29-2C, Chg 1 2/12/03
(ii) Inspection methods The designer should strive to ensure adequate inspectability of all structural parts to qualify them under the fail-safe flaw growth provisions. Inspection means that are appropriate for safe crack growth design follow:
(A) Routine Inspections. To support routine inspection programs, blind areas should be avoided, where practical.
(B) Special Inspections. These inspections will generally result from test results as well as the geometry of the design. Care should be given to special inspection techniques to be used in the field. Inspection techniques requiring facilities and resources beyond the capability of the small operator or not generally available in remote-area operations traditionally associated with rotorcraft operations should not be specified for field inspections. Conservative sizes for detectable cracks or other flaws should be used. Sufficient inspection intervals should be provided to detect cracks before they grow from a detectable size to a size that reduces the remaining strength below design limit strength.
(C) Pressurized Chambers. This design feature may be used to detect cracks that cause a chamber to lose its pressure (either positive or negative). Gauges can indicate the loss of pressure, or dye may be used if it is shown to be a dependable indicator.
(D) Vibration Generation. This characteristic should be considered both from the aspect of vibrations giving indications of a failure, and from the aspect of the increased fatigue loading resulting from the vibrations.
(E) Noise Generation. If initial failure will result in a clear and unmistakable noise that is sufficiently continuous and loud, this characteristic can be used in achieving flaw tolerance without additional special inspections.
(F) Crack Detection Wire, Foil, etc. Detection wire may be used in areas that are sufficiently well defined so that the wire can be properly located. This technique is appropriate in areas otherwise difficult to inspect.
(iii) Multiple Elements . Use of multiple element structures may be provided so that damage or failure occurring in one element of the member will be confined to that element and the remaining structure will still possess adequate load-carrying ability until the failed element is discovered by inspection. These provisions should be designed to provide adequate independence of each failure mode of multi-path constructions. The use of full -scale fatigue test articles is recommended in this evaluation. Examples of concurrent multiple damage to be avoided are:
(A) Simultaneous failure or partial failure of multiple path discrete elements working at similar stress levels.
(B) Failures or partial failures, in adjacent areas, due to redistribution of loading, following a failure of a single element.
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(iv) Partial Failures. The following are typical examples of the type of partial failures that may be considered in the flaw growth fail-safe evaluation:
(A) Detectable skin cracks in the trailing edge sections of rotor blades.
(B) Detectable failures of individual straps in “strap packs.”
(C) Detectable skin cracks emanating from the edge of structural openings or cutouts.
(D) Detectable circumferential or longitudinal skin crack in the basic fuselage or tail boom structure.
(E) Complete severance of interior frame elements or stiffeners in addition to a detectable crack in the adjacent skin.
(F) Presence of a detectable fatigue failure in at least the tension portion of the spar web or similar element.
(G) Detectable failure of a primary attachment, including blade attachment fittings and control surface hinge and fittings.
(H) Fretting, corrosion, and galling conditions expected in service.
(v) Initial inspection time. The initial crack size to be used for evaluati on of the initial inspection time should be the flaw size controlled by manufacturing quality and damage that can go undetected for the life of the part.
(vi) Evaluation of the inspection intervals. For multiple load paths, a minimum of three inspection intervals is recommended between the initially detectable damage time and the time when residual strength is reduced to design limit load by crack growth. For single element structures, a minimum of four inspection intervals is recommended. The repeated loads should be defined in the loading, temperature, and humidity spectra. The loading conditions should take into account the effects of structural flexibility and rate of loading where it is significant.
Tests of two or more specimens should be used to obtain crack propagation data using either a realistic load spectrum or an accelerated load (spectrum or single) associated with the use of propagation theory and data after cracks have been initiated. As far as applicable, crack growth tests should be carried out considering a fatigue crack naturally induced in testing. In the other cases, crack initiation points should be made by suitable methods, reference paragraph g(3)(viii). Unless a more rational method with an equivalent level-of-safety is applied for, the following methods of setting inspection intervals should be applied. In all cases, the inspection methods and intervals should Page MG 11 - 11
29-2C, Chg 1 2/12/03 adequately consider variables such as inspectability, type of inspection, crack growth behavior, and other scheduled maintenance considerations.
