effective Rule Date Configuration of production airplane Configuration at Approval Date of Extended LOV How are Maintenance Actions Mandated? Airworthiness Directive Placement in Airworthiness Limitations section Placement in Airworthiness Limitations Section When is Unpublished Service Information Due? By Date Indicated in the Binding Schedule By Date of TC Approval or Delivery of the First Airplane or Issuance of a Standard Certificate of Airworthiness or Date of LOV Approval, Whichever Occurs Latest By Date of Approval of the Extension Is Compliance Plan Required? Yes per § 26.21(d) Not Required by § 25.571 but Recommended as Part of the Normal Certification Process per Part 21 Not Required by § 26.23 but Recommended as Part of the Normal Certification Process Per part 21 301. APPLICATION FOR EXTENDED LOV. Any person may apply to extend an existing LOV established under § 25.571, § 26.21, or § 26.23. The applicant must demonstrate that WFD will not occur in the airplane up to the proposed extended LOV. The applicant should consider the age (flight cycles or flight hours or both) of high-time airplanes relative to the existing LOV to determine when to begin developing data to extend it. Because the data is likely to include additional full-scale fatigue testing, the applicant should allow sufficient time (e.g., four years) to 01/10/11 AC 120-104 16 complete such testing and to submit the compliance data for approval. An extended LOV is a major change to the type design of an airplane. Thus any person applying for an extended LOV must use the processes for an amended type certificate (ATC) (subpart D of 14 CFR part 21) or supplemental type certificate (STC) (subpart E of part 21). An extended LOV may also include specified maintenance actions, which would be part of the new LOV approval. Extended LOVs, along with any required maintenance actions for the extended LOV, would be incorporated into the Airworthiness Limitations section. 302. AIRPLANE STRUCTURAL CONFIGURATION FOR AN EXTENDED LOV. Persons seeking approval of an extended LOV must identify the airplane structural configuration for each airplane model they are evaluating for WFD and perform that evaluation. The applicant need not extend the LOV for the entire group of airplanes for which the existing LOV was established and instead may extend the LOV for only a subset of those airplanes. The structural configuration considered must be the configuration evaluated for the existing LOV plus any additional modifications and replacements mandated by airworthiness directive up to the approval date of the extended LOV. 303. ESTABLISHING AN EXTENDED LOV AND SUPPORTING DATA. The process for establishing an extended LOV, and the documentation required, is the same as that for an initial LOV. Depending on where in the life of the airplane the initial LOV was established, this may mean that full scale fatigue tests are necessary to substantiate the extended LOV. See AC 25.571-1D for guidance on performing full-scale fatigue tests. 304. AIRWORTHINESS LIMITATIONS SECTION. Only design approval holders may revise the Airworthiness Limitations section of the Instructions for Continued Airworthiness for their airplanes. Anyone else adding limitations must do so in the form of a supplement. The extended LOV, along with any service information that supports it, must be incorporated into the Airworthiness Limitations section. Type certificate holders would do this in the form of a revision, and other persons would add a supplement. Service information documented in the Airworthiness Limitations section or any supplement to it becomes airworthiness limitation items. 305. FAA APPROVALS FOR EXTENDED LOVS. For approval of an extended LOV the applicant and the FAA would follow the same procedures they use during approval of amended type certificates or supplemental type certificates. For an extended LOV, the Airworthiness Limitations section, with the LOV and any required service information incorporated (and supporting data), requires FAA approval. 01/10/11 AC 120-104 17 CHAPTER 4—COMPLIANCE INFORMATION FOR OPERATORS 400. GENERAL INFORMATION FOR OPERATORS. This chapter tells operators how to incorporate an LOV approved by the FAA Oversight Office into their maintenance programs. It applies to 14 CFR part 121operators of transport category airplanes and to foreign air carriers or foreign persons operating U.S.-registered transport category airplanes under 14 CFR part 129. Under § 121.1115 and § 129.115, no one may operate an affected airplane unless it has an Airworthiness Limitations section with an LOV approved under Appendix H to part 25 or § 26.21. Under § 91.403(c), an operator must comply with any Airworthiness Limitations section referenced in operations specifications issued to it under part 121 or 129. This chapter describes the process for incorporating the Airworthiness Limitations section with the LOV into the maintenance program and the procedure for approval of the maintenance program revisions by the principal maintenance inspector (PMI). If a design approval holder develops maintenance actions to support the LOV under § 26.21, those maintenance actions would be mandated by AD. This chapter also provides guidance to operators who choose to incorporate an extended LOV. If there are any maintenance actions developed to support the extended LOV, they will be included in the Airworthiness Limitations section. 401. AIRWORTHINESS LIMITATIONS SECTION. Depending on the airplane’s certification basis, a design approval holder would have either revised an Airworthiness Limitations section or established a new one to include the LOV for affected airplanes. The design approval holder must make the Airworthiness Limitations section available to operators (see paragraph 209) for incorporation into their maintenance programs. a. Airplanes Subject to § 26.21. For any existing airplane that does not have an Airworthiness Limitations section, the design approval holder must create one under § 26.21 to include an LOV. If an existing airplanes does have an Airworthiness Limitations section, the design approval holder would revise it under § 26.21 to include an LOV. b. Airplanes with Type Certificates Applied for After January 14, 2011, Subject to § 25.571. Unlike § 26.21, § 25.571 applies to all transport category airplanes, regardless of the rule under which they are operated. Under § 25.571, the FAA may issue a design approval for an airplane model before full-scale fatigue testing has been completed. The design approval holder would establish the LOV after completion of this testing, in accordance with an FAA-approved schedule. Operators may operate such airplanes while the design approval holder is performing the fatigue testing. To do this, they must incorporate into their maintenance program the Airworthiness Limitations section with a number of cycles equal to ½ the number of cycles accumulated on the fatigue test article. Under § 91.403(c), operators may not fly these airplanes beyond that limitation. The number of cycles in the Airworthiness Limitations section may be revised by the design approval holder as additional cycles are accumulated on the test article. Although it is not required, operators may then revise their maintenance program to reflect 01/10/11 AC 120-104 18 the new number. Once fatigue testing is complete and the LOV is established and approved, operators may revise their maintenance program to include the LOV. If they do not, the previously described number of cycles entered into the Airworthiness Limitations section will serve as the LOV. The LOV is an airworthiness limitation, and no airplane may be operated beyond the LOV stated in the Airworthiness Limitations section. 