U.S. Department of Transportation Federal Aviation Administration
Advisory Circular Subject: Geometry and Dimensional Date: AC No: 33-12 Considerations for Comparative Test and Analysis Initiated By: AIR-625for Turbine Engine and Auxiliary Power Unit (APU) Replacement, and Repaired Parts
1. PURPOSE.
This advisory circular (AC) provides guidance to support the development of technical data needed to show that an engine or APU replacement part complies with the airworthiness requirements under test and computation, per Title 14, Code of Federal Regulations (14 CFR) 21.303, using the comparative test and analysis approach. This guidance also supports the development of technical data needed for major repairs performed in accordance with 14 CFR Part 43.13(b), where parts fabrication and their implementation must be accomplished “in such a manner that the condition of the aircraft engine or appliance worked on will be at least equal to its original or properly altered condition.” This method supports showing the engine or APU still complies with the applicable airworthiness requirements of 14 CFR Part 33, and Technical Standard Order (TSO) C77.
This AC helps applicants identify potential dimensional differences caused by reverseengineering parts using only measurements, and in doing so, aids the applicant in meeting the “stay within” criteria for comparative test and analysis methodologies. This AC does not provide guidance for projects that intentionally introduce dimensional differences into replacement parts or minor repairs.
2. APPLICABILITY.
2.1 The guidance in this AC is directed to parts ma nufacturer approval (PMA) or major repair applicants, Federal Aviation Administration (FAA) engine type certification engineers, and FAA designees.
2.2 This is a guidance document. Its content i s not legally binding in its own right and will not be relied upon by the Department as a separate basis for affirmative enforcement action or other administrative penalty. Conformity with the guidance document is
AC 33-12 voluntary only. Nonconformity will not affect rights and obligations under existing statutes and regulations.
2.3 The FAA will consider other means of demonstrating compliance that an applicant may elect to present. Terms such as “should,” “may,” and “must” are used only in the sense of ensuring the applicability of this particular method of compliance when the acceptable method of compliance in this document is used. If the FAA becomes aware of circumstances in which following this AC would not result in compliance with the applicable regulations, the FAA may require additional substantiation or design changes as a basis for finding compliance.
2.4 The material contained in this AC does not change or create any additional regulatory requirement, nor does it authorize changes in, or permit deviations from, existing regulatory requirements.
3. RELATED READING MATERIAL.
The following materials are referenced in this document. Unless otherwise indicated, you should use the current edition.
3.1 14 CFR Regulations.
• Part 33, Airworthiness Standards: Aircraft Engines.
• Section 21.303, Application.
• Section 43.13, Performance rules (general).
• Section 43.15, Additional performance rules for inspections.
3.2 FAA Publications.
• AC 33-8, Guidance for Parts Manufacturer Approval of Turbine Engine and Auxiliary Power Unit Parts under Test and Computation.
• AC 33-9, Developing Data for Major Repairs of Turbine Engine Parts.
• AC 21.303-4, Application for Parts Manufacturer Approval Via Tests and Computations or Identicality.
• AC 33.70-1, Guidance Material for Aircraft Engine Life-Limited Parts Requirements.
4. BACKGROUND.
4.1 The type certificate (TC) holder will often modify an industry standard or elect to create their own standard to comply with the airworthiness requirements established by the engine certification basis. The TC holder’s part designs are based on regulated engine requirements and known engine effects from variation in the part’s dimensional characteristics, material properties, and manufacturing processes. Further, part designs 2
AC 33-12 evolve with engine specific experience and are produced and managed in a way that preserves compliance throughout the engine’s lifecycle.
4.2 Relying solely on industry accepted standards to design, manufacture, and select materials for replacement parts, and for repairs that substantially remanufacture type design parts, may not result in the same level of confidence in the compliance showing as data developed during engine certification. Testing verifies that any assumptions used in a reverse engineering process about the adequacy of industry standards are correct. Testing also verifies if the measurement data is interpreted and applied correctly. This data can provide supporting evidence that shows the engine will likely continue to meet its certificated performance requirements with the replacement part or repaired part installed in the engine.
