AC 33-12 Figure 7. Precision and Accuracy.
5.9.2 Compared to the rest of the part, certain features have minimal variation for various reasons. Maintaining the dimensional variation within a small range may be necessary to keep stresses below a design threshold, preserve the desired aeromechanical properties, control the location of the part within an assembly, or minimize wear between interfacing parts. Reproducing the dimensional variation ensures these properties and other functional properties are preserved in the replacement part.
5.9.3 The precision of a dimension on a drawing should not exceed the precision of the equipment used to acquire the measurement. Also, if a dimension on a drawing is a result of a stack-up of multiple dimensions that were acquired using various methods, then the precision of the resulting dimension should reflect the least precise measurement in the stack-up.
5.9.4 Normally, reverse-engineered measurement precision is determined by increasing the measurement precision until a threshold is reached where the measurements are consistent and no longer change with higher precision measurements. Measurement techniques that are less precise than the precision to which the type design part was manufactured will likely mask the actual variation, increase the variation in the replacement part, and may fail to reveal features that are in the type design.
5.9.5 Using industry standard manufacturing process yield alone to establish the precision of finished-part tolerances can reduce the dimensional accuracy of the replacement part if the type design part is qualified using inspection criteria that are tighter than the standard process yield. Similarly, reverse engineering techniques that offset nominal shapes or assign tolerances based on criteria from sources that are not associated with the type design part can affect the dimensional accuracy of the replacement part.
5.9.6 Selecting tolerances assuming there are common design practices among engine manufacturers and engine models is another way of introducing dimensional inaccuracies to replacement parts. Reverse engineering processes that use assumptions such as these can introduce substantial design differences to characteristics that affect the airworthiness of the part and result in a major design change to the engine.
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5.10 Choice of Tolerance Control.
5.10.1 There are several options available for characterizing the dimensional variation of a part, which could affect the functional properties of the replacement part. For example, if the type design part defines the dimensional variation of a load-bearing surface by tolerances that control its contour and its distance relative to a datum, and the reverseengineered surface is characterized only by a contour, then the position of the loadbearing surface relative to the datum can vary as much as the manufacturing process allows. This can cause loads to be concentrated or redistributed, among other features, resulting in stresses or wear that exceeds the maximum limits intended by the original part designers. Similarly, using a flatness requirement for a surface, instead of a profile, to control the variation in a load-bearing surface could change the load intensity, distribution, and wear properties at an assembly interface. Profile tolerance controls check all the measured points back to a datum (or datums), and flatness is the distance between two parallel, imaginary planes, which contains all of the points on the surface, independent of a datum. Figures 8 and 9 illustrate the flatness and profile tolerance controls.
Fi gure 8. Flatness Tolerance Control.
This flatness on a drawing can mean this or this Figure 9. Profile Tolerance Control.
This profile on a drawing means this 14
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5.10.2 Differences in tolerance controls that affect surface-to-surface transition radii can result in unintended stress concentrations if the differences allow sharp features in contact surfaces and non-uniform surfaces that do not exist in the type design part. Sometimes, the magnitude of the effects from stress concentrations is only apparent when assessing the minimum and maximum material condition of the reverse-engineered part and interfacing parts. Therefore, applicants should carefully examine the part and its functions to determine the appropriate dimensional controls.
5.10.3 Concentricity, circularity, and run- out are also examples of tolerance controls that can affect the dimensional similarity and the functions of cylindrically shaped parts. When measuring the run-out of a cylindrical part, the tolerance zone is on the surface, whereas the tolerance zone for concentricity is at the center of the cylinder. Circularity is a form control that defines how much a surface of revolution may vary from a perfect circle. Figure 10 shows an example of the total variation in a cylindrical part. In this example, the total variation for the part is the summation of concentricity and run-out measurements.
Fi gure 10. Total Variation from Run-out and Concentricity.
5.11 Statistical Dimensioning.
5.11.1 The drawing tolerances determined by reverse engineering for the proposed replacement part should not exceed the measured data taken from type design parts.
The measurement data might exhibit a good fit with a particular statistical distribution.
but this approach expands the range of dimensional variation beyond the measurements taken from the type design parts. To show a statistical distribution that accurately describes the full range of dimensional variation in a type design part, measurements from many more type design samples are necessary than the amount that is typically used for a replacement part project.
