Appendix 2 Appendix 2 Full-Scale Fatigue Testing
1. Overview. Section 25.571(b) requires that special consideration for WFD be included where the design is such that this type of damage could occur. Many such areas typically are found in metallic transport airplane structure. Refer to AC 120-104 for examples of WFD-susceptible structure. These areas have in common the same or similar structural deta ils over large areas that are subject to similar repeated tension-stress levels.
a. Without intervention, simultaneous fatigue cracking would eventually occur at multiple locations that would be very difficult to detect before the structure strength degrades below required levels. A strategy must be in place to prevent such fatigue cracking from occurring, as mere inspection of these areas is not reliable to maintain continued airworthiness of the airplane. Section 25.571(b) requires the app licant to demonstrate with sufficient full-scale fatigue-test evidence that WFD will not occur in any susceptible area within the LOV of the airplane.
b. As discussed in section 2 of this appendix, full-scale fatigue-test evidence may be obtained from in-service experience. Testing should involve subjecting a full-scale fatigue-test article to repeated loads followed by an evaluation to determine residual-strength capability. For new type certificates and derivative models, the applicant should use the results of the evaluation to help establish the LOV for the airplane as discussed in section 7 of this AC.
c. Section 3 of this appendix discusses the key elements of a full-scale fatigue-test program. The scope of the full-scale fatigue-test article required will depend on the type of the certification project as discussed in section 4 of this appendix. For the purposes of this AC, certification projects include: (1) new type certificates, (2) derivative models, (3) type design changes – service bulletins, (4) type design changes – supplemental type certificates, and (5) major repairs. In some cases, data from previous full-scale fatigue testing may exist and may be relevant to the certification project.
d. Applicants must consider the issues identified in section 5 of this appendix when using existing test data as evidence to support compliance.
e. Applicants must consider the issues identified in section 6 of this appendix when using in-service data as evidence to support compliance.
2. Full-Scale Fatigue-Test Evidence. Full-scale fatigue-test evidence in the context of § 25.571 makes up the body of evidence (which may also include service experience) that supports the WFD evaluation for a certification project. It includes data used in determining or bounding the time to develop MSD/MED cracks of a certain size, MSD/MED crack-growth scenarios, and residual-strength capability with MSD/MED cracks present. Types of data include strain-survey results, non-destructive and destructive inspection results, and residualstrength test results. The primary source of full-scale fatigue-test evidence is full-scale fatigue A2-1
Appendix 2 testing. The guidelines contained herein should ensure that sufficient test evidence is produced to provide a high degree of confidence that WFD will not occur within the LOV. This involves a laboratory test in which structure that is representative of the type design being considered is subjected to loading that simulates typical service operation. The test article may be the entire airframe or a portion of it. A source of data that may supplement full-scale fatigue-test evidence is data from operational airplanes. This “in-service” data includes maintenance findings and results of teardown inspections.
3. Elements of a Full-Scale Fatigue-Test Program. The following guidance addresses elements of a test program used in generating the data necessary to support compliance. It is generally applicable to all certification projects.
a. Article. The test article should be representative of the structure of the airplane to be certificated (i.e., a production article). The test article should be conformed in accordance with 14 CFR 21.23. Attributes of the type design that could affect MSD/MED initiation, growth, and subsequent residual-strength capability should be replicated as closely as possible on the test article. Critical attributes include, but are not limited to, the following:
material types and forms, dimensions, joining methods and details, coating and plating, use of faying surface sealant, assembly processes and sequences and, influence of secondary structure (e.g., loads induced due to proximity to the structure under evaluation).
b. Test setup and loading. The test setup and loading should result in a realistic simulation of expected operational loads.
(1) Test setup. The test setup dictates how the loads are introduced into the structure and reacted. Every effort should be made to introduce and react loads as realistically as possible. When compromises are made (e.g., wing-air loading), the resulting internal loads should be evaluated (e.g., using finite-element methods) to ensure that the structure is not being unrealistically underloaded or overloaded, locally or globally.
