(H) Critical areas will be assumed as a typical location of the damage, unless proper justification is provided to limit the applicability to specific areas or sections of the part.
(iii) Classification of Damage.
(A) The results of the threat assessment are used to classify the damage used in the fatigue tolerance evaluation. The process employed to classify the damage will depend on the fatigue tolerance evaluation method to be used. Depending on the method, a BDF, a CDF, a BEC, or an initial inspectable crack must be established.
(B) For each damage type identified, the sizes to be considered should be representative of the maximum sizes that might not be detected by the inspection techniques established for the component. Sizes exceeding those that are likely to occur do not need to be considered. Standard sizes of damage or standard level of aggression may be derived from previous experience. Each applicant will be required to present justification for damage and crack sizes to be used in the fatigue tolerance evaluations. Within the operational life, defect sizes that have been found in service should be correlated with the sizes used in the design certification.
(C) Barely Detectable Flaw (BDF). For retirement time analysis, flaw sizes that are “barely detectable” may be used to conservatively represent the worst case of undetectable flaws. Alternatively, when the detectable size is larger than the one identified by the threat assessment, a smaller size, but one not less than the flaw size likely to occur, can be used. Sometimes an “allowable” detectable size is established as acceptable for a specific manufacturing process, such as castings, to remain in place for the life of the structure. When it is impossible to simulate that maximum allowable size in the test specimen, the sizes available in the specimen may be used, provided the subsequent analysis of the test result conservatively accounts for the shortfall in the damage size.
(D) Clearly Detectable Flaw (CDF). For inspection intervals, flaw sizes that are “clearly detectable” may be used. The largest discrete size of a CDF to be considered may be limited to the maximum size of the CDF that is likely to remain in place for a significant period of time and not be detected during routine inspections for general conditions and normal observations by knowledgeable personnel. The damage size used may be limited to the maximum probable size identified in the threat assessment. For multiple load path structure, the number of failed load paths to be considered should be established.
(E) Bounding Equivalent Crack (BEC). A Bounding Equivalent Crack must be defined to determine a retirement time using the Crack Growth Retirement method.
The size of the BEC should bound the life reducing effect of damage that could occur as a result of manufacturing, maintenance, or the service environment. The size may be established by analytical back calculations from coupon or service fatigue life data accounting for material variability effects in the data. In any case, there should be no Page C - 113
AC 29-2C, Chg 4 12/2/2011 probable damage from any source that would lead to failure of the part in less time that it would take the BEC to reach critical size. Each applicant must justify the BEC sizes used in the analysis; however, there has been some limited experience that indicates that the following BEC sizes could be appropriate.
(1) 0.015 inch or 0.380 mm radius semicircular surface crack for precision-machined mechanical parts
(2) 0.050 inch or 1.270 mm radius quarter-circular corner crack in fastener holes for typical aluminum airframe structure (F) Initial Inspectable Crack. The size and shape of the initial inspectable crack (aDET) must be established when the Crack Growth Inspection approach is used.
The inspection interval is based on the time for the initial inspectable crack to grow to a size (aCRIT) that would result in catastrophic failure of the rotorcraft if limit loads were applied. The initial inspectable crack is a function of the inspection method that is used to detect it. Regardless of the inspection method, the probability of detecting this size crack should be high and it should be substantiated.
(6) Inspectability and Inspection Methods. This section provides guidance on selecting and substantiating damage detection methodology for use with the methods of paragraphs f.(8) (Inspection Intervals) and f.(10) (Approved Equivalent Means). The methods of paragraph f.(8) can result in a mandated inspection program that must be included in the Airworthiness Limitations Section (ALS) of the Instructions for Continued Airworthiness in accordance with § 29.1529 of the regulatory requirements. Qualified personnel must conduct these inspections at the specified interval using the approved method or methods. Additionally, § 29.571 allows that substantiation may be accomplished by “Approved Equivalent Means,” which is discussed in paragraph f.(10).
These Approved Equivalent Means may include actions that detect damage or flaws indirectly, and are substantiated using the methods of paragraph f.(8). These actions should be shown to be reliable and systematically conducted by knowledgeable personnel. The following are considerations for establishing inspections, inspection methods, or indirect damage detection.
(i) Inspectability. The ease of conducting an inspection should be a design goal for principal structural elements. Design features such as open construction, access panels or ports, or other easy access to fatigue critical areas for needed inspections should be considered. A design that requires disassembly in order to conduct a required inspection, other than during a scheduled maintenance disassembly, should be avoided.