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5.11.2 Inspection limits may restrict or bias dimensional properties of new parts to enhance performance, minimize wear, or improve reliability. Also, part-to-part bench tests usually do not include replacement parts that are made to the extent of their reverseengineered dimensional limits, nor are the interfacing fixtures or type design hardware that are used in the tests. Depending on the criticality and sensitivities of higher-level assemblies and engine systems to dimensional variation in the part, the associated bench tests might not extract all the information necessary to verify there are no dimensional differences that could affect fit and function. Staying within the measurements taken from type design parts will ensure any special inspection criteria applied by the TC holder are captured in the replacement part design.
5.11.3 Inspection criteria may restrict or bias dimensional properties of new parts to enhance their performance, minimize wear, or improve overall system reliability. Dimensional inspection criteria are also used to screen parts produced by a statistically controlled manufacturing process to account for important design considerations such as tolerance stacks with critical mating parts, dynamic properties, and maximum moment weight.
Figure 11 shows an example of how inspection limits can ensure the dimensional attributes of a part are compatible with the higher-level assembly.
Figure 11. Example of a Truncated Process Distribution.
5.11.4 If statistical methods are used to smooth tolerances across a feature, exercise caution to ensure a sufficient number of type design samples are used to acquire the tolerances that will be smoothed. Also, to perform a valid statistical analysis, evaluate the adequacy of measurement density and precision of the area being smoothed to account for any possible non-uniform, restricted, or biased tolerances that might be present in the type design for proper function throughout the certificated engine operating range.
AC 33-12
5.12 Surface Texture.
5.12.1 Surface texture may result from finishing processes or surface treatments that can affect the various functional properties of a part, such as fatigue, wear, performance, and reliability. For example, airfoil surface texture (also referred to as smoothness) affects airfoil fatigue properties and engine performance. The effect of differences in airfoil surface texture on fatigue in bench test conditions might be measurable with the proper comparative bench test, but the effect on engine performance is difficult to quantify without engine data or an analysis properly correlated to engine data. Therefore, the surface texture in a replacement part should be the same as the finish in the type design part, and the finishing processes that produce the type design surface texture should also be comparable.
5.12.2 Many techniques are available for measuring surface texture. The method used to quantify texture should be suitable for the part that is measured. New type design parts are the best source of information to characterize the texture and associated process for surfaces in contact. Figure 12 shows an example of various surface features that contribute to the texture. The measurement results will depend on how the surface is measured, the equipment that is used, and how the contributing parametric features are analyzed.
Fi gure 12. Various Parametric Features to Surface Texture.
5.12.3 Surface texture can result from a surface treatment that produces a compressive layer, such as peening or split-sleeve cold expansion of holes. A compressive surface layer improves fatigue resistance and the durability of a part and helps the engine achieve its certificated performance r equirements. The presence of a compressive layer is not always evident from an inspection of the surface texture. Careful examination of the sub-surface material condition might be necessary to detect a compressive layer, which could involve destructive tests with finished type design parts. Processes that remove material for smoothing surfaces that have a compressive layer to improve component performance can defeat the durability benefits of the compressive layer.
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5.12.4 Engine manufacturers develop their finishing methods, so there is no industry specification delineating the process and resulting surface condition or variation.
Sometimes the finishing process is necessary to meet higher assembly design requirements, such as bolted joint integrity under various operating loads and flight conditions. These circumstances warrant comparative testing to verify the finishing process selected for the replacement part results in the same level of safety as the type design part.
5.13 Influence on Critical Parts.
5.13.1 A non- critical part can influence the integrity of a critical part in varying degrees. The industry generally refers to these parts as “influencing parts.” The influence can be from direct physical contact with the critical part or by the effect the part has on the conditions around the critical part. Dimensional differences in non-critical, influencing parts can have a significant effect on critical parts when they affect contact stresses or the conditions that maintain the operating environment within certain limits. The validation procedures for showing equivalence to non-critical, influencing type design parts should provide enough insight to verify that the engine data that was developed from a direct showing of compliance to 14 CFR 33.70 is not invalidated.