402. AIRPLANES WITHOUT LOVs. Operators may be faced with a situation where an LOV for a certain model airplane is not available for compliance with the operating rule. This may happen because the design approval holder has not provided an LOV, or it can happen because the operator wants to operate an airplane on the excluded list for § 26.21 and the design approval holder wasn’t required to develop one.  If the design approval holder has not provided an LOV on an airplane for which it was required, the operator may use the default LOV published in Table 1 of § 121.1115 or § 129.115.  If an operator intends to operate an airplane on the exclusion list of § 26.21, that operator may develop its own LOV or use the default LOV published in Table 2 of § 121.1115 or § 129.115. Because these default LOVs are part of §§ 121.1115 and 129.115, there is no further need for FAA Oversight Office approval. However, the operator must still receive principal maintenance inspector approval to incorporate the default LOV into its maintenance program. 403. ADDING AN AIRPLANE TO AN OPERATOR’S FLEET AFTER THE COMPLIANCE DATE HAS PASSED. Before an air carrier adds an airplane to its operations specifications, the following applies: a. If the airplane was previously operated under an FAA-approved maintenance program, the new operator must ensure that the applicable LOV is incorporated into its maintenance program. b. If the airplane was not previously operated under an FAA-approved maintenance program, the operator must incorporate the applicable LOV. c. If the airplane is U.S.-registered, and will be in use in operations under 14 CFR part 129, the foreign owner or operator must incorporate the ALS containing the LOV into the maintenance program. 404. EXTENDED LOVS. Any person may apply for an extended LOV. Section 26.23 defines an extended LOV as a major change to the type design of an airplane. An applicant must use the amended type certificate or supplemental type certificate process when applying for an extended LOV. See Chapter 3 of this AC for further guidance. To operate an airplane to its extended 01/10/11 AC 120-104 19 LOV, operators may need to perform maintenance actions to prevent WFD from occurring. Those maintenance actions must be specified as airworthiness limitation items in the Airworthiness Limitations section of the Instructions for Continued Airworthiness. If an extended LOV is approved for an airplane, operators are not required to incorporate it unless they want to operate the airplane beyond the current LOV (initial or previously-approved extended LOV) in effect. The person with the approved extended LOV is not required to make it available to all operators of airplanes for which it is applicable. 405. PMI APPROVAL. Operators must submit maintenance program revisions, containing the Airworthiness Limitations section with the LOV, to the principal maintenance inspector or Flight Standards International Field Office for review and approval. After the principal maintenance inspector or Flight Standards International Field Office has found these changes acceptable, they would approve the revision through the existing approval processes for time limitations. Any subsequent revision to this limit will follow the same approval process. The Airworthiness Limitations section must be clearly distinguishable within the maintenance program. 406. OPERATOR REPORTING. Reporting requirements for operators remain the same. However, when reporting fatigue-related structural defects in compliance with existing rules, operators should include the following information:  Description (with a sketch) of the damage, including crack length, orientation, location, flight cycles/hours, and condition of structure.  Results of follow-up inspections by operators that identify similar problems on other airplanes in the fleet.  Findings from inspections performed during repair, modification, or replacement that identify additional similar damage sites.  Description of adjacent repairs within the same structure. Operators should also report the items above, and all suspected cases of multiple site damage/multiple element damage, to design approval holders and, if possible, provide them with the damaged structure for examination and further assessment. 01/10/11 AC 120-104 Appendix 1 A1-1 APPENDIX 1 RELATED REGULATIONS AND DOCUMENTS The regulatory basis of this AC is comprised of the following parts of 14 CFR:  Part 21, Certification Procedures for Products and Parts.  Part 25, Airworthiness Standards.  Part 26, Continued Airworthiness and Safety Improvements: Transport Category Airplanes.  Part 43, Maintenance, Preventive Maintenance, Rebuilding, and Alteration.  Part 119, Certification: Air Carriers and Commercial Operators.  Part 121, Operating Requirements: Domestic, Flag, and Supplemental Operations.  Part 121, Subpart AA, Continued Airworthiness and Safety Improvements.  Part 129, Foreign Air Carriers and Foreign Operators of U.S.-Registered Aircraft Engaged in Common Carriage, Subpart B, Continued Airworthiness and safety Improvements. You can download an electronic copy of 14 CFR from the Internet at http://www.gpoaccess.gov/cfr/. A paper copy may be ordered by sending a request to the U.S. Superintendent of Documents, U.S. Government Printing Office, Washington, D.C. 20402-0001, or by calling telephone number (202) 512-1800; or by sending a request by facsimile to (202) 512-2250. Below are the specific regulations pertinent to this AC: a. § 21.3, Reporting of failures, malfunctions, and defects. b. § 21.50, Instructions for continued airworthiness and manufacturer’s maintenance manuals having airworthiness limitations sections. c. § 21.101, Designation of applicable regulations. d. § 25.571, Damage-tolerance and fatigue evaluation of structure. e. § 25.1529, Instructions for continued airworthiness. f. Appendix H to part 25, Instructions for Continued Airworthiness. 01/10/11 AC 120-104 Appendix 1 A1-2 g. § 26.21, Limit of validity. h. § 26.23, Extended limit of validity. i. § 43.13, Performance rules (general). j. § 43.16, Airworthiness limitations. k. § 91.403, General (under Maintenance, Preventive Maintenance, and Alterations). l. § 121.703, Service difficulty reports. m. § 121.1115, Limit of Validity. n. § 129.115, Limit of Validity. REFERENCES. The following list of related documents is provided for information purposes and are not necessarily directly referenced in this AC. a. Advisory Circulars. Electronic copies of the following ACs can be downloaded from the Internet at http://www.faa.gov/rgl. Paper copies may be ordered from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785. (1) AC 20-107B, “Composite Aircraft Structure.” (2) AC 21.101-1, “Establishing the Certification Basis of Changed Aeronautical Products.” (4) AC 26-1, “Part 26, Continued Airworthiness and Safety Improvements.” (5) AC 25.571-1D, “Damage Tolerance and Fatigue Evaluation of Structure.” (6) AC 25.1529-1A, “Instructions for Continued Airworthiness of Structural Repairs on Transport Airplanes.” (7) AC 91-56B, “Continuing Structural Integrity Program for Airplanes.” (8) AC 91-82, “Fatigue Management Programs for Airplanes with Demonstrated Risk of Catastrophic Failure Due to Fatigue.” (9) AC 120-16E, “Air Carrier Maintenance Programs.” 01/10/11 AC 120-104 Appendix 1 A1-3 (10) AC 120-73, “Damage Tolerance Assessment of Repairs to Pressurized Fuselages.” (11) AC 120-93 “Damage Tolerance Inspections for Repairs and Alterations.” b. FAA Policy. An electronic copy of the following policy statement can be downloaded from the Internet at http://www.faa.gov/rgl. A paper copy may be ordered from the Federal Aviation Administration, Transport Airplane Directorate, Transport Standards Staff, Standardization Branch, ANM-113, 1601 Lind Avenue SW., Renton, WA 98055-4056.  