4.3 The FAA has observed significant variation in the way applicants define and substantiate the dimensional properties of replacement parts for turbine engines. This variation is typically caused by inconsistencies in the applicants’ understanding of the type design part and how it functions in the engine. In some cases, reverse engineering procedures resulted in unintended dimensional differences in replacement parts that influenced the engine environment and the integrity of other parts in the engine. The differences were not detected before the effects of the differences were experienced in the fleet after the part entered service.
4.4 Unintended dimensional differences in replacement parts have resulted in engine configurations that were not compatible with the TC holder’s Instructions for Continued Airworthiness (ICA). Sometimes unintended differences that are consequential to safety can result from imprecise evaluations of features that have a large effect on critical parts and engine systems. Sensitivities of interdependent parts are not an outcome of the FAA approval processes for replacement parts or major repairs. Under 14 CFR 21.303 and 33.75, applicants should know these sensitivities for developing suitable compliance plans that show functional equivalency and for conducting safety assessments so the project will have the oversight required by FAA approval processes. If these sensitivities are not recognized, the compliance plan might not detect important differences. Also, if differences are detected, but the sensitivities are not known, the compliance plan might need to include aspects of general compliance that will confirm these sensitivities.
4.5 Reverse engineering methods do not typically produce a design with the exact same dimensional characteristics as a type design part. This variance is because reverse engineering methods vary considerably in measurement techniques, how measurement data is interpreted and combined, and in dimensioning systems, all of which affect the dimensional similarity between type design and replacement parts. Conclusions about dimensional similarity without the comparative assessments that show equivalency is only valid when a single method is used to develop the dimensional properties of the type design and replacement parts.
AC 33-12
4.6 Since applicants will not likely duplicate the TC holder’s methods for measuring and dimensioning the type design part, dimensional differences in their replacement part can exist. Therefore, capturing the type design functional properties within the replacement part and safeguarding the interfacing parts and higher-level assemblies involves functional assessments that ensure the dimensional characteristics are equivalent. Depending on the extent to which the type design part supports the certificated performance of the engine, the functional assessments for the replacement part could involve both testing and analytical data that supplement the test. For example, to verify the full range of variation in a category 1 or 2 reverse-engineered part, testing could be used to cover a subset of the full range, and an analysis that is calibrated to the test data could be used to show functional equivalency through the full range of dimensional variation.
4.7 The applicant should provide compelling evidence that the replacement part is functionally equivalent to the type design part. Multiple FAA documents bring focus to the importance of staying within measurements taken from type design parts (unless substantiated) because dimensional similarity affects functional similarity. This AC helps applicants identify potential dimensional differences in reverse-engineered parts and meet the “stay within” criteria. The AC does this by providing examples for how dimensional differences might result from reverse engineering processes that use direct measurements taken from type design parts and encourages testing to verify similarity for what is not evident through direct measurement.
4.8 This AC does not provide guidance for projects that intentionally introduce dimensional differences into replacement parts or minor repairs. This AC helps applicants identify where unintended dimensional differences could exist in replacement parts and repairs and highlights the need to address them for the required compliance showing.
5. GUIDANCE.
5.1 Consideration for Dimensional Characteristics.
This AC shows several considerations that can help applicants develop the dimensional characteristics for replacement parts and major repairs and enhance their showings of similarity. It also helps applicants reduce the potential for unintended dimensional differences in their part by drawing attention to what is not known about the type design part. The following discussions indicate where part-to-part bench assessments might not fully address the potential for dimensional differences to exist in the replacement part, and when a project could require a supplemental type certificate. Due to the wide range of reverse engineering techniques and manufacturing processes used by the industry, there could be other causes of dimensional variation that are not mentioned in this AC. The applicant should develop substantiation procedures that account for the possibility of these and all the other potential sources of dimensional variability that are applicable to their methods.
AC 33-12
5.2 Geometric Relationships.
5.2.1 Some reverse engineering techniques generate a cloud of data from cumulative dimensional scans of multiple type design parts to determine the dimensional characteristics for their replacement part. However, assuming the replacement part shape can exist anywhere within the minimum, and maximum dimensional limits derived from measuring multiple type design parts might introduce unintended dimensional differences. For example, the type design could have dimensional requirements that maintain trends, positional dependencies, and relationships among interrelated part features within a data cloud. These relationships are not always obvious. Figure 1 provides an example showing the importance of recognizing the interrelated dimensions that affect the part’s functional design.