(2) Test loading. Test loading includes the sequence of statically balanced end conditions that are applied to represent some amount of operational usage. Typically, a test sequence representing 1/10 of an anticipated service life is repeated over and over again until the desired test duration is achieved. The sequence used should include loads from all sources (e.g., A2-2
Appendix 2 cabin pressurization, maneuvers, gusts, engine thrust, control-surface deflection, and landing impact) that are significant for the structure being evaluated. The applicant should provide supporting rationale when a load source is not included in the sequence. Differences between the test sequence and expected operational sequence should be justified. For example, standard practice eliminates low loads considered to be non-damaging, and clip high, infrequent loads that may bias the outcome, but care should be taken in both cases so that the test results are representative. Section 8b(6) of this AC provides guidance on resolving test loading to actual loading.
c. WFD (average behavior). 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. Widespread fatigue damage therefore can be statistically quantified.
Widespread fatigue damage cannot be absolutely precluded because there is always some probability of occurrence. The av erage time in flight cycles and/or flight hours to develop WFD is referred to as the WFD(average behavior) for WFD-susceptible structure. AC 120-104 provides guidance how to determine when structure should be modified or replaced to minimize the probability of having WFD in the fleet. The point at which a modification or replacement is undertaken is referred to as the “structural modification point” (SMP). The SMP is generally a fraction of the number representing the point in time when WFD(average behavior) will occur. 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 inspections are deemed effective, or by a factor of 3 if inspections are not effective. If an inspection is determined to be effective, the “inspection start point” (ISP) may be determined by dividing the number representing the timing of when WFD(average behavior) will occur by a factor of 3.
d. Test duration. The duration of the full-scale fatigue test varies depending on the test objective, and whether the test is to be used for a new type certificate, design change, or repair. It is standard practice to interpret th e unfactored fatigue life of one specimen as the average life. It follows, then, that if one full-scale fatigue-test article maintains the minimum residual-strength requirements of § 25.571(b) for test duration X, one can conservatively assume that the WFD(average behavior) of all WFD-susceptible structure is equal to X. Assuming inspection for MSD/MED is impractical, replacement or modification would be required at X divided by 3 per the guidance in AC 120-104. For areas where inspections for MSD/MED are practical, the applicant may defer replacement or modification until X divided by 2, provided inspections for MSD/MED start at X divided by 3. The applican t should consider these factors when deciding on the duration of a full-scale fatigue test.
(1) New type certificates and derivatives. The applicant must establish an LOV for new type certificates and derivative models. There are no requirements regarding the value of LOV. The full-scale fatigue-test duration shou ld not be less than 2 times the LOV. Although not required, a longer test may identify unanticipated fatigue-sensitive areas as well as help validate cracking sites, cracking scenarios, crack-growth lives, and residual-strength capabilities.
The applicant may also use a longer test to reduce or eliminate the need for WFD-related mandatory-maintenance actions. Consistent with the guidance in section (3)(d) of this appendix, and AC 120-104, a test duration of 3 times the LOV, without an occurrence of WFD, would eliminate the need for WFD-related mandatory-maintenance requirements. On the other hand, a A2-3
Appendix 2 test duration of 2 times the LOV may result in required inspections and/or replacements/modifications prior to the LOV.
(2) Repairs and type design changes. The test duration should support the installation of repairs and type design changes up to the LOV of the affected airplane model. If it is conservatively assumed that the repair or type-design change is implemented before an airplane has accumulated any time in service, the rationale discussed in section 3d(1), above, applies. A test duration of 3 times the LOV w ould be recommended. However, consideration of the age of the airplane being repaired or modified could reduce the test duration. For example, an applicant for a repair or type design change to an airplane that has reached an age equivalent to 75 percent of its LOV must demonstrate that the repaired or modified airplane will be free from WFD for at least the remaining 25 percent of the LOV. In this example, a test duration of 75% of the LOV would be recommended (i.e., 3 times the time remaining until the LOV is reached).
e. Post-test evaluation. One of the primary objectives of the full-scale fatigue test is to generate data needed to determine the WFD(average behavior) for each susceptible area or establish a lower bound. The definition of WFD(average behavior) is the average time required for MSD/MED to initiate and grow to the point that the static-strength capability of the structure is reduced below the residual-strength requirements of § 25.571(b). A residual-strength assessment is required at the end of the full-scale fatigue test to demonstrate that the structure meets this requirement.
(1) Residual-strength tests. The direct way to demonstrate freedom from WFD at the end of a full-scale fatigue test is to subject the article to the required residual-strength loads specified in § 25.571(b). If the te st article sustains these loads, the applicant has demonstrated that the structure has a minimum WFD(average behavior) equal to the number of flight cycles and/or flight hours tested. However, beca use fatigue cracks that might exist at the end of the test are not quantified, it is not possible to determine how far beyond the test duration WFD would occur in any of the susceptible areas without additional work (e.g., teardown inspection). Residualstrength testing could preclude the possibility of using an article for additional fatigue testing.
