Appendix 3 Report any cracks you find within (insert a reasonable time frame, 30 days is typical) days after the cracks are found or within 30 days after (insert the effective date of this AD), whichever occurs later. Send your report to (insert the name of the responsible ACO engineer, including pertinent contact information, e.g., address, telephone and facsimile number, and email address). The Office of Management and Budget (OMB) approved the information collection requirements contained in this regulation under the provisions of the Paperwork Reduction Act and assigned OMB Control Number 2120- 0056. Include in your report the following information:
(1) Aircraft model and serial number;
(2) Number of cycles (if applicable);
(3) Aircraft hours Time in Service (TIS);
(4) (insert part description, e.g., right wing front lower spar cap) hours TIS (and/or cycles, if applicable, if it is possible for the part to be removed from one aircraft and installed on another);
(5) Hours TIS (and/or cycles, if applicable) on the (insert part description) since last
inspection;
(6) Crack location and size;
(7) Procedure (visual, penetrant, magnetic particle, ultrasonic, eddy current, etc.) used for the last inspection (if the AD permits more than one inspection method);
(8) Description of any previous modifications and hours TIS when the modification was done, (list pertinent examples, if applicable, such as engine model change, installation of winglets, cold working procedures, repairs, or reinforcements of the area cracked area);
(9) Corrective action taken;
(10) Point of contact name and phone number; and
(11) Clearly identify the AD No., Docket No., and Directorate Identifier of the AD action requiring the report.
The above list should cover most situations. In some instances, the FAA may request photographs and/or reports of damage other than cracks (corrosion, working rivets, nicks, scratches, etc.). If this is the case, then add th at requirement to the list. If items on the list are not applicable, do not include them. Adjust these instructions accordingly if circumstances require inspectors to report additional information or a different report format. Exact wording should be coordinated with the accountable FAA Aircraft Certification Directorate’s AD writereditor staff.
In most instances, the FMP holder (typically the TC holder) will include an inspection reporting form in the FMP instructions. If the reporting form contains the necessary information, then the A3-2
Appendix 3 AD can specify that form be used to report cracks. The following language is suitable for stating this requirement:
Report any cracks you find within (insert a reasonable time frame, 30 days is typical) days after the cracks are found or within 30 days after (insert the effective date of this AD), whichever occurs later. Report the results to (insert the FMP/TC holder name here) using the form in the service bulletin (or the applicable FMP document). Send a copy of this report to (insert the name of the responsible ACO engineer, including pertinent contact information, e.g., address, telephone and facsimile number, and email address).
For the reporting requirement in this AD, under the provisions of the Paperwork Reduction Act, the Office of Management and Budget (OMB) has approved the information collection requirements and has assigned OMB Control Number 2120-0056.
In most cases, reporting is necessary only when cracks are found. However, there are some circumstances when it is important for the FAA to know when cracks are not found. The FAA can use this knowledge to help determine the frequency of cracking fleet-wide and the associated safety risk. The following language is suitable for either of the above instructions when all inspection results need to be reported:
Report all inspection results, positive or negative, within (insert a reasonable time frame, 30 days is typical) days after the cracks are found or within 30 days after (insert the effective date of this AD), whichever occurs later. Send your report to (insert the name of the responsible ACO engineer, including pertinent contact information, e.g., address, telephone and facsimile number, and email address). The Office of Management and Budget (OMB) approved the information collection requirements contained in this regulation under the provisions of the Paperwork Reduction Act and assigned OMB Control Number 2120-0056. Include in your report the following information:
FAA-IR-M-8040.1C, Chapter 8, explains the OMB information regarding collection requirements as they relate to the Paperwork Reduction Act.
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Appendix 4 APPENDIX 4. METHODOLOGY FOR DETERMINING REPEAT INSPECTION
INTERVALS The reliability of repeat inspections should assure crack detection before the residual strength degrades below the required level. The inspection method chosen will define the initial detectable crack that will be used to perform the damage-tolerance evaluation. Once the initial detectable crack is defined, crack growth and residual strength assessments must be performed to determine the time for the initial detectable crack (aDET) to grow to a critical size (aCRIT) that would result in failure if the required residual strength loads were applied. These assessments could be performed analytically or by test; however, in most cases they are performed analytically using fracture mechanics methods. It is the applicant’s option to determine how complex or simple the analysis should be. The applicant may determine that a simple damage tolerance analysis using conservative assumptions and inspection safety factors produces an acceptable inspection interval.
An applicant may also use a more detailed analysis to justify a longer inspection interval. The applicant should explain and justify all simplifying assumptions. The resulting time for aDET to grow to aCRIT is used to set the inspection interval. This general process applies to both single and multiple load path structure regardless of the level of inspection (e.g., for complete load path failure or less than load path failure in a multiple load path structure). The details are discussed further below.
1. Single Load Path Structure. The time for a detectable crack (a DET) to grow to critical size (aCRIT) in a structure is denoted as L in Figure 4-1. The inspection interval would be established as L divided by an inspection safety factor, K1, of not less than 2.0. This provides at least two opportunities to find a crack between the time it reaches a detectable size and before it grows to a critical size.