(A) For a single element (load path) structure, plot the data and set the inspection as shown in Figure AC 29.MG 11-1.
(1) Set the initial inspection at L1 /3.
(2) Set the repetitive inspection intervals at L2 /4.
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CRITICAL CRACK LENGTH FOR LIMIT LOAD
FLAW SIZE
FIELD DETECTABLE FLAW SIZE
FLAW SIZE CONTROLLEDT
BY MANUFACTURING QUALITY LIFE FIGURE AC 29.MG 11-1 CRACK GROWTH FOR SINGLE ELEMENT STRUCTURE Page MG 11 - 13
29-2C, Chg 1 2/12/03 (B) For a multi-element load path structure substantiated by test:
(1) Obtain a complete failure of the critical load path, either by initiating and propagating a crack or by artificially disabling it.
(2) Damage (i.e. crack growth starting from nominal flaw or crack initiation) accumulated in the structure prior to load path failure should be considered in establishing Lr .
(3) Note when the residual strength of the remaining elements decreases to limit load due to crack growth, provided with a load margin if appropriate.
(4) From Figure AC 29.MG 11-2, set repetitive inspection intervals at Lr /3.
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FLAVY SIZE
FAILURE OF
CRITICAL CRACK LENGTH FOR LIMIT LOAD CRITICAL MEMBER,
FOR REMAINING MEMBERS OR ELEMENT
AFTER FAILURE OF CRITICAL MEMBER
IN CRITICAL REMAINING
DETECTABLE FLAW SIZE FLAW GROWTH GROWTH FLAW LIFE FIGURE AC 29 MG 11-2 CRACK GROWTH FOR REMAINING ELEMENTS
OF MULTIPLE ELEMENT STRUCTURE
(vil) No/Benign Crack Growth Page MG 11 - 15
29-2C, Chg 1 2/12/03 (A) This method depends on replacement rather than inspection to ensure the continued airworthiness of a PSE. The replacement time is established based on consideration of crack growth characteristics.
(B) To substantiate a structure in the no/benign crack growth category requires demonstration either by analysis, testing, or both, that the rogue crack (aR), which is 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 replaced. The crack should be assumed at the critical location, as defined by the largest stress intensity factor range under the expected service loading range including the ground–air–ground cycle.
(C) To determine the replacement time, the rogue cracks should be assumed at the critical location and the crack growth characteristics should be determined for the expected load/environment spectrum. There are three different scenarios that could result from a crack growth assessment and be used for establishing a replacement time. These scenarios are illustrated in Figures AC 29.MG 11-3, -4 and –5.
(D) The no crack growth scenario is illustrated in Figure AC.29.MG 11-3.
In this figure, the rogue flaw does not grow or growth is insignificant. In this case the replacement time should not exceed the design service life (LDES).
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a
CRA CK S IZ E R
LDES TIME
(E) Figure AC 29.M G 11-4 illustrates the scenario where the rogue crack grows relatively slowly but becomes critical prior to becoming detectable (aDET). In this case the replacement 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 CRI T a R TIME Page MG 11 - 17
29-2C, Chg 1 2/12/03 (F) Figure AC 29.MG 11-5 illustrates the scenario where the rogue crack grows to a detectable size (at L1) before becoming critical (at L1+L2). In this case the replacement time should be set equal to the total crack growth life (L1+L2) divided by a factor N.
CRA CK S IZ E a R a DE T a CRI T L 1 L 2 TIME Page MG 11 - 18
(G) In determining the factor of N to be used for determining the replacement 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.).
(H) The minimum suggested N value should be (1) N=2 in the case where the conservative top-of-scatter crack growth data are used in the crack growth analysis, or (2) N=4 when the average crack growth data are used in the crack growth analysis, or 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).