For example, the application of loads in excess of representative operational loads on a metallic test article may result in local material yielding at existing crack tips that may slow or temporarily stop crack growth.
(2) Teardown inspections. The residual-strength capability may be evaluated indirectly by performing teardown inspections to quantify the size of any MSD/MED cracks that might be present, or to establish a lower bound on crack size based on inspection-method capability. After this is done, the residual-strength capability can be estimated analytically.
Depending on the results, crack-growth analyses may also be required to project backward or forward in time to estimate the WFD(average behavior) for an area. As a minimum, teardowninspection methods should be capable of detecting the minimum size of MSD or MED cracking that would result in a WFD condition (i.e., residual strength degraded to below the level specified in § 25.571(b)). Effective teardown insp ections required to demonstrate freedom from WFD typically require significant resources. They usually require disassembly (e.g., fastener A2-4
Appendix 2 removal) and destruction of the test article. All areas that are susceptible to WFD should be identified and examined.
4. Scope of the Full-Scale Fatigue-Test Article. The scope of the full-scale fatigue-test article is generally dependent on the type of certification project. Guidance on the expected scope for various certification projects is as follows:
a. New type certificates. As previously discussed, for any metallic airplane design, it is likely that multiple areas are susceptible to WFD based on the examples given in AC 120-104.
Likewise, testing a complete airframe is probably the most practical approach. Regardless, the applicant is expected to subject all susceptible areas to full-scale fatigue testing. The only exception involves existing full-scale fatigue-test evidence that could be used to support compliance.
b. Derivative models – amended type certificates. The default for a derivative model would be to test the entire airframe. However, it is considered likely that existing data was developed and is available to support compliance of the original model. If this is the case, the article may be limited to only a portion or portions of the complete airframe.
c. Type-design changes – service bulletins. The default for a type-certificate holder ’s type design change is to test an article consisting of the design change and any original structure that is susceptible to WFD, and which the design change could negatively affect in terms of fatigue. See appendix 4 for examples of types of alterations for which the typecertificate holder should perform full-scale fatigue tests, unless the type-certificate holder can show that fatigue testing is not required (e.g., an alteration is not susceptible to WFD, or sufficient test data already exists). It is ex pected that the type-certificate holder has existing relevant data. If this is the case, this type of certification project would not require a test, provided that the type-certificate holder demonstrates, with the existing data, that the design change and any of the original structure that it affects will be free from WFD up to the LOV of the airplane to which the design change applies.
d. Type-design changes – supplemental type certificates. Because supplemental type certificate applicants often do not have access to the type-certificate holder’s full-scale fatigue-test data, a full-scale fatigue-test article is expected. The article should consist of the design change and any original structure that is susceptible to WFD, and any structure that the design change could negatively affect in terms of fatigue. See appendix 4 for examples of types of alterations for which the applicant for an supplemental type certificate should perform fullscale fatigue tests, unless the applicant can show that fatigue testing is not required (e.g., an alteration is not susceptible to WFD, or sufficient test data already exists).
e. Major repairs. In general, major repairs do not require testing of a full-scale test article. For those repairs where testing is required , the test article should consist of the repair and any original structure that is susceptible to WFD, and which the repair could negatively affect in terms of fatigue.
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Appendix 2
(1) Published repair instructions (Amendment 25-96 or later). The applicant for a type certificate usually develops published repair instructions, such as a structural repair manual, for an airplane model. The applicant should perform analyses, such as fatigue and damage-tolerance analyses, that correlate to results of relevant full-scale fatigue tests to show the repairs in that manual are free from WFD up to the LOV. Alternatively, the applicant could perform component fatigue testing to show the published repairs are free from WFD up to the LOV.
(2) Non-published repair instructions (Amendment 25-96 or later). The applicant should correlate analysis methods to results of relevant full-scale fatigue tests, or perform component fatigue testing to evaluate the repair. Repairs for which correlated analysis is acceptable are those that are the same as, or comparable to, those found in FAA-approved published data, such as the type-certificate holder’s structural repair manual. For the purpose of this AC, a repair is comparable when the operating stress levels and fatigue characteristics are approximately the same as those of the FAA-approved published data. To achieve this, the repair needs to have design aspects, such as design details, airframe materials, and production processes that are similar to the FAA-approved published data. For other repairs and depending on the type of repair, the applicant may need to perform full-scale fatigue testing. Alternatively, the applicant may propose analysis methods, such as fatigue and damage-tolerance analyses, that compare the fatigue characteristics of the original type design to the repaired structure and affected areas.