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Appendix 4 FIGURE 4-1. CRACK GROWTH IN SINGLE LOAD PATH STRUCTURE CRACK SIZE aDET aCRIT L aCRIT: Critical Crack Size aDET: Detectable Crack Size
CRACK GROWTH LIFE
2. Multiple Load Path Structure. An acceptable, but potential ly conservative approach for multiple load path structure is to treat each element as a single load path and establish inspections, as required, to detect cracking before any single element would fail under the required residual strength loading. However, if the residual strength subsequent to a complete element failure exceeds the required residual strength loading, it might be possible to take advantage of redundancy. Any benefits (e.g., le ss onerous inspection method and/or longer repeat inspection intervals) would depend on inspectability and the remaining life of the structure with the element failed. When this is done, the likelihood of simultaneous cracking in multiple elements should be considered and accounted for as necessary.
When taking advantage of redundancy in a multiple load path structure there are two scenarios: when the inspection is for a completely failed load path, and when the inspection is for a cracked but not completely failed load path. In either case, the repeat inspection interval will be a function of the secondary load path remaining life (LS) after primary load path failure. It follows that the resulting interval is only valid out to the time that the cumulative fatigue damage and/or crack growth in the intact structure is accounted for. This issue is illustrated in a crack growth context in Figure 4-2.
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10/1/12 AC 91-82A Appendix 4 Change 1 FIGURE 4-2. DECREASING REMAINING LIFE IN SECONDARY LOAD PATH WITH
TIME FOR MULTIPLE ELEMENT STRUCTURE aCRITp: Primary Load Path Critical Crack Size aCRITs: Secondary Load Path Critical Crack Size aDETp: Primary Load Path Detectable Crack Size LS: Secondary Load Path Remaining Life t: Time in Service
CRACK SIZE aCRITp aCRITs aDETp A B 1 2 3 LS1 LS2 LS3 Primary Load Path Crack Growth Primary Load Path Failure Secondary Load Path Crack Growth – Intact Primary Load Path t1 t2 t3
CRACK GROWTH LIFE (TIME IN SERVICE (TIS)) Figure 4-2 illustrates the damage tolerance characteristics of a simple redundant structure consisting of two load paths. The primary load pa th is the one that is inspected for a crack or complete failure while the secondary load path provides the necessary redundancy. The residual strength of the structure with the primary load path failed and a crack in the secondary load path exceeds the required residual strength until the crack in the secondary load path grows to critical size (aCRITs). Three different crack growth scenarios are shown for a detectable size crack in the primary load path. Each scenario corresponds to the crack in the primary load path becoming detectable at a different point in time and then growing to critical size (aCRITp) and causing complete failure of the primary load path at different points in time (i.e., t1, t2 and t3). At the time of primary load path failure, loading on the secondary load path will increase due to load redistribution and crack growth will accelerate (e.g., subsequent growth from point 1, 2, or 3 depending on if the failure occurs at time t1, t2 or t3). Note that the amount of remaining life in the secondary load path (LS) has an inverse relationship to the time at which primary load path failure occurs. For example, L S3 is less than LS2, which is less than LS1 because the longer crack A4-3
Appendix 4 growth life of the primary load path means that a crack in the secondary load path will have also grown longer and thus have a shorter remaining crack growth life once the primary load path fails. Since the repeat inspecti on interval is a function of LS and LS decreases as the time in service increases, the interval will have to be adjusted when the actual time in service exceeds the time in service considered when the interval was originally established.
a. Inspection for Complete Load Path Failure. Detection of a crack prior to load path failure may be feasible, but difficult. If, on the other hand, a failed load path is easily detectable and the remaining life and residual strength of the structure after load path failure is sufficient, it may be desirable to establish an inspection for complete load path failure instead. Analysis or tests as described in the following paragraphs can be used to determine the inspection interval.
(i) Evaluation by Analysis. Figure 4-3 illustrates an ex ample of multiple load path structure for which a completely failed load path is easily detectable. The inspection interval is based on the life of the secondary load path (LS) after time to primary load path failure (NF).
Consistent with this, damage accumulated in the secondary load path prior to primary load path failure must be accounted for in the analysis. In order to do this within the context of a crack growth analysis it is necessary to assume some initial crack, of size ai, exists in the secondary load path at time zero. This initial crack size should be representative of normal manufacturing quality. Analytical crack growth in the secondary load path accumulated prior to an assumed primary load path failure is accounted for by calculating the amount of growth, (Δai), between time zero and NF. Load redistribution th at may occur prior to NF should be considered. The secondary load path remaining life (LS) is the time for a crack of size ai + Δai in the secondary load path to grow to critical size assuming a complete primary load path failure has occurred (i.e.
“failed” condition loads used – all load now carried by the secondary load path). It should be noted that the calculated time to primary load path failure (NF) would also represent an upper limit of applicability for any repeat inspection period based on LS. This is because if the primary load path failed shortly after NF the time for the assumed crack in the secondary load path to grow to its critical size is less than LS. (The analyst’s selection of L S and NF requires judgment based on a balance between a reasonable inspection interval and reasonable period of applicability of the inspection. A longer N F will result in a shorter LS and vice versa.) Based on the above discussion, the following applies:
• Inspection Interval = LS/K1.