(I) It should also be noted that, in the no/benign crack growth category, the validity of the crack growth threshold, ∆KITH, is especially important since there is no element of inspection to ensure continued airworthiness. Consistent with this, additional attention may be required relative to 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 to develop a consistent ∆KITH data base and the use of bibliographic data might require additional conservatism.
(viii) Testing of Principal Structural Elements.
(A) The nature and extent of tests on complete structures or on portions of the primary structure will depend upon applicable previous design, construction, tests, and service experience in connection with similar structures. For fail -safe testing considering crack propagation, simulated cracks should be as representative as possible of actual fatigue damage. Where it is not practical to produce actual fatigue cracks, flaws can be simulated by cuts made with a fine saw, sharp blade, guillotine, Electrical Discharge Machine, or other suitable means. The validity of saw cuts, etc., should be verified by comparison to coupon tests of a cracked specimen of the same material. In those cases where bolt failure, or its equivalent, is to be simulated as part of a possible flaw configuration in joints or fittings, bolts can be removed to provide that part of the simulation.
(B) Other test and inspection programs may be used if shown to have comparable or better probability of assuring that a catastrophic fatigue failure will not occur.
(ix) Analytical Evaluation The fail safe characteristics can also be shown analytically, by reliable and conservative methods, by demonstrating that the repeated loads and limit load stresses do not exceed those of previously verified fail safe designs of similar configuration, materials, and inspectability. Analytical models should be properly validated to assure that:
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29-2C, Chg 1 2/12/03 (A) Stress analysis is sufficiently accurate. This can be achieved by similarity with full scale strain surveys, thermography, or other suitable testing approaches.
(B) Fracture mechanics is properly applied.
(C) Load interaction models are verified for the specific load histories.
(D) Consistent material data base is used.
(x) Inspection plan. Detection of flaws before they become dangerous is the ultimate control in ensuring the flaw tolerance characteristics of the structure.
Therefore, the applicant should provide sufficient guidance information to assist operators in establishing the frequency, extent, and methods of inspection of the critical structure, and this kind of information should, under § 29.571(a)(2), be included in the maintenance manual required by § 29.1529. Due to the inherent, complex interactions of the many parameters affecting flaw tolerance, such as operating practices, environmental effects, load sequence on flaw growth, and variations in inspection methods, related operational experience should be taken into account in establishing inspection procedures. Comparative analysis can be used to guide the changes from successful past practice, when necessary. Therefore, maintenance and inspection requirements should recognize the dependence on experience and should be specified in a document that provides for revision as a result of operational experience, such as the one containing the operator’s FAA/AUTHORITY-approved structural inspection program developed through the Maintenance Review Board (MRB) procedures for FAR Part 121 operators.
(4) Safe-Life Demonstration
(i) Information to guide fatigue evaluation based on the safe-life approach is described in details in AC 27-1B MG 11. The safe- life approach is used when both the fail-safe and flaw-tolerant safe-life methods are verified to be impractical due to considerations of inspectability, geometry, or good design practice as described above in paragraph d(2).
(ii) The safe-life approach may not account for flaws and imperfections due to manufacturing and in service conditions as compared to the flaw tolerant approaches. Therefore, conservative factors that adjust for variations in both load and strength are generally utilized when substantiating by the safe-life method.
(5) Combining Methods.
(i) Components may be managed by a retirement time based on the Flaw Tolerant Safe-Life method, paragraph g(2); by an inspection program based on the Fail-Safe method, paragraph g(3); or, if approved, a retirement time based on conventional Safe Life, paragraph g(4). In some cases it may be appropriate to Page MG 11 - 20
establish a retirement time based on the lowest from several methods, and in some cases it may be appropriate to establish both a retirement time and an inspection program for a given structure.
(ii) Retirement Times. The conventional Safe- Life retirement time determined from as-manufactured parts and conventional working curves may be lower than that determined from a Flaw Tolerant Safe-Life evaluation. In this event, the lower of the two retirement times should be used. Additionally, inspection intervals resulting from a Fail-Safe evaluation may be used as retirement times if practical, which removes the need to conduct that specific inspection. In this event, the conventional safe life retirement time should be used if lower.
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