5. Use of Existing Full-Scale Fatigue-Test Data. In some cases, especially for derivative models and type-design changes the type-certificate holder accomplished, existing full-scale fatigue-test data may be used in supporting compliance and mitigating the need to perform additional testing. This is most probable when the data are from testing performed to support compliance with § 25.571 at Amendment 25-96 or later. This is less likely with other data.
a. Amendment 25-96 or later. Because the primary objective for developing the original full-scale fatigue-test data is to establish an absolute or lower-bound WFD(average behavior), for WFD-susceptible areas to support compliance, the original test article should be conformed and a rigorous post-test evaluation should be completed to demonstrate the remaining residualstrength capability. In addition to demonstrating residual-strength capability, the applicant must identify and reconcile any physical differences between the structure originally tested and the structure being considered that could affect its fatigue behavior. Differences that should be addressed include, but are not limited to, differences in any of the physical attributes listed in section 3a of this appendix. Additionally, the applicant should identify and reconcile differences in operational loading. As a minimum, th e applicant should address the following:
Gross weight (e.g., increases) Cabin pressurization (e.g., change in maximum cabin or operating altitude) Flight-segment parameters A2-6
Appendix 2
b. Other data. Performing full-scale fatigue testing to support compliance with § 25.571(b) was not required prior to Amendment 25-96. It was accepted that fatigue would be managed solely by inspection, and that inspection requirements adequately could be determined based on analysis supported by limited testing at the coupon and component level. Nevertheless, many OEMs performed their own full-scale fatigue tests with the primary objective to evaluate the economic life of the structure. However, most test articles were not conformed in accordance with 14 CFR 21.33. Since no regulation required such tests, applicants did not submit test plans or reports to the FAA. Testi ng philosophies and protocols were not standardized, and the rigor of methods used for load-sequence development varied significantly over the years and from one OEM to another. Post-test evaluations (if perf ormed) varied significantly and, in some cases, consisted of nothing more than limited visual inspections. An applicant may propose to use fullscale fatigue-test data from airplanes certificated to 14 CFR 25.571 (pre-Amendment 25-96) to support compliance with Amendment 25-96 or later requirements. In such cases, the applicant must consider the issues identified in section 5a of this appendix.
6. Use of In-Service Data. In-service data may be available to be used in supporting WFD evaluations. Examples of such data are as follows:
Documented positive findings of MSD/MED cracks that include location, size, and the time in service of the affected airplane, along with a credible record of how the airplane had been operated since original delivery.
Documented negative findings from in-service inspections for MSD/MED cracks on a statistically significant number of airplanes, with the time in service of each aircraft and a credible record of how each aircraft had been operated since original delivery. For this data to be useful, the inspection methods used should be capable of detecting MSD/MED crack sizes equal to or smaller than those sizes that could reduce the strength of the structure below the residualstrength levels specified in § 25.571(b).
Documented findings from the destructive teardown inspection of structure from inservice airplanes. This might be structure (e .g., fuselage splices) removed from airplanes that were subsequently returned to service or from retired airplanes. It would also be necessary to have a credible record of the operational loading the subject structure experienced up to the time it was taken out of service.
a. Prior to using in-service data, any physical and loading differences that exist between the structure of the in-service or retired airplanes, and the structure being certified, should be identified and reconciled as discussed in section 5a of this appendix.
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All Structure 1 r Step 1: Is the integrity of the No - I ) structure essential in maintaining the End overall structural inegrity of the - Step 10: Revise FAA airplane and does it contribute ,. ' . ,. approved ALS to include all significantly to the carrying of flight, inspections, modifications, and ground, or pressurization loads? replacements required before the LOV.
Yes '~ 1 f Step 2: Must comply with § 25.571(a) and (b) or (c), as applicable.
Yes No Step 3: Is the structure susceptible to No Step 9: Are any inspections, fatigue damage? modifications, or replacements required Yes before the LOV?
