Advisory U.S. Department of Transportation Federal Aviation Circular Administration Subject: Application For Parts Date: 3/21/14 AC No: 21.303-4 Manufacturer Approval Via Tests And Computations Or Identicality Initiated by: AIR-110 Change: 0
1. Purpose.
a. This advisory circular (AC) updates the Federal Aviation Administration’s (FAA) guidance to applicants for parts manufacturer approval (PMA) of articles via tests and computations or identicality without a license agreement. T his AC cites regulations in Title 14, Code of Federal Regulations (14 CFR) part 21, Subpart K that became effective April 16, 2011. In addition, this AC provides a convenient application and compliance checklist, adds a certifying statement of compliance, provides guidance for assessing an article’s impact on safety and describes how the FAA approves replacement parts for technical standard order (TSO) articles. This AC does not apply to the articles that are listed in 14 CFR 21.9(a)(1) through (6).
b. This AC refers to parts and components as articles per 14 CFR 21.1. This section defines an article as a material, part, component, process or appliance. These items may include sealants, modified standard parts, brake assemblies, etc. that are in a product’s type design.
Please note PMA is not for base materials, processes or inspection procedures.
c. This AC is not mandatory and does not constitute a regulation. This AC describes an acceptable means, but not the only means, to comply with 14 CFR Part 21, Subpart K. If you chose to use any of these best practices, we expect you to follow it completely. Adherence to the guidance for each applicable facet will show that an article’s design complies with the airworthiness requirements of its eligible products. Also consult other ACs when you need guidance on product specific requirements for showing compliance. For example, AC 33-8 has guidance for Parts Manufacturer Approval of Turbine Engine and Auxiliary Power Unit Parts under Test and Computation.
2. Audience.
a. This AC is for applicants seeking PMA through tests and computation or identicality without a license agreement. Consult AC 21-43, Production under 14 CFR Part 21, Subparts F, G, K, and O and Order 8120.22, Production Approval Procedures for further guidance on applications for PMA via identicality with a license agreement or via supplemental type certificate (STC). Recommend new applicant’s consult with the responsible ACO before applying for PMA. Early discussions will provide essential information on the PMA process, the
associated level of effort, its data requirements, potential pitfalls and timelines. These discussions will allow the FAA and you to focus our efforts in a productive and effective manner.
b. Qualified existing holders of PMA may use an expedited process to attain approval for non-safety significant articles. See Modification and Replacement Part Association (MARPA) Document 1100, Streamline Program for PMA Applications of Non-Safety Significant Articles Submitted by Experienced Applicants with a Qualifying Performance Record and FAA Order 8110.119, Streamlined Process for Parts Manufacturer Approval (PMA)
3. General Requirements. 14 CFR 21.8(a) lists PMA as one way to approve articles for typecertificated products. 14 CFR 21.9 requires any person representing a replacement or modification article as eligible for installation on a type-certificated product to get approval. A PMA is a combined design and production approval for replacement articles. However, if any replacement article alters a product by introducing a major change, then 14 CFR 21.113 requires an STC for the approval of these articles. See FAA Order 8110.4, Type Certification, for STC procedures. An STC approves the design and installation of modification articles in products. A subsequent PMA approves the production of these modification articles.
4. Getting a PMA for TSO Articles. PMA approves replacement and modification articles for specific type-certificated products. These articles may reside in an assembly approved under a TSOA on these specific products, but the PMA limits installation of these articles to those specific products. PMA cannot approve replacement articles solely for original articles approved solely under a TSOA.