PS-ANM110-7-12-2005, Policy Statement, “Safety – A Shared Responsibility - New Direction for Addressing Airworthiness Issues for Transport Airplanes,” issued July 6, 2005, effective July 12, 2005. c. FAA Orders. Electronic copies of the following Orders can be downloaded from the Internet at http://www.faa.gov/rgl. Paper copies may be ordered from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75 th Avenue, Landover, MD 20785. (1) Order 8110.54, “Instructions for Continued Airworthiness Responsibilities, Requirements, and Contents.” (2) Order 8900.1, “Flight Standards Information Management System (FSIMS).” (3) Order 8110.104, “Responsibilities and Requirements for Implementing Part 26 Safety Initiatives.” d. Related Documents. Electronic copies of the following reports can be downloaded from the Internet at http://www.faa.gov. (1) “Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Airplane Fleet,” Revision A, dated June 29, 1999 (A report of the Airworthiness Assurance Working Group for the Aviation Rulemaking Advisory Committee Transport Aircraft and Engine Issues). (2) “Widespread Fatigue Damage Bridging Task Multiple Element Damage,” dated July 23, 2003 (A report of the Airworthiness Assurance Working Group for the Aviation Rulemaking Advisory Committee’s Transport Aircraft and Engine Issues Group). (3) Final Report of the AAWG – Continued Airworthiness of Structural Repairs, September 12, 1996. (4) A Report of the AAWG – Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Airplane Fleet, March 11, 1999. 01/10/11 AC 120-104 Appendix 1 A1-4 (5) A Report of the AAWG - Recommendations For Regulatory Action To Enhance Continued Airworthiness Of Supplemental Type Certificates, Final Report, January 21, 2003. (6) ATA Report 51-93-01 - Structural Maintenance Program Guidelines For Continuing Airworthiness.* (7) A Report to the AAWG - Structures Task Group Guidelines Document, June 1996. (8) A Report to the AAWG – Recommendations Concerning ARAC Tasking FR Doc. 04-10816, Re: Aging Airplane Safety Final Rule, 14 CFR 121.370a and 129.16,Task 3 Final Report, March 22, 2007, Revised April 11, 2007. *For a copy of this report, please contact the ATA: Air Transport Association of America, Inc., 1301 Pennsylvania Avenue., NW, Suite 1100, Washington, DC 20004-1707; telephone (202) 626-4000. 01/10/11 AC 120-104 Appendix 2 A2-1 APPENDIX 2 DEFINITIONS For purposes of this AC, the following definitions apply: Airplane structural configuration—The approved type certificate design, which includes the original design, any model variations or derivatives, and alterations or replacements mandated by AD. Airworthiness limitation item (ALI)—A maintenance action item identified in the Airworthiness Limitation section of a design approval holder’s Instructions for Continued Airworthiness. These items may contain mandatory modification or replacement times, mandatory inspection times, or inspection procedures. Airworthiness Limitations section (ALS)—A collection of mandatory maintenance actions required for an airplane’s structure and fuel tank systems. The Airworthiness Limitations section is part of an airplane’s Instructions for Continued Airworthiness. For structural maintenance actions, the ALS includes structural modification times, replacement times, structural inspection thresholds and intervals, and related structural inspection procedures. Alteration or modification—A design change made to an airplane. Within the context of this AC, the two terms are synonymous. Amended type certificate (ATC)—An approved change to an airplane’s original type certificate made by the type certificate holder. Only the holder of the type certificate may apply for an amended type certificate. Baseline structure—The structure that is designed under the original type certificate or amended type certificate for that airplane model. Damage tolerance —The attribute of a structure that permits it to retain its required residual strength without detrimental structural deformation for a period of use after the structure has sustained a given level of fatigue, corrosion, or accidental or discrete source damage. 01/10/11 AC 120-104 Appendix 2 A2-2 Design approval holder—The term applied to the holder of any design approval, including type certificate, amended type certificate, supplemental type certificate, amended supplemental type certificate, parts manufacturer approval, TSO authorization, letter of TSO design approval, and field approval (FAA Form 337). Design service goal (DSG)—The period of time (in flight cycles or flight hours, or both) established at design and/or certification during which the principal structure of an airplane will be reasonably free from significant cracking. Fatigue-critical baseline structure—The part of the baseline structure of an airplane that is classified as fatigue-critical. Fatigue critical structure—Structure of an airplane that is susceptible to fatigue cracking that could lead to a catastrophic failure of the airplane. For the purposes of this AC, fatigue-critical structure refers to the same class of structure as the structure that would need to be assessed for compliance with § 25.571(a) at Amendment 25-45 or later. The term fatigue-critical structure may refer to fatigue-critical baseline structure, fatigue-critical alteration structure, or both. FAA Oversight Office—The aircraft certification office or office of the Transport Airplane Directorate having oversight responsibility for the relevant type certificate or supplemental type certificate, as determined by the Administrator. Flight Standards Service Offices—Offices located in FAA headquarters responsible for developing guidance and policy applicable to transport category airplanes for Aircraft Evaluation Group personnel and Flight Standards Service field personnel (maintenance, avionics, and operations Aviation Safety Inspectors) in the conduct of their responsibilities. Inspection start point (ISP)—The point in time when special inspections of the fleet are initiated because of a specific probability of having a multiple site damage/multiple element damage condition. Instructions for Continued Airworthiness (ICA)—Maintenance actions developed by the TC or STC holder in accordance with 14 CFR 25.1529 and delivered with the airplane in accordance with 14 CFR 21.50. Instructions for continued airworthiness are documentation that set forth instructions and requirements for the maintenance that is essential to the continued airworthiness of an aircraft, engine, or propeller. 01/10/11 AC 120-104 Appendix 2 A2-3 Limit of validity (of the engineering data that supports the structural maintenance program)—The period of time (in flight cycles, flight hours, or both), up to which it has been demonstrated by test evidence, analysis and, if available, service experience and teardown inspection results of high-time airplanes, that widespread fatigue damage will not occur in the airplane structure. Maintenance actions—Inspections, modifications, replacements, or any combination of these. Multiple site damage (MSD)—A source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in the same structural elements. Multiple element damage (MED)—A source of widespread fatigue damage characterized by the simultaneous presence of fatigue cracks in similar adjacent structural elements. Structural modification point (SMP)—The point in time when a structural area must be modified to preclude WFD. Scatter factor—A life reduction factor used in the interpretation of fatigue analysis and fatigue test results. Structures Task Group—A model-specific airplane industry group that consists of type certificate holders and operators responsible for developing aging airplane model-specific programs. Such a group also includes regulatory authorities who approve and monitor those programs. Supplemental Structural Inspection Program—A damage-tolerance-based inspection program. Structural inspection programs only address the structure identified by the type certificate holder using the guidance contained in AC 91-56. Teardown Inspection—The term used for the process of disassembling structure and using destructive inspection techniques or visual (e.g., magnifying glass and dye penetrant) or other non-destructive (e.g., eddy current, ultrasound) inspection techniques to identify the extent of damage within a structure caused by fatigue, corrosion, and accidental damage. 