(ii) The specific inspection methods that are used to accomplish fatigue substantiation should be:
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(A) Compatible with the threats identified in the threat assessment, paragraph f.(5), and provide a high probability of detection in the threat assessment and their development, under the operational loads and environment.
(B) Consistent with the capabilities, facilities, and resources of the potential operators of the helicopter. The need to conduct complex or difficult field- level inspections should be avoided, especially when the projected usage of the helicopter may include extended periods of operation in remote areas.
(C) Developed and substantiated for each specific application by means of a full-scale test program, or by experience with similar methods in similar applications.
(D) Included in the Airworthiness Limitations Section of the Instructions for Continued Airworthiness in accordance with § 29.1529 as required by § 29.571(g).
(iii) Detectable Damage Size Assessment.
(A) In the case where the substantiation is predicated on the detection of a specific flaw or crack size, an assessment should be conducted to assure that the selected inspection method would be highly reliable in detecting that size of damage in service. This assessment may be based on the known capability of currently available inspection methods and equipment, provided that this capability is verified by a full - scale test program or by experience with the method in service for similar structure and damage.
(B) If the current capability of a specific inspection method is in question, or if the capability of a specific method needs to be extended to a smaller damage size, then a systematic assessment and substantiation of the method for the intended purpose is appropriate. This assessment could include the determination of the Probability of Detection (POD) as a function of damage size and should consider the capabilities of the potential operators of the helicopter and the environment in which the inspections will be conducted.
(iv) Indirect Detection of Damage. Several damage detection procedures are available that could be used as “Approved Equivalent Means” to support substantiation of a structure [reference paragraph f.(10)]. These procedures, if systematically required and conducted by knowledgeable personnel, can be used in conjunction with the methods presented in paragraph f.(8) to achieve the substantiation. Examples of this type of substantiation are:
(A) In-flight damage detectable by vibration, noise, or observing a bladeout-of-track tip path plane. Consideration should be given to the background levels of noise and vibration, as well as whether the indication is of a different character (more detectable) rather than just a change in level (less detectable).
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AC 29-2C, Chg 4 12/2/2011 (B) Damage that is obvious in a preflight check or routine visual examination. This could include obvious flaws or cracking, but also could include structure that is found to be loose, broken, or soft when deflected by hand. Other obvious damage detection could include fluid leaks, missing fasteners, structure bent or out of alignment, or jamming of mechanical parts.
(C) Damage that is indicated following flight completion. Spectrographic oil analysis would be an example.
(D) Damage detection by automated means. This includes crack detection by foil, fiber, or wire break, load monitoring (to detect a change in internal load distribution), acoustic emission monitoring, or other on-board sensors that meet the goals of damage detectability and reliability.
(7) Retirement Times. Each of the four methods below provides a means to establish a retirement time for each PSE. The determination of the fatigue tolerance characteristics should include an assessment using the conventional Safe-Life methodology. In addition, this serves as a baseline for comparison to retirement times determined with flaws and defects included, and should be used as the structure’s retirement time if it is the lowest calculated time.
(i) The conventional Safe-Life methodology accounts for damage induced by fatigue loading but does not account for flaws and defects due to manufacturing and in-service conditions. If the retirement time is established using this method, then the damage identified in paragraph f.(5) (as required by § 29.571(d)(iii)) must be addressed by inspections or other equivalent means. Information to guide a fatigue evaluation based on a conventional Safe-Life approach is provided in detail in AC 27-1B MG 11.
The method consists of:
(A) Establishing mean fatigue curves (e.g., stress-life or strain-life) based on crack initiation in constant-amplitude or spectrum testing of as-manufactured structure;
(B) Establishing working fatigue curves with strength and life margins; and (C) Conducting a cumulative damage working life calculation using known flight loads and estimated usage.
(ii) A Safe-Life retirement time substantiation with BDF provides a safe period of operation of a structure with probable flaws that may remain in place without detection for that period. Barely detectable flaws are intended to conservatively represent a worst-case of undetectable flaws. The substantiation is accomplished by testing and analysis employing conventional Safe-Life methodology except that an intrinsic and discrete critical flaw in critical locations on the structure is considered. It should be noted that this method, since it is a Safe-Life (crack initiation) method, is not appropriate for use when the flaw being considered is already a crack.