5. What are the Exceptions to PMA?
a. Materials, Procedures and Processes. Manufacturers use a variety of raw and refined materials to produce their finished articles. The manufacturing process integrates these materials into articles for installation on products. A PMA is not for the base materials that compose an article. These m aterials include sheet, bar and plate stock, rough forgings, extrusions, etc. Also PMA is not for the approval of inspection procedures or processes. Any specific procedures or processes (such as hardening, plating, or shot peening) integral to a PMA are valid only for that particular article. Any person performing only specialized processes or procedures on articles intended for installation on type-certificated products must do so within a production approval or as a supplier to a production approval holder (PAH).
b. ‘One-Time Only’ STCs. Articles produced under a “one-time only” STC or a field approval are ineligible for a PMA. Holders of these design approvals may produce replacement articles for installation in their modifications or alterations without PMA.
c. Other Production Approval Holders (PAH). Holders of production certificates or TSO authorizations do not need a PMA to produce replacement parts for their products or TSO articles. A supplier to a PAH needs direct ship authorization to sell an article to any other entity than that PAH. A direct-ship authorization is a written permission from a production approval holder to a supplier for shipment of articles directly to a user or operator. See Order 8130.21, Procedures for Completion and Use of the Authorized Release Certificate, FAA Form 8130-3, Airworthiness Approval Tag.
d. Aircraft Owners or Operators. Owners and operators may produce articles for installation on their own product without a PMA. The installation of these articles must comply with applicable airworthiness standards to preserve the regulatory compliance of their products and ensure continued safe operation. If an owner or operator intends to sell an article for installation on another owner’s aircraft, then they need a PMA.
e. Air Carriers Operating Under 14 CFR Parts 121 or 135. Carriers may produce articles for installation on their own products without a PMA when their approved maintenance program contains the appropriate instructions and procedures. Also, the installation of the article must comply with 14 CFR part 43. If air carriers intend to sell an article to other owners or operators, then they need a PMA.
f. Repair Stations. An FAA-certificated repair station may fabricate an article for installation on a type-certificated product provided it is consumed within a repair authorized under their operations specification. See AC 43-18, Fabrication of Aircraft Parts by Maintenance Personnel, for further guidance. They cannot sell these fabricated articles to others.
g. Producing and Selling Standard Parts. Production and sale of standard parts for typecertificated products do not require a PMA. These parts conform to established industry or U.S.
specifications. However, a PAH may buy standard parts, subject them to more restrictive inspection criteria and then assign new part numbers. These parts are no longer standard parts.
If questions arise, contact the certificating Aircraft Certification Office (ACO), Manufacturing Inspection District Office (MIDO), or both to determine if the part design meets the criteria for standard parts. See appendix H for details of acceptable standards.
h. Commercial Parts. These articles are on an FAA-approved commercial parts list (CPL) in a design approval holder’s instructions for airworthiness (ICA). Companies that produce articles on the CPL do so without PMA. See AC 21-45, Commercial Parts, for more information.
i. Imported Modification and Replacement Articles. Under 14 CFR 21.502, foreign manufacturers may export modification and replacement articles to the United States if it is within the scope of an applicable bilateral agreement and is marked in accordance with 14 CFR part 45. This agreement defines the scope and manner of the FAA’s acceptance. These foreign manufacturers will include documentation of airworthiness with these exported articles.
Other acceptable replacement and modification articles from foreign countries with bilateral agreements include:
(1) Articles produced by a foreign holder of an FAA TC, STC, or letter of TSO design approval (LODA) on a product with a foreign State Of Design, or
(2) Articles produced under a licensing agreement with the U.S. design approval holder or by a foreign manufacturer holding a production approval from the bilateral partner civil aviation authority (CAA) or,
(3) Articles that have PMA or equivalent approval from a foreign airworthiness authority and are covered by a bilateral agreement with the FAA.
6. PMA and Older Products. Approval of replacement articles in older products may present potential problems due to incomplete design data, out-of-production products, and inactive type certificate holders. Regardless, applicants must provide enough information to show that the prospective PMA design meets applicable airworthiness standards. Applicants may introduce changes to these legacy designs that include later industry-adopted standard practices and specifications. Additionally, applicants still must produce articles conforming to an approved design.
7. How to Submit an Application.
a. Apply by letter to the ACO or MIDO in the geographic area of your manufacturing facility depending on the basis of your PMA. The FAA may reassign review of your application to another ACO or MIDO at its discretion. See appendix F for ACO addresses. Use appendix B, PMA Application Checklist, as an aid to your application. The following guidance assumes that the listed supporting data is complete at the time of application. However, use a Part Specific Certification Plan (PartSCP) and discussions with the project ACO to schedule significant showings of compliance and associated test reports.
b. If your PMA basis is identicality with a license agreement or STC, see AC 21-43 and Order 8120.22 for further guidance.
c. If the PMA basis is identicality without a license agreement, show the design of your replacement article is identical to the design of an article approved under a type certificate.