01/10/11 AC 120-104 Appendix 2 A2-4 Test-to-structure factors—A series of factors used to adjust test results to full-scale structure. These factors could include, but are not limited to, differences in—  Stress spectrum,  Boundary conditions,  Specimen configuration,  Material differences,  Geometric considerations, and  Environmental effects. Type design—The engineering definition of a particular product. The type design consists of the following (see 14 CFR § 21.31):  Drawings and specifications,  Dimensions, materials, and processes,  Airworthiness limitations,  (for primary category aircraft, if desired) A special inspection and preventive maintenance program designed to be accomplished by an appropriately rated and trained pilot/owner, and  Other data to describe the product design and to determine the airworthiness, noise characteristics, fuel venting, and exhaust emissions (where applicable). WFD(average behavior)—The point in time when, without intervention, 50% of the fleet is expected to develop WFD for a particular structure. Widespread fatigue damage (WFD)—The simultaneous presence of cracks at multiple structural locations that are of sufficient size and density that the structure will no longer meet the residual strength requirements of § 25.571(b). 01/10/11 AC 120-104 Appendix 3 A3-1 APPENDIX 3 ACRONYMS USED IN THIS AC AAWG Airworthiness Assurance Working Group AC Advisory Circular AD Airworthiness Directive ALS Airworthiness Limitations section ARAC Aviation Rulemaking Advisory Committee ATC Amended Type Certificate DAH Design Approval Holder FAA Federal Aviation Administration ICA Instructions for Continued Airworthiness LOV Limit of Validity of the Engineering Data that Supports the Structural Maintenance Program ISP Inspection Start Point MED Multiple Element Damage MSD Multiple Site Damage MSG Maintenance Steering Group PMI Principal Maintenance Inspector SMP Structural Modification Point SSID Supplemental Structural Inspection Document SSIP Supplemental Structural Inspection Program STC Supplemental Type Certificate TC Type Certificate WFD Widespread Fatigue Damage 01/10/11 AC 120-104 Appendix 4 A4-1 APPENDIX 4 BACKGROUND Structural fatigue is the degradation of a material subjected to repeated structural loads. Fatigue of metallic structure is recognized as a significant threat to the continued airworthiness of airplanes. This is because even small fatigue cracks can significantly reduce the strength of airplane structure. For over 50 years, the airworthiness standards for certification of new transport category airplanes have addressed fatigue. These airworthiness standards are meant to prevent catastrophic failures due to fatigue throughout the anticipated operational life of the airplane. These standards have evolved over the years and have changed as the relevant knowledge base has increased. This knowledge includes service experience, specific incidents and accidents, and technological advances in design, analysis, testing, manufacturing, and inspection of airplanes. One of the first significant changes in the airworthiness standards occurred in March 1956, with the revision of the fatigue evaluation requirements contained in Civil Air Regulations (CAR) 4b.270. This revision added “fail-safe strength” as an option to the “fatigue strength” approach for addressing fatigue. Motivation for this change was the realization that precluding the occurrence of fatigue cracking might not always be possible and, therefore, as an option, the structure may be designed to survive cracking. The fatigue strength approach aims for a design where fatigue cracking is not probable within the operational life of the airplane. The fail-safe approach assumes that cracking could occur, but that a specified minimum strength could be maintained after a “fatigue failure or obvious partial failure.” The efficacy of the fail-safe approach was not only dependent on the structure keeping the specified minimum strength with the fatigue damage present, but also on finding the damage during normal maintenance. As applied, the fail-safe approach emphasizes redundancy as opposed to fatigue performance, and inspectability is assumed and not quantified. The fail-safe option was the predominant approach chosen for most large transport category airplanes certified in the 1960s and 1970s. Another significant change in the airworthiness standards for fatigue occurred in October 1978 with Amendment 25-45, when § 25.571 was revised and § 25.573 was deleted. This change involved removing the fail-safe option entirely and establishing a new requirement to develop damage-tolerance-based inspections wherever practical. The fatigue strength approach, as a default option, is used only if the damage-tolerance approach is impractical. The motivation for the 1978 change was the recognition, based on mounting evidence, that the fail-safe approach that had been applied up to that point was not reliable and would not achieve the desired level of safety. Specific areas of concern with the fail-safe approach included loss of “fail-safety” with age. This was because of the increased probability of cracking in the structure adjacent to the fatigue failure, or obvious partial failure, and the lack of directed inspections and quantification of residual life with the assumed damage present. It was agreed at the time that more emphasis was needed on where and how fatigue cracking could occur in the structure, and on quantifying crack growth and residual strength characteristics. Such an approach includes knowledge of damage tolerance characteristics and development of effective inspection protocols, such as 01/10/11 AC 120-104 Appendix 4 A4-2 where, when, how, and how often to inspect. The 1978 changes achieved this for certification of new transport category airplanes. The same events and reasoning that drove the changes to airworthiness standards for new airplanes also influenced the strategy adopted to ensure continued airworthiness of the existing fleet. There was increasing concern about existing older airplanes that had been certified according to the fail-safe requirements of CAR 4b.270. Eleven large transport models were specifically identified as needing the most attention. The FAA determined a need to develop damage-tolerance-based inspection programs. These inspections supplemented existing maintenance inspections, so they were referred to as Supplemental Structural Inspection Programs (SSIPs). The inspection requirements for these programs were documented in supplemental structural inspection documents (SSIDs). It was also agreed that SSIDs would be developed by the original equipment manufacturers on a voluntary basis and then mandated by airworthiness directives. The Civil Aviation Authority for the United Kingdom published guidance for developing the SSIPs in Airworthiness Notice No. 89, Continuing Structural Integrity of Transport Aeroplanes, dated August 23, 1978, and the FAA published guidance for developing the SSIPs on May 6, 1981 in AC No. 91-56, Supplemental Structural Inspection Program for Large Transport Category Airplanes. SSIPs were subsequently developed and documented in SSIDs and mandated by airworthiness directives for the eleven aging model airplanes. In April 1988, one of the eleven aging model airplanes suffered major structural damage to its pressurized fuselage because of undetected fatigue cracking of the baseline primary structure. Although that airplane had an SSIP that was mandated by an airworthiness directive, there were no special directed inspections for fatigue cracks at multiple structural locations. This was because it was believed that the link-up of multiple fatigue cracks