Fi gure 1. Example of a Dimensional Relationship.
5.2.2 Similarly, if an applicant reverse-engineered a surface tolerance using only a minimum and maximum material condition without also controlling the surface transitions within the material envelope, the reverse-engineered surface may allow trends and step changes that are not present in the type design part. If the reverse-engineered dimensional characteristics allow these anomalies to exist in the surface of the replacement part, but they are not observed in the surface of the type design parts, then the criteria that characterize the reverse-engineered surface is not complete.
AC 33-12
5.2.3 An understanding of how the part functions in the engine is fundamental to preserving the dimensional relationships intended by the original engine manufacturer. These relationships can influence the functional properties of the part and the integrity of associated critical parts. Therefore, reverse engineering criteria and measurement techniques that establish geometric relationships should reflect an understanding of the functional requirements of the part, and not be based solely on the total variation within a cloud of data.
5.3 Datum Selection and Reference Dimensions.
5.3.1 Datum selections influence the dimensional characteristics of a part. Datums are typically established based on how the part functions in the engine. Datums locate features on the part relative to other features and locate the part relative to other parts in the engine. Datums also ensure the part fits in the assembly and that it functions properly in the engine. Proper datum selection will take into account considerations, such as fit with adjacent hardware, function, and the state of the part while it is in operation.
5.3.2 Figure 2 shows how the selection of a datum can affect the interface between adjacent hardware. The figure on the left represents the type design, and the figure on the right represents a reverse engineering technique that places the datum in a location that could result in an interference fit with adjacent hardware.
Fi gure 2. Example of How Datum Selection Can Affect the Interface Between Parts.
Type Design Datum Reverse-Engineered Datum
5.3.3 Reference dimensions are theoretical distances that locate features constrained by dimensions elsewhere on the part. They are usually calculated from a stack-up of many other dimensions and placed on the drawing solely for convenience. The accuracy of reference dimensions can affect the contours and transitions between features that support the proper function of the part. For example, reference dimensions could be used to check features that interface with critical hardware during the production process; therefore, an inaccurate reference dimension can adversely affect safety.
AC 33-12
5.3.4 Placing pin gauges on external surfaces of the part to establish datums will not necessarily result in a part with the same functional design. Though TC holders sometimes use pin gauges to qualify used or repaired type design parts, pin gauges do not provide enough information to reverse engineer a part. A pin gauge contacts a particular location on several surfaces and averages the variation of the surfaces touched by the gauge. This technique does not provide any insight into tolerances, tolerance controls, datums, reference dimensions, or coordinate systems that the TC holder used to define the dimensional attributes of the individual surfaces touched by the pin gauge. Depending on how the applicant uses these measurement data for reverse engineering, this technique can create a different datum system for the replacement part that decouples the type design and replacement part dimensional attributes.
5.3.5 Measuring the dimensional variation of a type design part using the reverse-engineered datums created for a replacement part can make the type design part appear to have more variation compared to the replacement part. Further, if applicants use their replacement part datums to align the replacement and type design parts for a dimensional comparison, the variation in important features in type design parts can appear to be greater than it really is. For example, datums for replacement parts established by touching the external surfaces of type design parts will be different. In this case, the replacement parts will exhibit less variation than they would if they were compared using the datums that were developed for the type design part, but they will in fact have more variation.
5.3.6 Depending on the part, unintended expansion of the dimensional variation beyond the variation measured in type design parts can adversely affect the functional design of the replacement part and the integrity of interrelated hardware. For example, more variation in highly stressed turbine blade dovetails can increase the stress at the disk and blade contact surfaces, resulting in fracture of the blade dovetail or disk from accelerated wear or fatigue at the contact surfaces. More variation in adjoining rabbets at the interface between combustor hardware and casings can increase wear rates and shorten the maintenance interval. More variation in airfoil contours and load-bearing shafts can substantially affect their dynamic properties causing excessive flexure, stress, and failure from fatigue.
5.3.7 The applicant should clearly portray their selection of datums and reference dimensions in the data package and support their choices with evidence showing that their selections safeguard the part’s functions and its interface with codependent critical parts, thereby preserving the engine’s airworthiness.