Identical means the same in every respect including dimensions, tolerances, processes, etc.
Provide sufficient data to show your design is identical. Send this application and supporting data to the geographic ACO.
d. If the PMA basis is test reports and computations, send the application letter to the geographic ACO. Test reports and computations employ two strategies to show compliance with applicable airworthiness requirements for eligible products: general analysis and comparative analysis. This AC provides guidance on each of these strategies and aids you in providing the necessary data to show compliance with the appropriate airworthiness requirements.
8. The application letter regardless of basis must:
• Provide the name and address of the manufacturing facility for the article;
• Identify the replacement article and its original counterpart from a type certificate (TC, STC or Amended Type certificate (ATC));
• Identify the makes, models/series of aircraft, engines or propellers eligible for the proposed article or refer to the eligibility listing in an enclosed draft supplement;
• State the method of showing compliance with airworthiness requirements (i.e., test reports and computations using either general analysis, or comparative analysis, or a combination of both methods; or identicality without a license agreement);
• Stipulate use of a quality system that meets the requirements of 14 CFR 21.137 per 14 CFR 21.307. If using a previously approved quality system, cite the associated 4
quality manual, its latest revision, and date. See AC 21-43, Production under 14 CFR Part 21, Subparts F, G, K, and O, for further guidance.
9. Also enclose with your letter the following supporting data:
a. For each PMA basis supply:
• Drawings and specifications necessary to show the configuration of the article;
• Information on dimensions with tolerances, materials and processes necessary to define the structural strength of the article;
• The marking scheme for the article that complies with 14 CFR 45.15;
• A draft PMA supplement that will facilitate entry into our PMA database (see appendix C). Send the draft supplement in a Microsoft Word compatible file to the project ACO;
• If establishing a new quality system per 14 CFR 21.307, provide your manual to your geographic MIDO for FAA approval per 14 CFR 21.308. Get the specific requirements from a MIDO representative.
b. If the PMA basis is test and computation, add the following as enclosures:
• A PartSCP when warranted or requested by the ACO;
• A compliance checklist that lists the specific 14 CFR requirements, the associated methods of compliance and the reports that document these compliances (see the sample in appendix A);
• Add a statement on your checklist about any initial showings of compliance per AC 21-51, Applicant's Showing of Compliance and Certifying Statement of Compliance;
• Test reports and computations that show the article’s design meets the airworthiness requirements of its eligible product(s) as called out in the compliance checklist;
• A safety assessment that characterizes the nature of the article and the impact of its failure modes on safety;
• A continued operational safety plan reflective of the article’s safety significance;
• If applicable, a list of proposed designated engineering representatives (DER) and their respective authorizations.
10. Applicant’s Statement of Compliance for PMA via Test Reports and Computations.
Include a signed statement of compliance upon completion of all showings to applicable airworthiness requirements of the eligible products. See AC 21-51 for guidance on complying with 21.303(a)(5). You may include the statement in the text of your application letter, on an 5
enclosed compliance listing or as a separate letter with a signature. See appendix E for a sample PMA application that has this statement as an enclosure. Make reference in your statement to the associated compliance listing accompanying the PMA data package.
11. Installation Eligibility.
a. Identify the eligible aircraft, engines or propellers for proposed installation of your article. List each product by make, model or series, and if appropriate serial numbers, as specified on the applicable type certificate data sheet (TCDS). Usually the “make” of an aircraft, engine or propeller is the common name of the entity that built the respective product. The data plate on the product (14 CFR 45.11) declares its builder. The “make” designation, builder and type certificate holder are usually the same on a product’s TCDS. A block in the upper right hand corner of the TCDS consolidates and abbreviates this information in one place. However, type certificates are transferable. The FAA updates the holder information on the TCDS to cite the new holder. In this case, refer to the record of ownership for the make and model designations of the original builder and TC holder of the product.
b. Also identify the corresponding article from the type design by a descriptive name and part number. Show this article’s location in its respective products. Consider using an IPC along with other data like drawings, purchase orders, service bulletins, maintenance manuals, technical publications indexes, or master drawing lists from holders of design and production approvals.