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(A) The types, sizes, and locations of flaws to be considered are determined by the threat assessment (paragraph f.(5)). These flaws may be represented by “equivalent flaws” if it is demonstrated that they have the same or a more severe strength-reducing effect than the corresponding representative flaws.
(B) The mean fatigue strength of the structure with flaws may be determined by one of the following three methods:
(1) Testing a full-scale structure with flaws:
(i) Representative flaws as determined by the threat assessment, or equivalent flaws if substantiated, are imposed at the critical locations on the structure where flaws are likely to occur.
(ii) S-N or spectrum safe-life fatigue testing is conducted; see paragraph e of AC 27-1B MG 11.
(iii) A mean S-N curve with flaws is derived directly from this data.
(2) As-manufactured structure strength modified by the effect of flaws.
(i) A mean strength for as-manufactured structure (without flaws) can be determined using full-scale S-N or spectrum safe-life fatigue testing.
(ii) The effect of flaws may be determined by analysis, by similarity to components where the effect of the flaws has previously been determined, or by a specimen test program incorporating the pertinent features of the full -scale component.
Consideration should be given to the material form, geometric features, surface finish, and steady and vibratory load levels, in combination with flaws representative of those identified in the threat assessment.
(iii) The effect of the flaws is combined with the fatigue result determined on the as-manufactured structure without flaws.
(3) Analytical mean strength modified by the effect of flaws:
(i) A mean strength for as-manufactured structure (without flaws) can be determined analytically, provided that correlation with a similar design can be accomplished, or if additional conservatism is included in the working curve reductions employed in paragraph f.(7)(ii)(C).
(ii) The effect of flaws may be determined by analysis, by similarity to components where the effect of the flaws has previously been determined, or by a specimen test program incorporating the pertinent features of the full -scale component.
Consideration should be given to the material form, geometric features, surface finish, Page C - 117
AC 29-2C, Chg 4 12/2/2011 and steady and vibratory load levels in combination with flaws representative of those identified in the threat assessment.
(iii) The effect of the flaws is combined with the fatigue result analytically determined for the as-manufactured structure without flaws.
(C) Working Curve Determination. Reduction factors should be applied to the mean curve determined above to derive a working fatigue curve. As outlined in AC 27-1B MG 11, working curve reduction factors should include consideration of the number of specimens tested, variability (scatter), previous test data on the same materials or similar structures, as well as service experience. Different reduction factors from those used for conventional Safe-Life methodology may be employed if appropriately justified.
(D) Retirement Time Determination. The working fatigue curve, flight loads (paragraph f.(3)), and usage spectrum (paragraph f.(4)) are used with a cumulative damage analysis such as shown in AC 27-1B MG 11, to calculate a safe retirement time.
(iii) Safe-Life Retirements with Clearly Detectable Flaws.
(A) A retirement time may also be based on flaws larger than the BDF case, up to the clearly detectable size described in paragraph f.(5), if the applicant chooses. This could be the case, for example, if it was desired to allow a specific manufacturing-related flaw of detectable size to remain in place for the life of the structure without further inspection.
(B) The substantiation for this case can be the same as described in paragraph f.(7)(ii), except that the larger flaws selected for the replacement time substantiation are used instead of the BDFs.
(iv) Crack Growth Retirement.
(A) General.
(1) This approach depends on retirement rather than inspection to ensure the continued airworthiness of a PSE. The retirement time is established based on consideration of crack growth characteristics. Fatigue with damage is addressed by timely retirement and there are no explicit inspection requirements that are derived from this approach.
(2) This approach requires demonstration either by analysis, testing, or both, that the BEC (aBEC), the most severe crack consistent with manufacturing, maintenance, and service environment, will not grow or will not grow to critical size (aCRIT) under the service loading and environment before the structure is retired. The critical crack size (aCRIT) is established by limit load. The crack should be assumed at Page C - 118
the critical location, as defined by the largest stress intensity factor range under the expected service loading range including the ground–air–ground cycle. It is recommended that full scale fatigue testing be undertaken to provide an understanding of the fatigue behavior of the component in support of the chosen methodology. In particular it ensures hot spots are identified, which experience has shown analysis often fails to identify.
(3) A threat assessment (see paragraph f.(5)) should be performed to support establishing the BEC size to be used. It is intended that the BEC conservatively bounds the most severe defect resulting from manufacturing, maintenance, or the service environment. That is, there should be no probable defect, from any source, that would lead to failure of the part in less time than it would take the BEC to reach critical size. It should be noted that the resulting crack is a mathematical expedient that may not represent a true physical crack. If the BEC is defined by analytical back calculations from coupon or service fatigue life data, it will be highly dependent on the predictive tool used (i.e., growth algorithm, material data, etc.).