AC 33-12
5.4 Establishing Coordinate Systems.
5.4.1 Applicants can use various coordinate systems to define a part dimensionally.
Examples of coordinate systems are Cartesian, Polar, Cylindrical, and Spherical.
Choose the coordinate system appropriate for the part’s shape, function, and features that require dimensional controls. Figure 3 shows examples of coordinate systems.
Fi gure 3. Examples of Coordinate Systems.
Cartesian Cylindrical Spherical
5.5 Non-Uniform Tolerances.
5.5.1 Measurements taken from type design parts should be sufficient in precision and density to detect the presence of non-uniform tolerances. Non-uniform tolerances in a part might indicate the presence of unique dimensional controls that support engine compliance throughout the type certificated operating range. They may also indicate where variation in local areas on a part has little or no effect on engine compliance.
5.5.2 Some reverse engineering processes assume the largest measured tolerance on a particular feature on a type design part applies evenly across the feature. This technique for replicating a tolerance is suitable if the maximum measured tolerance occurs over the entire feature or if a smaller measured tolerance is an artifact of how a part is seated in a fixture used for taking the measurements. However, non-uniform tolerances in a type design part may occur for various reasons. Reverse engineering methods should be capable of distinguishing non-uniform tolerances, and they should be explained in the safety assessment.
5.5.3 In some cases, TC holders build-in non- uniform tolerances into their type design parts, so they have inherent dynamic properties that accommodate certain engine system responses. In other cases, non-uniform tolerances can result from hand-blending procedures that TC holders allow in specific areas of the part, such as the trailing edge of some airfoil designs. Therefore, applicants should not automatically apply the largest measured variation in the type design part to the entire area or feature of the replacement part, because the resulting variation in the replacement part could exceed the range of variation intended by the TC holder for the type design part. Depending on the interrelationships with critical parts and complexities of the engine systems it affects, the effects of increased dimensional variation on engine compliance can be difficult to assess without engine experience or testing.
AC 33-12
5.6 Combining Measured Variation.
5.6.1 The applicants’ reverse engineering technique will determine how separate measurements taken from type design parts are combined on the replacement part drawing. There are numerous ways to combine measurements, and the various combinations can result in different amounts of dimensional variation in the part. For some design features, small variations in geometry can affect important functional properties, such as operating stresses, fatigue, vibration, wear, and failure modes. The way applicants combine separately measured tolerances can affect any of these functional properties. Therefore, to produce a credibly reverse-engineered part, applicants should ensure that their reverse engineering technique combines measured tolerances to account for the various stack-ups that can affect the part’s function and interrelationships with other parts.
5.6.2 For example, a surface could have a tolerance for its position relative to a datum, and another tolerance for its profile. In this example, the surface profile tolerance is meant to be within the upper and lower bounds of the positional tolerance in the type design part. If the profile and positional tolerances are added together instead, the replacement part will have more variation than the type design part. This example shows there are circumstances where separate measurements from type design parts need to be combined in a specific way to reproduce the total variation in the replacement part.
5.6.3 Where assumptions are used in response to what is not known about the type design tolerances, it is appropriate to make conservative decisions that minimize the dimensional v ariation in a stack-up. It is also appropriate to implement validation methods to ensure the combined tolerances result in a replacement part that is functionally equivalent to the type design part. Contact stress between two parts is an example of a functional property that can be affected by small variations in dimensional characteristics. Combining measurements in a way that minimizes the total dimensional variation at the contact interface can help ensure the range of stress in the replacement part, and the part it contacts does not exceed the stresses intended for the type design part.
5.6.4 Figure 4 shows two measurements taken separately that are combined in different ways.
An understanding of the functional design is necessary to determine which method duplicates the type design dimensional properties.
AC 33-12 Figure 4. Combining Separate Measurements and Functional Design.
5.6.5 Physical differences from the type design part are determined by comparing the stackup of dimensions that establish the total variation in the replacement part to the total variation measured in a sample population of type design parts. For example, separating part features such as nominal shape and contour and comparing them individually will not show if the replacement part geometry is within the type design part geometry.