Other evidence of installation eligibility includes:
• FAA airworthiness approval tag (FAA Form 8130-3, Authorized Release Certificate, Airworthiness Approval Tag),
• Other PMA supplements, and
• Other evidence including attributions from operators along with enough information from various supporting sources to show eligibility.
12. Approval of Critical and Life-Limited Parts.
a. Approval of critical and life-limited parts usually requires an STC. Small differences in the designs of these replacement parts may affect associated life limits and represent a major change to its product. However, PMA of critical and limited parts is possible when a replacement part’s design produces only a minor change in its product. PMA will require the same rigor of compliance showings as an STC process. Use a tailored PartSCP to propose a compliance framework and plan for approval of these parts (see appendix G).
b. Refer to product specific criteria for rotorcraft and turbine engine critical parts in 14 CFR 27.602, 29.602 and 33.75, respectively. Refer to any specific accountable directorate guidance and consult the specific guidance for critical parts on rotorcraft in AC 27-1 and 29-2 as necessary. For turbine engines, refer to AC 33.70-1. Otherwise, see appendix H for a general criteria for critical parts.
13. PMA and TSO Assemblies.
a. PMA for replacement articles in TSO assemblies occurs when these assemblies are in the type designs of aircraft, engines or propellers. As a result, the replacement article is for the typecertificated product, not the TSO assembly. Additionally, the installation eligibility of these replacement articles is tied to the type-certificated product.
b. Assess the effects of your replacement article on the minimum performance requirements of the applicable TSO. Assessments may range from qualitative evaluations to quantitative analyses and performance testing. For example, replacement articles that maintain the same interfaces in their assemblies as the respective original parts probably preserve the minimum performance requirements of the TSO assembly. A qualitative evaluation may attest that these articles do not affect an assembly’s performance. A replacement for a dynamic component in a TSO assembly may need quantitative analyses and tests to confirm continued compliance with the minimum performance requirements of its TSO.
c. Describe the effect of the replacement article on the TSO assembly. If in stallation of a replacement article prevents a TSO assembly from meeting its minimum performance requirements, but it still complies with the associated product’s airworthiness requirements, then direct installers of your replacement article to remove the assembly’s TSO markings in a supplemental ICA.
14. Compliance Listing. Use a compliance listing in the suggested tabular format of Appendix A to support your statement of compliance. The listing presents the applicable airworthiness requirements, their associated means of compliance and respective reports that document such.
This template aids coordination with the responsible ACO by defining the project scope from the various showings.
15. Test Reports and Computations. Test reports and computations show that an article’s design meets the applicable airworthiness requirements of its respective product. Use the certification basis of the eligible product from its TCDS to establish the relevant airworthiness requirements for your article. An article’s nature lies in its purpose, physical characteristics, interfaces with its products and how its failure modes impact safety. The scope and rigor of each test and computation vary with the nature of the article and includes at least the following:
• A safety assessment that characterizes the nature of the article and its effect on safety;
• Computations that show regulatory compliance or substantiate the comparative analyses;
• Test results that show direct regulatory compliance or verify the comparative analyses;
16. The Safety Assessment.
a. The safety assessment establishes the nature of an article and its impact on safety. The safety assessment looks at the consequences of an article’s failure modes on the next higher assembly and associated interfaces. Then it assesses any resulting degradation or failure of this higher assembly on the product (i.e., aircraft, engine or propeller). One way to characterize an 7
article is through a failure modes and effects analysis (FMEA). This analysis is at a minimum qualitative and demonstrates an understanding of where the article goes and what it does. Use the applicable product criteria in 14 CFR 33.75, 29.602, 27.602, 25.1309, or 23.1309 as a guide.