in one skin frame bay would result in safe decompression by skin flapping.3 It was thought that the damage to the fuselage skin would be obvious by inspection or by the inability to pressurize the fuselage. The accident was attributed, in part, to the aging of the airplane involved. This aging included the simultaneous presence of small fatigue cracks at multiple locations in the fuselage skin lap splice. Instead of being obvious, those cracks grew undetected. Then they linked up quickly to cause catastrophic failure of a large section of the fuselage. That accident precipitated actions that culminated in regulations aimed at avoiding catastrophic failures from fatigue in existing and future airplanes. In response to the April 1988 accident, the FAA sponsored a conference on aging airplanes and established a task force representing the interests of the airplane operators, airplane manufacturers, regulatory authorities, and other aviation representatives. This task force was later renamed the Airworthiness Assurance Working Group (AAWG). They specifically recommended establishment of an Aging Aircraft Program to address long-term airworthiness issues in airplane structure that result from aging. The AAWG also recommended that the program include an element for addressing fatigue cracking at multiple structural locations. The National Transportation Safety Board recommended that design approval holders discontinue classifying fuselage skin as “malfunction evident” or “damage obvious” in SSIDs. 3 Flapping is a phenomenon that occurs in cracks in fuselage skin subjected to cabin pressure. When the two tips of the crack meet stiffened structure, they change direction and turn away from the stiffened structure. 01/10/11 AC 120-104 Appendix 4 A4-3 The April 1988 accident also precipitated Congressional legislation. In October 1991, Congress enacted Title IV of Public Law 102-143, the Aging Airplane Safety Act of 1991 (AASA). The AASA had two key elements: (1) It required “the Administrator to make such inspections and conduct such reviews of maintenance and other records of each airplane used by an operator to provide air transportation as may be necessary to determine that such is in a safe condition and is properly maintained for operation in air transportation.” (2) It specified that an operator must be able to demonstrate, as part of that inspection, “that maintenance of the airplane’s structure, skin, and other age sensitive parts and components have been adequate and timely enough to ensure the highest level of safety.” The SSIPs were revised to remove the methodology for classifying certain fatigue cracking in structures as “malfunction evident” or “damage obvious” and to include damage-tolerance-based inspections for those structures. The FAA issued airworthiness directives to mandate those changes. The supplemental structural inspection program is one element of the overall Aging Aircraft Program for structures. We also adopted changes to operating rules and issued airworthiness directives to mandate, in part, other elements of the Aging Aircraft Program, including:  The Mandatory Modification Program.  The Repair Assessment Program.  The Corrosion Prevention and Control Program. For further discussion on the Aging Aircraft Program for structures, see AC 91-56B. In response to the AASA, in part, the FAA issued the Aging Airplane Safety Final Rule,4 and the Damage Tolerance Data Rule, 5 and guidance material. The Damage Tolerance Data Rule is the design-approval-holder component that facilitates operator compliance with the Aging Airplane Safety Final Rule. For further background information and discussion on these rules, see AC 120-93. In 1998, the FAA amended § 25.571 (Amendment 25-966 ) of the aircraft certification requirements for transport category airplanes. Under this amendment, we introduced the term widespread fatigue damage (WFD) into § 25.571. Widespread fatigue damage is a condition that occurs when there is simultaneous presence of fatigue cracks at multiple structural locations that are of sufficient size and density that the structure will no longer meet the residual strength requirements of § 25.571(b). As part of the certification process, § 25.571 requires full-scale fatigue test evidence to demonstrate that WFD will not occur before an airplane reaches its design service goal. 4 Aging Airplane Safety: 70 FR 5518; February 2, 2005 5 Damage Tolerance Data for Repairs and Alterations: 72 FR 70486; December 12, 2007 6 Damage-Tolerance and Fatigue Evaluation of Structure: 63 FR 15707; March 31, 1998 01/10/11 AC 120-104 Appendix 4 A4-4 Another significant change in the airworthiness standards for fatigue occurred in November 2010 with the issuance of Amendment Nos. 25-132 and 26-57 when the FAA revised § 25.571 and added §§ 26.21 and 26.23. We determined that this change was necessary because the structural fatigue characteristics of airplanes are understood only up to the point where analyses and testing of the structure are valid. Until Amendment Nos. 25-132 and 26-5,, there were no requirements to limit the operation of airplanes based on existing engineering data to prevent catastrophic failures from WFD. We adopted these amendments to require that design approval holders establish a limit of validity of the engineering data that supports the structural maintenance program (LOV) and demonstrate that WFD will not occur in the airplane before it reaches LOV. Under this change, we also added §§ 121.1115 and 129.115 in Amendment Nos. 121-351 and 129-48, to prohibit operation of an airplane beyond its LOV. Section 26.23 provides an option for any person to extend the LOV and to develop the maintenance actions that support the extended limit. Thereafter, to operate an airplane beyond the existing LOV, an operator must incorporate the extended LOV and associated maintenance actions into its maintenance program. The airplane may not be operated beyond the extended LOV. These amendments, which specifically addresses WFD, is intended to be the last element of the overall Aging Aircraft Program for structures. 7 Aging Aircraft Program: Widespread Fatigue Damage: 75 FR 69746, November 15, 2010. 01/10/11 AC 120-104 Appendix 5 A5-1 APPENDIX 5 EXAMPLES OF STRUCTURE SUSCEPTIBLE TO WFD STRUCTURAL AREA SEE FIGURE Longitudinal Skin Joints, Frames, and Tear Straps (MSD/MED) 5-1 Circumferential Joints and Stringers (MSD/MED) 5-2 Lap Joints with Milled, Chem-milled or Bonded Radius (MSD) 5-3 Fuselage Frames (MED) 5-4 Stringer to Frame Attachments (MED) 5-5 Shear Clip End Fasteners on Shear Tied Fuselage Frames (MSD/MED) 5-6 Aft Pressure Dome Outer Ring and Dome Web Splices (MSD/MED) 5-7 Skin Splice at Aft Pressure Bulkhead (MSD) 5-8 Abrupt Changes in Web or Skin Thickness — Pressurized or Unpressurized Structure (MSD/MED) 5-9 Window Surround Structure (MSD, MED) 5-10 Over-Wing Fuselage Attachments (MED) 5-11 Latches and Hinges of Non-plug Doors (MSD/MED) 5-12 Skin at Runout of Large Doubler (MSD)—Fuselage, Wing or Empennage 5-13 Wing or Empennage Chordwise Splices (MSD/MED) 5-14 Rib-to-Skin Attachments (MSD/MED) 5-15 Typical Wing and Empennage Construction (MSD/MED) 5-16 01/10/11 AC 120-104 Appendix 5 A5-2 Figure 5-1 Longitudinal Skin Joints, Frames, and Tear Straps (MSD/MED) Figure 5-2 Circumferential Joints and Stringers (MSD/MED) 01/10/11 AC 120-104 Appendix 5 A5-3 Figure 5-3 Lap joints with Milled, Chem-milled or Bonded Radius (MSD) Figure 5-4 Fuselage Frames (MED) 01/10/11 AC 120-104 Appendix 5 A5-4 Figure 5-5 Stringer-to-Frame Attachments (MED) Figure 5-6 Shear Clip End Fasteners on Shear-Tied Fuselage Frame (MSD/MED) 01/10/11 AC 120-104 Appendix 5 A5-5 Figure 5-7 Aft Pressure Dome Outer Ring and Dome Web Splices (MSD/MED) Figure 5-8 Skin Splice at Aft Pressure Bulkhead (MSD) 01/10/11 AC 120-104 Appendix 5 A5-6 Figure 5-9 Abrupt Changes in Web or Skin Thickness — Pressurized or Unpressurized Structure (MSD/MED) Figure 5-10 Window Surround Structure (MSD, MED) 01/10/11 AC 120-104 Appendix 5 A5-7 Figure 5-11 Over Wing Fuselage Attachments (MED) Figure 5-12 Latches and Hinges of Non-plug Doors (MSD/MED) 01/10/11 AC 120-104 Appendix 5 A5-8 Figure 5-13 Skin at Runout of Large Doubler (MSD) — Fuselage, Wing, or Empennage Figure 5-14 Wing or Empennage Chordwise Splices (MSD/MED) 01/10/11 AC 120-104 Appendix 5 A5-9 Figure 5-15 Rib-to-Skin Attachments (MSD/MED) Figure 5-16 Typical Wing and Empennage Construction (MSD/MED) 01/10/11 AC 120-104 Appendix 6 A6-1 APPENDIX 6 WIDESPREAD FATIGUE DAMAGE EVALUATION This appendix tells you how to perform a WFD evaluation as part of the overall process for establishing an LOV. The WFD evaluation may be simple or very complex. The situation will vary from model to model and area to area on any given airplane structural configuration. The evaluation of the identified WFD-susceptible structure has two objectives—  To predict when WFD is likely to occur.  