5.13.2 As described in this AC, the way an applicant dimensionally defines a part involves decisions for selecting datums and coordinate systems, how measurements are combined, and choosing measurement techniques. As shown in paragraphs 5.2 and 5.5 of this AC, some assumptions can cause the variation in replacement parts to exceed the variation in the type design part. Understanding the effect that these choices have on the part, and its influence on other parts is important. For example, preserving the stress at the interface between an influencing and critical part could require measurements from both parts at the interface. These measurements will provide important information about the total variation in the type design assembly at the interface and can help determine the dimensioning method that results in an equivalent amount of variation in the replacement part.
5.13.3 Dimensional differences in influencing parts tend to result in major changes in design because of the direct effects the differences have on critical parts. Assessing the magnitude of the effect dimensional differences have on engine systems and critical parts will likely require data from specific locations in the engine under certain operating conditions or from validated analytical techniques that are correlated to engine data.
5.13.4 Dimensional differences will likely exist in critical replacement parts and repairs that substantially remanufacture type design parts. Dimensional differences in critical parts can affect their ability to meet the prescribed integrity requirements of 14 CFR 33.27,
33.70, 33.76, 33.94, and TSO-C77b, paragraph 5.9, for example. If the critical part is life-limited, the applicant will need a method for assessing the life of the part with the differences, which involves data that is normally acquired by engine tests. Though not covered in this AC, differences in manufacturing, materials, engine configurations, and engine system environments also influence the integrity of critical parts.
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5.13.5 Applicants usually demonstrate the extent of their knowledge about the interrelationships between non-critical parts and critical parts in their safety assessments. Also, some engine manuals provide a list of influencing parts.
5.14 Minimum and Maximum Material.
5.14.1 The replacement part design encompasses the geometry of the part manufactured to its dimensional limits. The minimum and maximum material condition of the part corresponds to the minimum and maximum dimensional tolerances. These material conditions include the cumulative effect of any dimensional differences, tolerances, and offsets from the nominal shape.
5.14.2 The results from replacement part assessments that show the part is functionally equivalent to the type design part pertain to the replacement part when it is manufactured to its reverse-engineered minimum and maximum material limits.
Typically, measurements from a sample of type design parts are used to establish the dimensional limits for the replacement part. However, the methods that define the dimensional characteristics of the replacement part (datum selection, combining measured variation, choice of tolerance controls, etc.) will affect the reverse-engineered dimensional limits. Uncertainties in reverse engineering methods that can cause replacement parts to exceed the dimensional measurements taken from a sample of type design parts could expand the scope and expense of a project significantly.
5.14.3 A sample of parts produced for compliance testing does not usually include parts manufactured at the extremes of their drawing limits. However, these limits are important to consider for dimensional characteristics that interface with critical parts.
When the integrity of a critical part or critical engine system is sensitive to variation in a part, the compliance showing should include assessments that extrapolate the results from the tested samples to account for the minimum and maximum material conditions.
These data ensure there is a similarity in functional properties for the entire reverseengineered design.
5.14.4 Applicants should examine the essential properties of a replacement part at its maximum material dimensions. For example, to properly apply the comparative test and analysis methodology, replacement turbine blades should not exceed the maximum moment weight measured from a sample of type design turbine blades. However, if the sample population of tested replacement blades pass this criterion, but the nominal reverse-engineered design is inadvertently biased toward the maximum material condition, then a maximum material replacement blade could exceed the maximum moment weight criterion. Supplemental data from an analysis correlated to the bench test data, in addition to the moment weight test data, can ensure the replacement part does not exceed the moment weight criterion when the part is manufactured to the maximum material condition.
AC 33-12
5.14.5 The dimensional characteristics of a replacement part manufactured to the minimum material condition should also be evaluated. The minimum material condition can influence the stress in these parts in various ways. For example, the stress at loadbearing surfaces in replacement parts will be adversely affected if the minimum material condition causes the load-bearing area to drop below the minimum allowable area in the type design. Similarly, high stresses can result when the minimum material condition increases a stress concentration or results in unintended sharp features that do not exist in type design parts. Sharp features can result when a feature is defined without considering the transition geometry between surfaces in various combinations of minimum and maximum material conditions. If a sharp surface transition is in contact with another surface, it will likely result in improper seating between the parts that are in contact and high stresses that can cause localized fretting, premature wear, or fatigue failures.