b. The nature of an article is also inherent in its design and function. A design’s sophistication varies greatly, as does its functions, which are often vital to a product’s continued safe operation. The safety assessment considers how these attributes of design and function interface with the product to determine the effect of an article’s failure or improper function on safety in its various applications. For example, if an article, such as an o-ring, has a variety of uses on the same product, then the safety assessment should catalog these uses and evaluate the ones with the greatest effect on safety.
c. The safety assessment also considers the service history of the original article or its next higher assembly on the type-certificated product. If the next higher assembly has reported failures, assess whether the original article contributed to those failures. The assessment summarizes the investigation of the original article’s service difficulties, any airworthiness directives against the article and its role in any accidents. The following available databases can help you with your investigations:
• Service Difficulty Report (SDR) database (http://av-info.faa.gov/sdrx) for any service problems with the original article;
• Airworthiness Directive (AD) database (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgAD.nsf/Frameset?OpenPage) for unsafe conditions in the original articles;
• Special Airworthiness Information Bulletins (SAIB) that concern the article (http://rgl.faa.gov/Regulatory_and_Guidance_Library/rgSAIB.nsf); and
• National Transportation Safety Board (NTSB) accident and synopses database (http://www.ntsb.gov/aviationquery/index.aspx) for causal factors; and
• Also review pertinent safety bulletins and airworthiness directives from other airworthiness authorities.
17. Manufacturing Inspection and Test Procedures. The Quality System Manual describes the processes and procedures that ensure each article conforms to its approved design. These processes and procedures refer to inspections and tests during manufacture that confirm the attributes of the article’s design. If the article is critical o r life-limited, cite these procedures in your quality manual or provide them in your data package to the ACO. The project ACO may require demonstration of these procedures as an element of finding compliance with applicable airworthiness requirements. These tests and procedures include process controls, finished product performance, and incoming material controls. The associated data include elements of the manufacturing cycle such as raw material purchase, evaluation of material chemistry and grain structure, fabrication processes, melt practices, forging processes, machining and surface treatments, other material properties, and required inspections. If the approval basis for the critical part or life-limited article is identicality, show that article’s manufacturing processes, inspection and test procedures produce a part that is identical to the original from a type certificate. Consult product specific guidance for critical parts from the accountable directorate.
18. Test Results. Coordinate test plans with the responsible ACO at the onset of your project.
Propose the anticipated level of FAA involvement in the testing. Use the PartSCP for substantial test plans. Upon establishment of FAA involvement, provide test results commensurate to the article’s design, function and its basis for approval. If the article is critical, the project ACO may require specific FAA inspections, tests and the resulting reports to confirm the airworthiness of the conforming design. For critical parts, use a FAA-approved test plan which mandates FAA review and approval of the resulting data. If the basis of the critical part or life-limited article is identicality, submit test results that show your article has the same attributes and life-limits as the original.
19. Airworthiness Limitations. If the TC holder’s article has a life limit, assess the proposed replacement article’s fatigue life using an appropriate method that considers material property distributions, loads, frequency of loads, mission profiles, stress and temperature distributions, and results from fatigue testing. Consult product specific guidance from the responsible directorate for acceptable methods to determine life limits. Provide data that supports your article’s replacement times, inspection intervals or related procedures. Develop and submit a supplemental ICA with the updated airworthiness limitations section to the project ACO.
Approval of life-limited parts in turbine engines usually requires an STC and a preapproved methodology to establish the part life. Pertinent product specific guidance is in various ACs on our regulatory and guidance library (RGL) (http://rgl.faa.gov).
20. System Effects. Consider the effects of your article on the environment of its system and any associated life-limited parts. Assess how differences in your article’s design and resulting characteristics impact its interfaces. Include this assessment in any engineering plan involving life-limited parts. Articles that affect these life-limited parts in turbine engines are influencing parts. See AC 33.70-1 for specific guidance on these articles.