To establish additional maintenance actions, as necessary, to ensure continued safe operation of the airplane. The information contained in this appendix was derived in part from the 1999 and 2003 ARAC Working Group reports referenced in Appendix 1. THE EVALUATION The WFD evaluation of susceptible structure is a process for determining when WFD is likely to occur in the structure, what maintenance actions might be necessary to prevent it from occurring before the LOV, and when those maintenance actions should be begun. This evaluation should be based on service history, test data, fatigue analyses, damage-tolerance analyses, teardown inspections of high-usage airplanes, or any combination of these. 1. Predicting When WFD is Likely to Occur. a. Characterization of Events Leading to WFD. The fatigue process that leads to WFD is shown in Figure 6-1. This figure is applicable both to damage that occurs in multiple sites (multiple site damage) and damage that occurs in similar structure at more than one location (multiple element damage). For any susceptible structural area, it is not a question of whether WFD will occur—but when it will occur. In Figure 6-1, the “when” is illustrated by the line titled “WFD (average behavior),” which is the point when, without intervention, 50 % of the airplanes in a fleet would have experienced WFD in the considered area. (Note that the probability density function for flight cycles or flight hours to WFD has been depicted for reference.) The WFD process includes this phase of crack initiation and a crack growth phase. During the crack initiation phase, which generally spans a long period of time, there is little or no change in the basic strength capability of the structure. The actual residual strength curve depicted in Figure 6-1 is flat, and equal to the strength of the structure in its pristine state. However, at some time after the first small cracks start to grow, residual strength begins to degrade. Crack growth continues until the capability of the structure degrades to the point of the minimum strength required by § 25.571(b). In this context, the line in Figure 6-1 called WFD (average behavior) represents a point when, without intervention, 50 % of the airplanes in a fleet fall below the minimum strength requirements of § 25.571(b). 01/10/11 AC 120-104 Appendix 6 A6-2 Probability Density Function for Cycles to NOTE: No Scale Implied. For Definition Use Only REQUIRED RESIDUAL STRENGTH CRACK LENGTH ACTUAL RESIDUAL STRENGTH MSD/MED CRACK GROWTH PROBABILITY DENSITY FUNCTION FOR CYCLES TO WFD CONDITION WFD (AVERAGE BEHAVIOR) FLIGHT CYCLES/HOURS MSD/MED CRACK INITIATION MSD – MULTIPLE SITE DAMAGE MED – MULTIPLE ELEMENT DAMAGE Figure 6-1 MSD/MED Residual Strength Curve 01/10/11 AC 120-104 Appendix 6 A6-3 b. Widespread Fatigue Damage (average behavior). WFD average behavior should be estimated for each susceptible structural area. Such an estimate may be based on full-scale fatigue test evidence and analyses at a minimum, and if available, in-service history. In making this estimate, consider the following: (1) A complete review of the in-service history of the susceptible areas (including loads, mission profiles, environment, and operational statistics of the fleet, stated as a number of accumulated flight cycles or flight hours). (2) Significant production variants (material, design, assembly method, and any other change that might affect the fatigue performance of the detail). (3) Relevant full-scale and component fatigue test data. (See Appendix 1 of AC 25.571-1D for guidelines on full-scale fatigue test evidence, including addressing cracks found during testing.) (4) Relevant data from teardown inspections of full-scale and component fatigue test data, including teardown inspections of high-time airplanes. (5) Any fractographic analysis available. The evaluation of test results for the reliable prediction of the time when WFD might occur in each susceptible area should include appropriate test-to-structure factors (See definition in Appendix 2 of this AC). The WFD evaluation may be analytically determined, supported by test or in-service information. c. Cracking Pattern. Regardless of whether the assessment of WFD average behavior is based on in-service data, full-scale fatigue test evidence, analyses; or a combination of any of these, the following should be considered: (1) Initial cracking scenario. This is the size and extent of multiple location cracking expected at the initiation of multiple site damage or multiple element damage. To determine the time to multiple site damage or multiple element damage initiation you will need empirical data or an assumption of the crack locations and sequence, plus a fatigue evaluation. Alternatively, analysis can be based on either: (a) The distribution of equivalent initial flaws as determined from the regression to zero cycles of flaws found during fatigue testing, teardown inspections, or both. (b) The distribution of fatigue cracking determined from relevant fatigue testing or service experience, or both. (2) Final cracking scenario. This is an estimate of the size and extent of multiple location cracking that could cause residual strength to fall to the minimum required 01/10/11 AC 120-104 Appendix 6 A6-4 level (WFD condition), as shown in Figure 6-1. There are techniques for 3-D elastic- plastic analysis of such problems; but there are also several alternative test and analysis approaches that provide acceptable estimates. One such approach is to define the final cracking scenario as not yet critical—a point in time before the cracking has developed into the WFD condition. An example of a multiple site damage problem would be the occurrence of the first crack link-up in a row of fastener holes. An example of a multiple element damage problem would be simultaneous cracking of two or three adjacent structural elements. Using a sub-critical scenario reduces the complexity of the analysis and, in many cases, will not greatly reduce the estimate of the total time to WFD(average behavior) because the majority of the total time is generally associated with crack initiation. (3) Crack growth. Progression of the crack distributions from the initial cracking scenario to the final cracking scenario should be developed. You can develop crack growth predictions in two ways— (a) Analytically: typically based on linear elastic fracture mechanics; or (b) Empirically: from test or service fractographic data. (4) Differences between multiple site damage and multiple element damage. We expect details of the approach used to characterize events leading up to WFD to be different. The differences will depend on whether you are considering multiple site damage or multiple element damage. This is especially true for crack interaction. (a) Crack Interaction. Multiple site damage has the potential for strong crack interaction, and the effect of multiple cracks on each other needs to be addressed. Multiple element damage, in most cases, does not have the same potential for strong crack interaction. The differences between interaction effects for multiple site damage and multiple element damage are illustrated in Figure 6-2. 