• Inspection Interval Period of Applicability = NF/KSF.
Note 1: Inspection safety factor, K 1, should normally be 2.0 or greater.
Note 2: Life safety factor, K SF, should normally be 1.0 to 2.0 depending on the overall conservatism in the analysis.
Note 3: L S is remaining life in secondary load path.
Note 4: N F is time to primary load path failure.
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Appendix 4 FIGURE 4-3. MULTIPLE LOAD PATH STRUCTURE ANALYTICAL EVALUATION
TO SUPPORT INSPECTION FOR A COMPLETELY FAILED LOAD PATH LS: Secondary Load Path Remaining Life ai: Initial Crack Size NF: Time to Primary Load Path Failure N F L S Primary Load Path Failure Assumed Δ ai Secondary Load Path Failure at Limit Load
CRACK SIZE Analytical Crack Growth in Secondary Load Path ai
CRACK GROWTH LIFE
(ii) Evaluation by Test. Figure 4-4 illustrates some key points if an inspection for a complete load path failure is to be developed based on testing. The inspection interval is based on the test demonstrated remaining life (LS) subsequent to load path failure. Because the remaining life decreases with the time accumulated prior to a load path failure as previously discussed, there will be a period of applicability for the LS and it will be dependent on the time at which a primary load path failure (NF) is simulated.
(iii) The test article should consist of as-manufactured production parts.
Representative “well” condition loading should be applied for some predetermined period of time (NF). At the end of this period the load path that is to be inspected for complete failure should be disabled (e.g., saw cutting, attachment(s) removal, member removal) to simulate its failure. The test should then be restarted with a representative “failed” condition loading. (Note A4-5
Appendix 4 that the external loads may be the same as for the “well” condition if the member failure simulation results in the correct “failed” condition internal load redistribution.) The test should continue until the desired remaining life has been achieved or to the time at which the secondary load path can no longer support the required residual strength loads (NS), whichever is less.
Based on the above discussion, the following applies:
• Demonstrated remaining life = LS = NS-NF.
• Inspection interval = LS/K1.
• Inspection Interval Period of Applicability = NF/KSF.
Note 1: Inspection safety factor, K 1, should normally be 2.0 or greater.
Note 2: Life safety factor, K SF, should be 2.0 for test.
Note 3: L S is remaining life in secondary load path.
Note 4: N F is time to primary load path failure.
Note 5: N S is time to secondary load path failure.
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_I + Appendix 4 FIGURE 4-4. MULTIPLE LOAD PATH STRUCTURE EVALUATION BY TEST TO
SUPPORT INSPECTION FOR A COMPLETELY FAILED LOAD PATH LS Secondary Load Path Not Capable of Limit Load
CRACK SIZE Primary Load Path Intentionally Failed “Well” Condition Loading “Failed” Condition Loading Region of Crack Nucleation & Undetected Crack Growth LS: Secondary Load Path Remaining Life NF: Time to Primary Load Path Failure NS: Time to Secondary Load Path Failure
TEST DURATION NF NS
b. Inspection for Less Than a Complete Load Path Failure. Inspection for less than a load path failure may require special NDE procedures. However, it should result in a longer inspection interval than intervals for detecting more obvious complete load path failure as discussed in paragraph 2a. Figure 4-5 illustrates how inspection intervals could be established based on crack growth and residual strength evaluation.
In this case, the inspection interval is based on the life of the secondary load path (LS) subsequent to primary load path failure at NF plus the time (LP) for a detectable crack size (aDET) in the primary load path to grow to critical size under in-service loads. The determination of LS is the same as discussed in paragraph 2(a)(i) above. Based on the above discussion, the following applies:
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--· ------- Appendix 4
• Inspection Interval = (LP + LS)/K1.
• Inspection Interval Period of Applicability = NF/KSF.
Note 1: Inspection safety factor, K 1, should normally be 2.0 or greater.
Note 2: Life safety factor, K SF, should normally be 1.0 to 2.0 depending on the overall conservatism in the analysis.
Note 3: L P is life from aDET to primary load path failure.
Note 4: L S is remaining life in secondary load path.
Note 5: N F is time to primary load path failure.
FIGURE 4-5. MULTIPLE LOAD PATH STRUCTURE ANALYTICAL EVALUATION
TO SUPPORT INSPECTION FOR LESS THAN A FAILED LOAD PATH
LP: Primary Load Path Life LS: Secondary Load Path Remaining Life NF: Time to Primary Load Path Failure Detectable Crack Size aDET:
ai: Initial Crack Size aDET ai NF LS
CRACK SIZE LP Crack Growth in Primary Load Path Crack Growth in Secondary Load Path after Primary Load Path Failure Failure of Secondary Load Path at Limit Load Failure of Primary Load Path Under In-Service Loads Crack Growth in Secondary Load Path with Primary Load Path Intact Δai