21. Life Assessment. If your article is not life-limited, but is in a cyclic load environment, evaluate its life relative to the corresponding article approved via a type certificate. This evaluation typically involves fatigue tests or analyses. Consider other factors in the cyclic load environment, such as temperature. The results verify equivalent article life and support the proposed ICA and continued operational safety plans. Also, assess the impact of your article on mating life-limited assemblies. See pertinent product specific guidance in various advisory circulars on our RGL website (http://rgl.faa.gov).
22. Airworthiness Directives (AD).
a. Identify all ADs and service difficulties involving the original article from the product.
Show your design does not reproduce the unsafe conditions noted in ADs. Also avoid the known service difficulties of the respective original articles when feasible.
b. Approvals of articles that replace those under ADs require additional showings and FAA coordination. Provide a sound technical rational for your design and show how it preserves the level of safety in the AD. Then the replacement article still needs a subsequent AMOC prior to installation. The ACO that finds your design compliant and the ACO that issues the AMOC are not always the same entity. The design of the article will require the approval of both these ACO’s. The responsible ACO may include this provision in your notification letter of PMA supplement.
23. Continued Operational Safety Responsibilities. PMA holders are responsible for the integrity of their designs throughout their articles’ service lives. Their continued operational safety depends on three principles: monitoring an article’s performance in service, investigating its problems and then providing remedies and problem prevention. The nature of the article and its effect on safety determines the scope and scale of these efforts. Articles that have little impact on safety may rely on unsolicited feedback from users to monitor their in-service performance. Other articles, however, may need more robust plans. An example of an acceptable COS plan and its principles is found in the Modification and Replacement Parts Association (MARPA) Guidance Material for a PMA Continued Operational Safety (COS) System. Go to http://www.pmamarpa.com/government/cos.s html for this guidance. The application of these principles enables applicants to fulfill the reporting requirements of 14 CFR
21.3, ICA per 14 CFR 21.50(b) and resolve unsafe conditions in compliance with 14 CFR 21.99.
24. Compliance with Airworthiness Standards. Applications based on test reports and computation, either comparative or general test and analysis, must show the design of an article complies with applicable airworthiness requirements of 14 CFR Subchapter C, Aircraft. These applicable requirements stem from the certification bases of eligible products. They may include special conditions that apply to these respective products. See the relevant TCDS and certification basis for each eligible product. If you desire eligibility to more than one product category (i.e. small aircraft and rotorcraft), show compliance with applicable airworthiness requirements for each category. However, the project ACO may stipulate that showing compliance with the more rigorous airworthiness requirements from one product as satisfying that of another product. Use minimum performance standards in applicable TSOs and airworthiness requirements in the following 14 CFR parts:
• 14 CFR part 23, Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter Category Airplanes.
• 14 CFR part 25, Airworthiness Standards: Transport Category Airplanes.
• 14 CFR part 27, Airworthiness Standards: Normal Category Rotorcraft.
• 14 CFR part 29, Airworthiness Standards: Transport Category Rotorcraft.
• 14 CFR part 31, Airworthiness Standards: Manned Free Balloons.
• 14 CFR part 33, Airworthiness Standards: Aircraft Engines.
• 14 CFR part 34, Fuel Venting and Exhaust Emission Requirements for Turbine Engine Powered Airplanes.
• 14 CFR part 35, Airworthiness Standards: Propellers.
• 14 CFR part 36, Noise Standards: Aircraft Type and Airworthiness Certification.
25. General and Comparative Tests and Analyses.
a. General tests and analyses show that an article directly complies with all airworthiness regulations applicable to the product affected by article installation. Regulations require this direct showing of compliance. For example, 14 CFR 25.853 requires demonstration of flammability characteristics for articles in cabin interiors. Consult the project ACO along with relevant ACs for specific guidance on showing compliance through article and product testing.
b. Comparative tests and analyses substantiate that the PMA article is at least equal to the original article approved under a type certificate. Replacement articles replicate the functionality and airworthiness of original articles from respective type certificates. The methods of showing compliance may vary from those of a type certificate, but meet applicable airworthiness requirements. Thus, the PMA article preserves the certification basis of its eligible products.
Include data from back-to-back comparisons and testing of your article with an article from a type certificate. For example, showing compliance with 14 CFR 33.83 for replacement blades in turbine engines entails comparing their vibratory characteristics to those from a type certificate.