Step 4: Is it reasonably foreseeable that ·~ .~ fatigue cracking will occur before the LOV? Fatigue cracking in this context No includes all fatigue-cracking scenarios, including fatigue cracking at unique design features as well as MSD and MED.
Yes Step 8: Establish No - modifications or Step 5: Can the structure be reliably - replacements, inspected? including time for Yes modifying or replacing structure.
Step 6: Are inspections looking at ,.
multiple details (e.g., rivet/fastener holes, adjacent frames/stringers) over Yes (proceed to steps 7 and 8) large areas for relatively small cracks (MSD/MED)?
No 1r Step 7: Establish requirements for
- inspections, including thresholds - and repetitive inspection intervals.
1/13/11 AC 25.571-1D Appendix 3 Appendix 3
Process for Compliance with 14 CFR 25.571(a), (b) and (c) for Fatigue Damage This chart applies only to § 25.571(a), (b), and (c) for fatigue damage (FD), relative to LOV.
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Appendix 3 For the purposes of this flowchart “All Structure” refers to all structure within the scope of § 25.571. This includes fatigue critical st ructure, as defined in 14 CFR 26.41.
Step 1: Is the integrity of the structure essential in maintaining the overall structural integrity of the airplane, and does it contribute significantly to the carrying of flight, ground, or pressurization loads?
If Yes, go to Step 2. Structure meeting these criteria are within the scope of § 25.571.
See section 6c of this AC for examples.
If No, then the structure under consideration is not within the scope of § 25.571.
Terminate the process.
Step 2: Evaluate each structure in accordance with § 25.571(a) and (b) or (c), as applicable.
Structure identified to be within the scope of the § 25.571 requires a showing of compliance with § 25.571 (a) and (b) or (c), as applicable, relative to fatigue damage.
Corrosion damage and accidental damage, including manufacturing defects, are not addressed in this flowchart.
Corrosion typically has been addressed by maintaining corrosion to Level 1 or better, and to corrosion control and accidental damage programs. These programs are developed under the Air Transport Association of America, Inc. (ATA) Maintenance Steering Group’s MSG-3 or other accepted version of the “Operator/Manufacturer Scheduled Maintenance Development” procedures.
As airplanes are subjected to these programs, operators should evaluate the programs and adjust them to fit their operations.
Note: Section 25.571 requires applicants to evaluate the strength, detail design, and fabrication to show that catastrophic failure due to fatigue, corrosion, manufacturing defects, or accidental damage will be avoided throughout the operational life of the airplane. It al so requires that, based on the evaluations, inspections, or other procedures be established, as necessary, to prevent catastrophic failure, and be included in the ALS of the ICA, as required by § 25.1529.
As discussed in section 6i(3) of this AC, the applicant may use the Maintenance Review Board process to develop maintenance tasks that require inspections or other procedures for addressing accidental and environmental (corrosion) damage. If this process is used as a means of compliance, the applicant should reference, in the ALS, the maintenance documents that contain those tasks.
Step 3: Is the structure susceptible to fatigue damage?
If Yes, go to Step 4.
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Appendix 3 If No, no further showing of compliance is necessary. Terminate the process.
Structure that is susceptible to fatigue cracking, which could, without intervention, eventually lead to catastrophic failure of an airplane, typically includes structure critical to carrying flight, ground, or pressurization loads that are subjected to tensiondominated repeated loads during operation (as discussed previously in this document).
This structure also includes structure which, if repaired or altered, could be susceptible to fatigue cracking and contribute to a catastrophic failure.
Step 4: Is it reasonably foreseeable that fatigue cracking will occur before the LOV? Fatigue cracking in this context includes all fatigue-cracking scenarios, including fatigue cracking at unique design feature as well as MSD and MED.
If No, no further showing of compliance is necessary. Terminate the process. Note that sufficient evidence must support a high degree of confidence that WFD will not occur within the LOV.
If Yes, go to Step 5.
To address this question, estimate the fatigue life for all susceptible structure. The evaluation should be based on analysis, test, or service experience, or a combination of these, as necessary. An applicant may screen out areas that exhibit long fatigue lives.
This means that the average fatigue life is a multiple of the LOV. Typically, the predicted fatigue life should be at least 3 times the LOV to justify eliminating the structure from further consideration. This ma y be the case for basic fuselage structure, away from discontinuities such as splices, windows, or doors, where the working-stress levels are relatively low. For structure that cannot be eliminated, additional evaluations are required.