01/10/11 AC 120-104 Appendix 6 A6-5 Figure 6-2 Difference between MSD and MED Interaction Effects MED a (mm) N (cycles) a (mm) N (cycles) MSD Single crack Multiple cracks Single crack Single crack Multiple cracks Multiple cracks 01/10/11 AC 120-104 Appendix 6 A6-6 (b) Multiple Site Damage and multiple element damage interaction. Some areas of an airplane are potentially susceptible to both multiple site damage and multiple element damage. Simultaneous occurrence of multiple site damage and multiple element damage is possible, even though it’s not common. A comparison of inspection start points or modification start points might indicate the possibility of this occurring. If so, your evaluation should consider interaction between multiple site damage and multiple element damage. (5) Multiple Site Damage. When you are assessing multiple site damage, certain assumptions or methods may have a greater impact than others on the final outcome of the WFD evaluation.8 The following items were found to have such an impact: (a) The flaw sizes assumed at initiation of the crack growth phase of analysis. (b) Material properties used (static, fatigue, fracture mechanics). (c) Ligament 9 failure criteria. (d) Crack growth equations used. (e) Statistics used to evaluate fatigue behavior of the structure (e.g., time to crack initiation). (f) Methods of determining the structural modification point (SMP). (g) Detectable flaw size assumed. (h) Initial distribution of flaws. (i) Factors used to determine lower-bound behavior as opposed to mean behavior. (6) Multiple Element Damage. When considering multiple element damage in your evaluation, you need not consider interaction between cracks in different elements. You should consider the following: (a) There is not a high probability that, after a crack initiates in an element, a second crack will initiate in an element right next to it. If this does happen, however, the consequences to the overall structure may be severe. This is because having two structural members fail right next to each other can completely negate any ability of the structure to tolerate additional damage. 8 “Recommendations for Regulatory Action to Prevent Widespread Fatigue Damage in the Commercial Airplane Fleet,” Revision A, dated June 29, 1999 (a report of the Airworthiness Assurance Working Group for the Aviation Rulemaking Advisory Committee’s Transport Aircraft and Engine Issues Group). 9 Ligament is the material between discontinuities in a given structure. Types of discontinuities include holes, cutouts, or cracks. 01/10/11 AC 120-104 Appendix 6 A6-7 Consequently, when performing your evaluation, you should make conservative assumptions and assume failures to be adjacent to each other. (b) When an element fails completely, the load that has to be redistributed onto the non-failed structure can be large and can have a significant impact on evaluation results. You should therefore consider the effects of load redistribution and include it in your evaluation. 2. Establishing Maintenance Actions. a. Structural Modification Point and Inspection Start Point. Fatigue damage is the gradual deterioration of a material subjected to repeated loads. This gradual deterioration is a function of use and can be statistically quantified. The term widespread fatigue damage is used, and can be statistically quantified, at the end of the deterioration process—when the structure is no longer able to carry the residual strength loads. As depicted in Figure 6-1, WFD can never be absolutely precluded because there is always some probability, no matter how small, that it will occur. Therefore, modifying or replacing structure at a pre-determined, analytically-derived time stated in flight cycles or flight hours, minimizes the probability of having WFD in the fleet. Modification or replacement is the most reliable method for precluding WFD. The point at which a modification is undertaken is referred to as the “structural modification point” (SMP) and it is illustrated in Figure 6-3. The SMP is generally a fraction of the number representing the point in time when WFD(average behavior) will occur, and should result in the same reliability as a successful two-lifetime fatigue test. This level of reliability for setting the SMP is acceptable if inspections for multiple site damage or multiple element damage are shown to be effective in detecting cracks. If the inspections are effective, they must be implemented before the SMP. The implementation times for these inspections are known as the “inspection start points.” Repeat inspections are usually necessary to maintain this effectiveness in detecting cracks. If inspections for multiple site damage or multiple element damage are not effective in detecting cracks, then SMP should be set at the time of inspection start point. For the purposes of this AC, an inspection is effective if, when performed by properly trained maintenance personnel, it will readily detect the damage in question.10 The SMP should minimize the extent of cracking in the susceptible structural area in a fleet of affected airplanes. In fact, if this point is appropriately determined, a high percentage of airplanes would not have any multiple site damage or multiple element damage by the time the SMP is reached. 10 The cracking identified in airworthiness directive (AD) 2002-07-09 is an example of the type of cracking that MSD inspections are effective in detecting. These cracks grow from the fastener holes in the lower row of the lower skin panel in such a way that the cracking is readily detectable using non-destructive inspection methods. The cracking identified in AD 2002-07-08 is an example of places where MSD inspections are not effective. These cracks grow in the outer surface and between the fastener holes in the lower row of the lower skin panel in such a way that the cracking is not readily detectable using non-destructive inspection methods. Modification is the only option to address this type of cracking. 01/10/11 AC 120-104 Appendix 6 A6-8 (1) Structural modification point. The SMP should be established as a point in time when structure should be modified or replaced to prevent WFD from occurring. This is typically established by—  Calculating when WFD would first occur in the structure (predicted using the WFD(average behavior)).  Setting a time before the predicted occurrence of WFD to perform modifications or replacements that will prevent it. (a) As an example, the SMP may be determined by dividing the number representing the timing of when WFD(average behavior) will occur by a factor of 2 if there are effective inspections, or by a factor of 3 if inspections are not effective. (b) A design approval holder may find that the SMP for a particular structural area has been exceeded by one or more airplanes in the fleet. In that case, the design approval holder should evaluate those high-time airplanes to determine their structural condition and notify the airworthiness authorities and propose appropriate maintenance actions specific to those airplanes. (2) Inspection start point. If an inspection is determined to be effective, you will need to establish when those inspections should start. This point is illustrated in Figure 6-1. The start point is determined through a statistical analysis of crack initiation based on fatigue testing, teardown, or in-service experience of similar structure. The inspection start point is assumed to be equivalent to a lower-bound value with a specific probability in the statistical distribution of cracking events. Alternatively, you may establish an inspection start point by applying appropriate factors (e.g., dividing by a factor of 3) to the number representing WFD average behavior. (3) Inspection interval. The interval between inspections depends on the detectable crack size and the probability that it will be detected with the specific inspection method. (4) Adjustment of SMP. The SMP may be extended or reduced, based on the following: (a) Extension of SMP. You can extend the SMP if you can show freedom from WFD up to the new SMP. The tasks required to extend the SMP may include any or all of the following: 1 Additional fatigue or residual strength tests, or both, on a full-scale airplane structure or a full-scale component followed by detailed inspections and analyses. 