5.14.6 Figures 13 and 14 illustrate how dimensional differences between a replacement part and a type design part can lead to functional differences and latent engine effects. The issues shown in these figures may not be detectable from a sample of replacement parts.
Measurements from a sample of interfacing type design parts will help ensure the dimensional stack-up with the replacement part will result in proper fit and function when it is manufactured to its reverse-engineered dimensional limits. This consideration is especially important for parts that can influence the integrity of critical parts and engine systems.
Fi gure 13. Minimum Material Replacement Part Resulting in a Higher Contact Stress.
AC 33-12 Figure 14. Maximum Material Replacement Part Resulting in a Higher Contact Stress.
5.15 Configuration Management.
5.15.1 TC holders develop configuration management programs based on the assumption that their customers will use their products and services throughout the engine lifetime. The TC holder has an internal design, quality, and manufacturing systems that flow into the configuration management programs they provide to their customers. Therefore, the TC holder publishes just the portion of their engine lifecycle management information that will be accomplished by their customers. This information ensures their engines are in a condition for safe operation when used with their maintenance instructions and other airworthiness programs. Providers of replacement parts have procedures for design, quality, and manufacturing systems that may or may not be compatible with the TC holder’s configuration management program. Therefore, the following considerations could be important to applicants that intend to stay within the dimensional characteristics of a type design part.
AC 33-12
5.15.2 An example of a TC holder quality system that safeguards their engine configurations is the Material Review Board. This system assesses non-conforming parts for compensating factors to determine if the part and the engine that uses the part can be managed with the existing ICA. Non-conforming TC holder parts are not within the intended type design, but they might be among the parts used for reverse engineering a PMA design. Nonconformities can include dimensional characteristics that are outside the TC holder’s drawing limits. The TC holder does not publish the nonconformity or compensating factors that make the part eligible for installation because these details are captured and managed under their quality system. If the applicant measures a sufficient quantity of the TC holder’s parts to perform a statistical analysis, they should test the population for outliers that may indicate parts that were accepted into service despite being outside the approved design tolerance.
5.15.3 Configuration control programs can involve grouping parts for assemblies, and selling them together, based on certain dimensional characteristics that maintain the integrity of higher-level assemblies. Sometimes, part suppliers will provide individual parts that are normally grouped with other parts by the TC holder. Failure to recognize higherlevel assembly design criteria for certain propulsion systems has resulted in safety concerns. Examples of parts that could be managed this way are piston and sleeve combinations, interfacing gears, and parts that assemble with an interference fit.
5.15.4 The TC holder can also hand- select parts for certain engine models based on where the parts fall in the range of manufacturing variation, but should ensure that doing so does not invalidate the maintenance and configuration management programs they provide to their customers. Depending on their internal engine configuration management systems, the TC holder may not indicate that they segregate these parts at assembly.
Typically, repairs to these parts are classified as major. Sometimes the dimensional characteristics of parts change after they are installed in higher-level assemblies. These changes can result from plastic deformation or machining after assembly. Physical changes such as this can preclude reverse engineering some parts that are extracted from higher-level assemblies.
5.15.5 TC holders often use the same part in different engines or multiple locations within engines, components, and accessories. This approach to managing engine configurations can help reduce the logistics costs for customers and lower the production costs for engine manufacturers. However, the upward compatibility of approved reverse-engineered parts to more demanding engine environments or across engine models where the same part number is used is not inevitable. For example, validation procedures for a replacement part eligible for installation in multiple engine models and assemblies should account for the full certificated performance of the most demanding engine and the various assemblies that could stress and fail the part in different ways. Assessments and tests that account for everywhere the part is used will help mitigate the risk of underestimating the criticality of the part, overlooking important reverse engineering criteria, and ensure any differences in the part are minor.
5.15.6 The compliance methods under test reports and computations provide alternatives to basic engineering analysis and side-by-side comparisons. General analysis, or direct compliance, allows applicants to address uncertainties about TC holder parts and how engine configurations are managed.
AC 33-12
5.16 Coatings.
5.16.1 Some type design parts are coated, so the coating becomes part of the reverseengineered design for a replacement part. In this case, applicants duplicate the coating as part of the overall effort to copy the type design part.