Therefore, the same predictive tool must be used to perform the fatigue tolerance evaluation. When the BEC is based upon test or service data, it must account for material variability in initiation and growth.
(4) To determine the retirement, the BEC should be assumed at the critical location and the crack growth characteristics should be determined for the expected load and environment spectrum. There are three different scenarios that could result from a crack growth assessment and be used for establishing a retirement time. These scenarios are illustrated in Figure AC 29.571B-2, Figure AC 29.571B-3, and Figure AC 29.571B-4.
(B) No Growth. The no crack growth scenario is illustrated in Figure AC 29.571B-2. Here the BEC does not grow when using top- of-scatter crack growth rate data. In this case the retirement time should not exceed the design service life (LDES).
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AC 29-2C, Chg 4 12/2/2011 CRA CK S IZ E a R
L TIME DES Figure AC 29.571B-2: No Growth (C) Slow Growth of Undetectable Crack. Figure AC 29.571B -3 illustrates the scenario where the BEC grows relatively slowly but becomes critical prior to becoming detectable (aDET). In this case, the retirement time should be set equal to the total crack growth life (LT) divided by a factor N.
CRA CK S IZ E a DE T L T a CRIT a R TIME Figure AC 29.571B-3: Slow Growth of Undetectable Crack Page C - 120
(D) Slow Growth of Detectable Crack. Figure AC 29.571B -4 illustrates the scenario where the BEC grows to a detectable size (at L1) before becoming critical (at L1+L2). In this case, the retirement time should be set equal to the total crack growth life (L1+L2) divided by a factor N.
CRA CK S IZ E a CRIT a DET a R L 1 L 2 TIME Figure AC 29.571B-4: Slow Growth of Detectable Crack (E) Life Factors for Crack Growth Retirement.
(1) In determining the factor of N to be used for determining the retirement time, consideration should be given to the crack growth data used (e.g., top of scatter data versus average data, number of specimens used to generate data, etc.).
(2) The minimum suggested N value should be N=2 in the case where the conservative top-of-scatter crack growth data are used in the crack growth analysis, or N=4 when the average crack growth data are used in the crack growth analysis, or N=4 when the crack growth life is obtained from the crack growth test of one specimen (for two or more full scale specimens, N=3 of the shortest crack growth life can be used).
(3) It should also be noted that with this approach, the validity of the crack growth threshold, Δ Kth, is especially important since there is no element of inspection to ensure continued airworthiness. Consistent with this, additional attention may be required for validating the crack growth threshold value(s) used in the analyses.
Consideration should be given to the influence of the test procedure used to develop values, microstructure, heat treatment, crack size, loading conditions, environment, grain size and orientation, etc. In general, a coupon- testing program may be necessary Page C - 121
AC 29-2C, Chg 4 12/2/2011 to develop a consistent ΔKth database and the use of published data may require additional conservatism.
(8) Inspection Intervals. Each of the following three methods provides a means to establish inspection intervals for detectable damage or detectable damage growth.
The time of the first inspection should coincide with the repetitive interval established unless the applicant can substantiate an alternate time.
(i) Safe-Life Inspection for a CDF provides a safe interval of operation between repetitive inspections for the presence of probable detectable flaws. The substantiation is accomplished by testing and analysis employing conventional Safe-Life methodology except that intrinsic and discrete critical flaws are considered.
The size of flaws considered should be “clearly detectable”, which is intended to be a conservative representation of detectable flaws that could remain in place for the entire interval in spite of routine inspections for general condition. It should be noted that this method, since it is a Safe-Life (crack initiation) method, is not appropriate for use when the flaw being considered is already a crack.
(A) The method described in paragraph f.(7)(iii), Safe-Life Retirements with Clearly Detectable Flaws, may be employed for this case, except that the calculated retirement time is used as a repetitive inspection interval.
(B) The repetitive inspection consists of examination of the structure for the presence of the flaw using the substantiated inspection method. If no flaw is found, the structure may be returned to service for another inspection interval period, up to the established retirement time. If the flaw is found, the structure is retired; or, if a repair procedure for the specific flaw type has been substantiated, the structure is repaired and returned to service for another inspection interval period, up to the established retirement time for the structure.