Applicants should stack the nominal shape and contour from the replacement part drawing and compare the resulting geometry to the corresponding measurements taken from the type design part. An overlay of the nominal shape, together with the contour around the nominal shape, will show the extent of physical differences between the part designs. Figure 5 shows an example of differences when considering total variation around nominal shapes.
Fi gure 5. Example Showing Differences in Total Variation.
AC 33-12
5.6.6 The method used to compare the geometry can also affect the results, showing the extent of dimensional differences in the replacement part. For example, using a “best fit” approach to quantify differences among features that are located by a datum on the part will not provide an accurate accounting of the physical differences and could result in unintended differences and, therefore, unsubstantiated functional differences in the replacement part.
5.6.7 Also, other dimensional controls within contours and tolerance bands could affect the stack-up of physical differences, like non-uniform tolerances and rates of dimensional changes on a surface. If present in the type design, these additional controls should be included in the assessments that determine the extremes of physical differences.
5.7 Transition Features.
5.7.1 Corners, fillet radii, and transition radii are transition features. They can contribute substantially to the function of a type design part, interfacing parts, and higher-level assemblies. The dimensional variation in transition features might not be directly controlled. Instead, they might be an outcome of interrelated dimensions and tolerances. Conversely, the dimensional variation in some important features, such as load-bearing surfaces, is sometimes defined by the transition geometry. Figure 6 shows an example of how the dimension of a feature can depend on interrelated features.
Differences in transition features have the potential for concentrating stresses, accelerating wear, and initiating fatigue cracks from typical mechanical and thermal loads. Therefore, to produce a valid design when using the comparative test and analysis method, the reverse engineering process should be capable of accurately identifying the dimensional characteristics for transition features, which could control or be controlled by adjacent features.
Fi gure 6. Dimension “L” is an Outcome of Other Dimensional Controls.
5.7.2 Compound fillets are also important features associated with static and rotating parts.
They are used to reduce stress in specific areas and ensure parts, such as life-limited and life-assessed parts, meet their durability requirements. Therefore, to ensure that there are no changes inadvertently introduced in the proposed design, reverse engineering processes for replacement parts and repairs should be able to identify and replicate compound fillets that exist in type design parts.
AC 33-12
5.7.3 To stay within the dimensional characteristics of the type design part, reverseengineered transitions, or blends, care should be taken not to inadvertently introduce new features that do not exist in the type design part. For example, surfaces in contact with critical hardware are designed to preclude high-localized stress from raised features or sharp contact edges at interfaces. Therefore, to successfully apply comparative test and analysis methodology, applicants should evaluate the transition feature for detrimental geometry throughout the range of the reverse-engineered tolerance band.
5.8 3D to 2D Translation.
5.8.1 Some reverse engineering methods involve equipment and analytical techniques that develop a three-dimensional model from measurements taken from multiple type design parts. The three-dimensional model is subsequently converted to a twodimensional drawing that defines the dimensional characteristics of the replacement part. The drawing dimensioning system incorporates datums, reference dimensions, tolerances, and geometric controls that are intended to represent the three-dimensional model. However, the choices applicants make to define the part on a drawing influence the amount of variation in individual features and the relative position of individual features.
5.8.2 The replacement part manufacturer develops its dimensioning system for th eir part.
Since they do not have the TC holder’s drawing, their drawing could implement different dimensional associations. These differences can potentially increase the variation in replacement parts beyond the limits specified on the TC holder’s drawing.
Dimensional differences can have undesired effects on the function of the part and interfacing parts. Therefore, applicants should not base their replacement part drawing definition solely on goals to facilitate manufacturing and inspection processes. Though there may be a need to develop drawings that correspond to in-process manufacturing requirements for the part, finished-part drawings are grounded in design criteria.
Therefore, applicants should base their reverse-engineered dimensioning system on the part’s various functional requirements. Applicants should also verify their dimensioning system is correct by assessments that consider it to be a potential source of variation.
5.9 Precision and Accuracy.
5.9.1 Precision is the number of digits used to define a value. Accuracy is the closeness of the measurement to the actual value. The precision of a measurement taken from a type design part should be adequate to capture the variation in the measured feature. The precision to which a feature is measured will affect the resulting variation in the reverse-engineered part. Figure 7 illustrates the difference between precision and accuracy.