5.16.2 If an applicant is copying a type design part in parallel with its type design coating, the total stack-up with the coating is included in the assessment for dimensional similarity.
The stack-up includes the part’s nominal shape and tolerances, the coating thickness and tolerances, and the coating coverage tolerances. Paragraph 5.6 of this AC provides additional information about assessing replacement and type design parts for dimensional differences. Examples of dimensional properties that involve coatings are thickness, taper at the interface between two parts, transitions from coated to noncoated surfaces, and airfoil leading edge contours.
5.16.3 Parts that operate in the gas-path are highly customized designs that evolve with experience. Airfoil contours, moment weights, tolerances, contact areas, and aeromechanical properties can be affected by coatings. Differences in the dimensional characteristics of gas-path hardware present substantial challenges for replacement parts and repairs because the performance data acquired from engine certification tests and engine experience, such as erosion rates, are directly affected by the coating. For example, some highly-loaded blade dovetail coatings have special dimensional requirements to prevent failure from overstressed conditions at the interface with a lifelimited part. Therefore, to meet § 21.303(a)(4), compliance data for replacement gaspath parts with coatings should provide enough insight to verify that the engine data that was developed from a direct showing of compliance is not invalidated.
5.16.4 New coatings are sometimes developed to replace the type design coating, or coatings are added to a repair to improve resistance to environmental effects, such as corrosion.
Replacement part designs might also change the coating areas or recondition surfaces before applying the coating. It is not appropriate to rely on assumed engine effects from dimensional differences or assume the applicability of ICA for projects that add coatings or change the distribution of coatings. These projects are outside the scope of this AC because assessing the effect of the coating on various airworthiness requirements often involves direct compliance.
5.17 Test Part Conformity.
5.17.1 Applicants should accomplish conformity for all replacement parts that will be used in the comparative assessments. Conformity ensures the tested parts are representative of the proposed design.
5.17.2 In accordance with14 CFR 21.310(b), parts that conform to the replacement part drawing are considered eligible for certification testing. Parts that do not fully conform to the replacement part drawing and are proposed for certification testing must be substantiated by showing that the non-conforming feature, or features, will not affect the property being assessed by the test.
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5.17.3 If certification tests show performance differences between the type design part and the replacement part, the replacement part manufacturer should verify that the dimensional attributes of the tested parts meet their drawing requirements. If they find their part to be conforming, they should then determine if there are other reasons for the performance differences. For example, the performance differences might be from differences in manufacturing processes, material properties, or how samples were tested. If the test involves comparative destructive evaluations, such as fatigue testing, the conformity assessment should be accomplished before the test to qualify the test data for certification purposes and to confirm the effects of any corrective design or manufacturing remedies.
5.17.4 Applicants might find their parts conform and do not detect any anomalies in their manufacturing and quality systems that explain the performance differences. In this case, applicants should reexamine the reverse engineering process they used to develop the design of the replacement part for potential sources of variation in features that could have influenced the outcome of the test.
5.18 Dimensional Development.
5.18.1 Some engine parts have internal cavities and labyrinth passageways that allow cooling air to pass through the core of the part to remove heat from external surfaces that are exposed to hot gas temperatures. These internal features provide a thermal balance in the part that inhibits deterioration and thermally induced failure modes, such as cracks and creep. The manufacturing techniques used to create internal features can involve processes that result in residual stresses, such as casting.
5.18.2 When applicants use destructive methods to inspect and quantify the dimensional characteristics of internal features in parts that have residual stresses, the resulting geometry of the sectioned part could be different from what it was when the part was in one piece. There could be a tendency for the part to shift or spring back when residual stress is relieved as the part is progressively sectioned for the dimensional analysis.
When this occurs, the resulting measurements acquired from the type design part will not provide accurate dimensional information for the replacement part definition.
5.18.3 Applicants that use destructive methods to reverse engineer a type design part should use suitable methods that ensure the parameters being acquired are not affected by geometric shifts that can occur from relieving residual stress. Figure 15 shows an example of surfaces that can shift during a destructive evaluation. Figure 16 shows some dimensional properties that are independent of the physical distortion that occurs when the residual stress is relieved.