Step 5: Can the structure be inspected reliably?
If Yes, go to Step 6.
If No, go to Step 8.
To address this question, crack-growth and residual-strength evaluations (damagetolerance evaluation) must be performed. These evaluations typically are based on analyses supported by test evidence. Sections 6, 7, and 8 of this AC contain discussions of damage-tolerance evaluations, and fatigue and widespread-fatigue evaluations. Use the results to determine if inspections reliably can detect cracking before it reduces the material strength below the required strength level.
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Appendix 3 For the purposes of this AC, an inspection is “effective” if, when performed by properly trained maintenance personnel, the inspection readily detects the damage in question.1 Step 6: Are inspections looking at multiple details (e.g., rivet/fastener holes, adjacent frames/stringers, etc.) over large areas for relatively small cracks (MSD/MED)?
If Yes, go to Step 7 to establish inspections, then to Step 8 to establish modifications or replacements.
If No, go to Step 7, then to Step 9.
This step determines whether inspections by themselves are adequate for precluding a catastrophic failure, or whether they are supplementary to modifying or replacing structure.
When inspections are focused on details in small areas and have a high probability of detection, they may be used by themselves to ensure continued airworthiness, unless or until there are in-service findings. Based on findings, these inspections may need to be modified, and it may be necessary to modify or replace structure.
When inspections examine multiple details over large areas for relatively small cracks, they should not be used by themselves. Rather, they should be used to supplement the modification or replacement of structure. This is because it would be difficult to achieve the probability of detection required to allow inspection to be used indefinitely as a means to ensure continued operational safety.
Step 7: Establish requirements for inspections, including thresholds and repetitive inspection intervals.
Using the results of your damage-tolerance evaluation, select an inspection method (or methods), and establish the inspection threshold and repetitive inspection interval.
Inspection thresholds must be based on crack-growth analyses and/or tests, with the assumption that the structure contains an initial flaw of the maximum probable size that could exist as a result of manufacturing or service-induced damage.
Thresholds established for cracking at multiple details that are over large areas (WFD-related cracking) should be determined through a statistical analysis of crack 1 The cracking identified in airworthiness directive 2002-07-09 is an example of the type of cracking that MSD inspections effectively detect. These cracks grow from 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 airworthiness directive 2002-07-08 is an example of where MSD inspections are not “effective.” These cracks grow in the outer surface and between 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. Therefore, modification is the only option to address this type of cracking.
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Appendix 3 initiation based on fatigue testing, teardown, or in-service experience of similar structure. If an inspection is determined to be effective, the “inspection start point” (ISP) may be determined by dividing the number representing the timing of when WFD(average behavior) will occur by a factor of 3.
If the answer to the question in Step 6 was Yes, go to Step 8 to establish modifications or replacements.
If the answer to the question in Step 6 is No, go to Step 9 to determine if the inspection requirements established in this step need to go into the ALS.
Step 8: Establish modifications or replacements, including time for modifying or replacing structure.
Based on the results of the fatigue and damage-tolerance evaluations performed, determine the replacement time or SMP (i.e., replacement time or modification time) for affected structure.
For structure that is not susceptible to damage at multiple sites and where the applicant has shown that the damage-tolerance-based inspections are impractical, a replacement time must be established per § 25.571(c). See section 8 of this AC for further guidance.
For structure that is susceptible to damage at multiple sites and where the applicant has shown that damage-tolerance-based inspections are practical, a SMP must be established per § 25.571(b). The Step 6 in spections supplement the required SMP.
An acceptable approach for determining the SMP is to divide the WFD(average behavior) by a factor of 2. To determine ISP, divide the WFD (average behavior) by a factor of 3.
See AC 120-104 for further guidance on establishing SMP and ISP.
For structure that is susceptible to damage at multiple sites and where the applicant has shown that damage-tolerance-based inspections are not reliable in Step 5, an SMP must be established per § 25.571(b). An acceptable approach would be to divide the WFD(average behavior) by a factor of 3.
Go to Step 9 to determine if the modification or replacement requirements need to be included in the ALS.
Step 9: Are any inspections, modifications, or replacements required before the LOV?
If Yes, go to Step 10.
If No, terminate the process.
Based on the results of Steps 7 and 8, determine if any maintenance actions need to be included in the ALS of the ICA. Any inspection with thresholds less than the LOV, or A3-5