01/10/11 AC 120-104 Appendix 6 A6-9 2 Fatigue tests of new structure or structure from in-service airplanes on a smaller scale than full component tests (i.e., sub-component or panel tests, or both). If a sub-component test is used, the SMP would be extended only for that sub-component. 3 Teardown inspections (destructive) on structural components that have been removed from service. 4 Teardown inspections (non-destructive) accomplished by selected, limited disassembly and subsequent reassembly of specific areas of high- time airplanes. 5 Analysis of in-service data (e.g., inspections) from a statistically significant number of airplanes. (b) Reduction of SMP. If an inspection program for WFD is in place, and if cracks are found in the structure during an inspection (or during a modification or replacement), the SMP should be reevaluated to determine if it provides sufficient freedom from WFD. If it does not, the point in the life of the airplane at which it is accomplished should be shortened, and you should revise the existing service information accordingly. 01/10/11 AC 120-104 Appendix 6 A6-10 Figure 6-3 MSD/MED Residual Strength Curve (Including WFD Inspections) REQUIRED STRENGTH RESIDUAL ACTUAL RESIDUAL STRENGTH NOTE: No Scale Implied. For Definition Use Only CRACK LENGTH MSD/MED DETECTABLE FLIGHT CYCLES/HOURS ISP SMP I WFD I WFD I WFD I WFD adet a PROBABILITY DENSITY FUNCTION FOR CYCLES TO WFD CONDITION REDUCTION FROM AVERAGE BEHAVIOR TO PROVIDE EQUIVALENT PROTECTION TO A TWO-LIFETIME FATIGUE TEST WFD (AVERAGE BEHAVIOR) acrit WFD SPECIAL MSD/MED INSPECTIONS 01/10/11 AC 120-104 Appendix 7 A7-1 APPENDIX 7 EXAMPLE OF HOW TO ESTABLISH AN LOV This appendix provides a simplified example of how to establish an LOV for a specified airplane structural configuration. The process for establishing an LOV involves four steps— Step 1. Identifying a candidate LOV (204.a. of this AC) for the airplane structural configuration (203). Step 2. Identifying WFD-susceptible structure (204.b.). For this evaluation example, it was determined that the airplane structural configuration had six areas with WFD- susceptible structure. Step 3. Performing a WFD evaluation of each of the six areas of WFD-susceptible structure (204.c. and Appendix 6) to determine whether there are inspection start points and structural modification points for the candidate LOV identified. This allows evaluation of the candidate LOV. Figure 7-1, shown below, shows the WFD behavior for one WFD-susceptible area. The figure also shows three different candidate LOVs. Candidate LOV1 is at a point that occurs significantly before the WFD(average behavior) line. This LOV won’t require any maintenance actions. Candidate LOV2 occurs before the WFD(average behavior) line, but closer to it. As a result, inspection will need to start before the LOV. Although candidate LOV3 occurs before the WFD (average behavior) line, with this LOV the probability of WFD in the fleet is unacceptable and an inspection and subsequent modification or replacement is required before the airplane reaches LOV3. Note that for LOV2 and LOV3, if inspections were determined to be unreliable, then the SMP would occur at the point on the chart where the ISP is. Using this example, this decision process needs to be repeated for all six WFD- susceptible areas. Evaluate candidate LOVs and results of WFD evaluations for each susceptible area. 01/10/11 AC 120-104 Appendix 7 A7-2 Figure 7-1 Comparison of WFD-Susceptible Structure to Airplane LOV Step 4. Finalizing the LOV. Once all susceptible areas have been evaluated, the final step is to determine where to establish the LOV that you will propose for compliance (204.d.). Figure 7-2 shows the results of the WFD evaluation of the six WFD-susceptible areas. As shown, there are inspections and modifications or replacements that must be performed over time to preclude WFD. Any LOV can be valid as long as it is demonstrated that, based on its inherent fatigue characteristics and any required maintenance actions, the airplane model will be free from WFD up to the LOV. The example in Figure 7-2 includes three LOVs that could be proposed for compliance.  LOV1: Maintenance actions are not required to address WFD.  LOV2: Inspection and modification or replacement of area four are required to address WFD. WFD (Average Behavior) SMP ISP Flight Cycles/Hours Candidate LOV1 Candidate LOV2 Candidate LOV3 Crack length Residual Strength 01/10/11 AC 120-104 Appendix 7 A7-3  LOV3: The design approval holder may propose an LOV that is greater than LOV2. However, as shown in Figure 7-2, that would result in more maintenance actions than identified for LOV2. Operators would be required to perform maintenance actions in four out of the six WFD-susceptible areas. Areas 1, 2, and 4 would have to be inspected prior to the LOV. Areas 3 and 5 would be free from WFD maintenance actions. Area 4 would be required to be inspected and modified, and then the modification would be required to be inspected prior to the LOV. Area 6 would require only modification prior to reaching the LOV because it was determined that inspections in this area would not be reliable. Some of the maintenance actions required for the LOV may have already been issued in a service bulletin and mandated by airworthiness directive. For the rest, airworthiness directives would need to be issued. Figure 7-2 Airplane Maintenance Actions Airplane Line Time Susceptible Structure LOV3 1 5 4 3 2 6 SMP ISP LOV2 LOV1 (Flight Cycles/Hours) 01/10/11 AC 120-104 Appendix 8 A8-1 APPENDIX 8—SAMPLE COMPLIANCE PLAN This appendix provides an example of what the compliance plan required by § 26.21(d) might look like. It tracks the steps necessary to develop a compliance plan and shows how the AAC Airplane Company prepares the compliance plan for its AAC1 airplanes. Design approval holders are not required to use the same format as the one used in this example. References to compliance/completion/meeting dates are tied to the effective date of the rule, and some may differ based on the design approval holder’s schedules. 01/10/11 AC 120-104 Appendix 8 A8-2 Limit of Validity Widespread Fatigue Damage 14 CFR Part 26 Compliance Plan Document No. AAC1-26.21-CP-01 AAC Airplane Company Airplane Model: AAC1 REVISION X (DATE) 01/10/11 AC 120-104 Appendix 8 A8-3 Table of Contents I. Design Approval Holder: AAC Airplane Company ...................................... II. Airplane Model Applicability [§ 26.21(a)]...................................................... a. Airplane Model: AAC1 ........................................................................... b. Airplane Model Description..................................................................... c. Certification Basis (amendment level for § 25.571) ................................ III. Applicable Part 26 Requirements ................................................................ IV. Airplane Structural Configurations [§ 26.21(b)(1)(i) &(ii)] ............................. V. Establishing Limit of Validity [§ 26.21(b)(1)]................................................. a. Candidate LOV b. Widespread-Fatigue-Damage-Susceptible Structure ............................. c. WFD Evaluation of Susceptible