(C) Substantiation of repairs should include careful consideration as to whether undetectable cracks may now exist and whether the original certification approach is still applicable.
(ii) Crack Growth Inspection. This approach depends on detection of cracks before they become critical to ensure the continued airworthiness of a PSE. While any inspections that are capable of detecting cracks with high reliability may be used with this approach, the criteria stated in paragraph f.(6), Inspectability and Inspection Methods, should be considered in making the selection. The inspection method chosen will define the initial inspectable crack that will be used to perform the fatigue tolerance evaluation. Once the initial inspectable crack is defined, crack growth, and residual strength assessments must be performed to determine the time for the initial inspectable crack (aDET) to grow to a size (aCRIT) that would result in a catastrophic failure of the rotorcraft if limit loads were applied. This assessment could be theoretically done analytically or by test; however, in most cases it is performed analytically using fracture mechanics methods. The resulting life for a DET to grow to Page C - 122
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 of defining the interval once the crack growth life has been determined are discussed later.
(A) Single Load Path Structure. The time for a detectable crack (a DET) to grow to critical size (aCRIT) in a structure is denoted as L2 in Figure AC 29.571B-4. If this were a single load path structure, the inspection interval would be established as L2 divided by N. (See paragraph f.(8)(ii)(C) for guidance on values of N.) This interval is valid until the part is retired.
(B) Multiple Element Structure.
(1) Depending on inspectability considerations and residual life characteristics of the structure following a load path failure, it may be beneficial to take advantage of the redundancy of a multiple load path structure. On the other hand, the safety of a multiple load path structure can be managed without taking advantage of its redundancy. In this case, each load path would be considered independently and inspection intervals established for each load path consistent with paragraph f.(8)(ii)(A).
This may be necessary for similarly stressed load paths when damage according to the threat assessment could occur in each element at the same time.
(2) When considering redundancy in a multiple load path structure, two scenarios might be possible; one where the required inspection is for a completely failed load path and one where the inspection is for less than a load path failure. In either case, the remaining life of the secondary load path after primary load path failure is used to determine the inspection interval. Consistent with this, the resulting intervals are only valid until the cumulative fatigue damage or crack growth in the intact structure is taken into account. This issue is illustrated in a crack growth context in Figure AC 29.571B-5. Crack growth in the secondary load path from an initial crack as detailed in paragraph f.(8)(ii)(B)(3)(i) will proceed along curve A-B as long as the primary load path remains intact and load redistribution is negligible. However, at the time of primary load path failure, loading on the secondary load path will increase due to load redistribution and crack growth will be accelerated (e.g., subsequent growth from point 1, 2, or 3 depending on if the failure occurs at time t 1, t2 or t3). Note that the residual life, Lr, in the secondary load path is inversely proportional to the time at which primary load path failure occurs. This should be considered whenever L r is used in establishing repeat inspection intervals.
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........ ---·-· _ ..... -----·-·-· .... -•-· ·----------------------·-------------------------- _____ .... ---· ------- ----- )t_ --- ---------- .... -·
.---
_________ .,.
AC 29-2C, Chg 4 12/2/2011 A B 1 2 3 Lr1 Lr2 Lr3
CRACK SIZE Primary Load Path Crack Growth Primary Load Path Failure Secondary Load Path Crack Growth - Intact Structure t1 t2 t3
CRACK GROWTH LIFE Figure AC 29.571B-5: Decreasing Residual Life in Secondary Load Path for Multiple Element Crack Growth with Inspections.
(3) Inspect for Load Path Failure. If a failed load path is easily detectable and the residual life and strength of the remaining structure is sufficient, this approach may be optimum. Analysis or tests as described in the following paragraphs can determine the inspection interval.
(i) Evaluation by analysis. Figure AC 29.571B -6 illustrates an example 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 (L r) after primary load path failure at time 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 a normal manufacturing quality unless the threat assessment indicates that larger damage could exist. Crack growth accumulated prior to a load path failure is accounted for by calculating the amount of growth, (Δai), between time zero and NF. Load redistribution that may occur prior to N F should be considered. The residual life, (L r), then becomes the time for a crack of size ai + Δai to grow to critical size, assuming a complete load path failure has occurred (i.e., “failed” condition loads used). It should be noted that the assumed time of load path failure would also represent an upper limit of validity for any repeat inspection period based on Lr. It is therefore recommended that N F be assumed equal to the retirement Page C - 124