See AC 33.83-1 for specific guidance.
c. A combined approach uses both general and comparative analyses for different aspects of an article’s design. This combined approach confirms that the functional design of a proposed article is at least equal to that of an original article approved under a type certificate. The original articles provide the standard for determining the adequacy of the proposed replacement articles. Dimensions and material characteristics are normally acquired through the reverse engineering process. However, some article attributes need destructive analyses and laboratory testing to discern their material composition and vibratory response respectively. Analyses and tests confirm that any differences meet the applicable airworthiness requirements. C omparative analyses show that the physical characteristics of an article (i.e., dimensions, tolerances, materials, etc.) are at least the same as or better than the original article. Assess these improvements to avoid possible adverse effects on interfacing articles. You may also extend the comparative approach to back-to-back testing of the replacement and original articles. This back-to-back testing shows the relative durability and performance of the articles in the same operating environment.
d. Often showings of compliance by either general or comparative analyses require sound statistical principles. Use these principles to establish an appropriate approach for comparing article attributes and variability.
26. Reverse Engineering. Reverse engineering is but one way to obtain an article’s design. The reverse engineering process uses techniques that vary widely, produce diverse results and often require validation. The challenge lies in selecting the processes and techniques that are 11
appropriate to the article’s attributes and functions. Dimensional analysis by itself is usually inadequate to fully characterize an article and support a comparative analysis. However, dimensional and material analyses may adequately define some non-critical articles where their uses and attributes are well known and defined. Other articles need other substantiating information to show equivalent attributes and performance to an original article. One usually considers the following when using reverse engineering:
a. Sample Size. Typically, these samples are new or unused articles from approved sources with traceable documentation (e.g., purchase orders or FAA airworthiness tags). The sample size varies with the key attributes that define an article. Usually one needs at least three articles to ascertain the essential characteristics of a design. This minimum sample size can establish the maximum, minimum and nominal dimensions of its geometry. Other characteristics include tolerances, material properties and fabrication processes. Reverse engineering based on used articles is acceptable only for characteristics that do not deteriorate from use. Use the appropriate number of samples that will minimize the effects of inaccuracies inherent in the sampling method. Less than three samples may constrain your replacement’s design to narrow or impractical limits. Support the validity of your sampling approach and consult available product specific guidance. Use measurements from new, original articles when the datum and dimensional relationships are fundamental for an article’s function and interfaces.
b. Sample Sources. When possible, get samples from separate lots, billets, production runs or other sources to find the variability in key characteristics of the original design. If production - tracking data for an article is unknown, obtain articles from different sources at different times to capture additional potential sources of variability. Testing ma y include more samples to show equivalency between a new original and the PMA article.
c. Dimensional Tolerances. Variations in the sample measurements and accepted engineering practices determine the tolerances in article dimensions. Ensure the measurement techniques and dimensioning systems of the article preserve the dimensional relationships in the original design. Potential differences in the datum and reference dimensions of a replacement article from that of the original may require testing to validate functional design and interfaces to the next higher assembly and product. The resulting tolerances for the PMA article should not exceed the minimum and maximum dimensions measured on the sampled approved articles.
Exceeding these limits requires further substantiation via analyses, fit checks and/or functional tests.
d. Key Dimensional Characteristics and Datum. Consider the functional design of an article, its interfaces with other hardware and systems, and its operating environment to properly replicate the needed dimensional characteristics and relationships. Tolerances establish the physical boundaries that support the article’s functional design. Tolerances can vary throughout an article and across individual features. Control tolerances that affect functional design (i.e. , concentricity, circularity, run-out, etc.). Maintain a system of dimensioning compatible with that of the original article. Identify, at a minimum, the primary and secondary datum, reference dimensions and tolerances in the drawings for the replacement article.
e. Materials. Various tests and documentation establish the material composition of an article from a type certificate. Usually materials for PMA articles are the same as the original article including the base material, added welds and coatings. However, one may propose and substantiate alternate materials and processes that have at least equivalent properties. Obtain the 12