# AC 21.101-1A: Establishing the Certification Basis of Changed Aeronautical Products - Type: Advisory circular (guidance, not mandatory unless a rule or AD requires it) - Status: Historical - Version in force: [AC 21.101-1B](https://openairdocs.org/ac/21.101-1), approved March 11, 2016. This is a historical version - Approved: September 3, 2010 - Last reviewed: July 24, 2015 - Family: AC 21.101-1 - Cancels: [AC 21.101-1 CHG 1](https://openairdocs.org/ac/21.101-1-CHG-1) - Cancelled by: [AC 21.101-1B](https://openairdocs.org/ac/21.101-1) - 14 CFR parts: 21 - 14 CFR sections: [21.101](https://www.ecfr.gov/current/title-14/section-21.101), [21.19](https://www.ecfr.gov/current/title-14/section-21.19) - Pages: 102 - PDF: https://openairdocs.org/files/sha256/1a/dc/1adc971f0971bbea2615759be41e64f0ca77f66677edccaf310580c61fa4ac71 - FAA DRS: https://drs.faa.gov/browse/excelExternalWindow/2FE5D630D76D6DBF86257798004609BC.0001 - DRS record last changed: November 22, 2025 - Page: https://openairdocs.org/ac/21.101-1A ## Versions - [AC 21.101-1B](https://openairdocs.org/ac/21.101-1) (Current), approved Mar 11, 2016, 160 pages - [AC 21.101-1A](https://openairdocs.org/ac/21.101-1A) (Historical), approved Sep 3, 2010, 102 pages - [AC 21.101-1 CHG 1](https://openairdocs.org/ac/21.101-1-CHG-1) (Historical), approved Apr 28, 2003, 78 pages - [AC 21.101-1](https://openairdocs.org/ac/21.101-1?v=23B499F129998F0086256AA2007156FF) (Historical), approved Aug 3, 2001, 30 pages ## ADs that cite it (1) - [AD 2020-24-02](https://openairdocs.org/ad/2020-24-02): Auto flight, Flight controls, and Indicating/recording systems · Boeing 737-8 and 737-9. Effective Nov 20, 2020 ## Text Each page's text from the PDF; open the PDF at a page with #page=N. ### Page 1 U.S. Department of Transportation Federal Aviation Administration Advisory Circular Subject: Establishing the Certification Basis of Changed Aeronautical Products Date: 09/03/2010 Initiated by: AIR-110 AC No: 21.101-1A Foreword This advisory circular (AC) provides guidance for the application of the Changed Product Rule, Title 14 of the Code of Federal Regulations (14 CFR) §§ 21.101 and 21.19, for changes made to type certificated aeronautical products. David W. Hempe Manager, Aircraft Engineering Division AIR-100 ### Page 2 ii Contents Paragraph Page Chapter 1. Introduction … 1 1. Purpose and Content of this Advisory Circular … 1 2. Audience … 2 3. Applicability … 2 4. Definitions … 2 Chapter 2. Overview of 14 CFR §§ 21.19 and 21.101 … 3 1. 14 CFR § 21.19 … 3 2. 14 CFR § 21.101 … 3 Chapter 3. The Process for Establishing the Certification Basis for Changed Products 14 CFR § 21.101(a) and (b) … 5 1. Overview … 5 2. Step 1 of Figure 1. Identify The Proposed Type Design Change To An Aeronautical Product … 7 3. Step 2 of Figure 1. Is the Change Substantial? (14 CFR § 21.19) … 9 4. Step 3 of Figure 1. Will the Latest Re quirements be Used? (14 CFR § 21.101(a)) … 10 5. Step 4 of Figure 1. Relation of Changes … 10 6. Step 5 of Figure 1. Is the Pr oposed Change/Grouping Significant? (14 CFR § 21.101(b)(1)) … 11 7. Proposing an Amendment Level for a Significant Change … 13 8. Proposing an Amendment Level for a Not Significant Change … 15 9. Step 6 of Figure 1. Is the Area Affected By the Proposed Change? (14 CFR § 21.101(b)(2)) … 16 10. Step 7 of Figure 1. Are the New Require ments Practical and Do They Contribute Materially to the Level of Safety? (14 CFR § 21.101(b)(3)) … 18 11. Step 8 of Figure 1. Is the Proposed Type Certification Adequate? … 20 Chapter 4. Other Considerations … 22 1. Design Related Operating Requirements … 22 2. Excepted Products under 14 CFR § 21.101(c) … 22 3. Special Conditions, 14 CFR § 21.101(d) … 22 4. Effective Period for an Application to Ch ange a Type Certificate. (14 CFR § 21.101(e)) . … 23 5. Other Category Aircraft (14 CFR § 21.101(f)) … 23 6. Clarification of 14 CFR 21.101 (g) (Part 26 Requirements) … 25 7. Documentation … 25 Appendix A. Classification of Changes … A-1 Appendix B. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product … B-1 1. Introduction … B-1 2. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product … B-2 ### Page 3 iii 3. Examples of How to Certify Changed Aircraft … B-6 Appendix C. The Use of Service Experience in the Certification Process … C-1 1. Introduction … C-1 2. Guidelines … C-1 3. Example: 14 CFR § 25.1141(f) Powerplant Controls for Transport Airplanes … C-3 Appendix D. Definitions and Terminology … D-1 Appendix E. Related Code of Federal Regulations Sections … E-1 1. Related Code of Federal Re gulations Paragraphs … E-1 2. How to Get Publications … E-1 ### Page 4 Chapter 1. Introduction 1. Purpose and Content of this Advisory Circular. a. The Federal Aviation Administration (FAA) wrote this advisory circular (AC) to provide guidance for establishing the certification basis for changed aeronautical products in accordance with Title 14 of the Code of Federal Regulations (14 CFR) § 21.101 and to help identify if it will be necessary to apply for a new type certificate (TC) under 14 CFR § 21.19. The guidance describes the process for establishing the certification basis for amended TCs, supplemental type certificates (STC) and amended STCs, detailing evaluations, classifications, and decisions made throughout the process. b. The content of this AC is divided into 4 chapters and 5 appendices: (1) Chapter 1 explains the purpose of this AC, describes its content, specifies the intended audience, and clarifies which changes are within the scope of applicability of this AC. (2) Chapter 2 provides a general overview of 14 CFR §§ 21.101 and 21.19, clarifies the principals and safety objectives and directs applicants to the applicable guidance contained in subsequent chapters of this AC. (3) Chapter 3 contains guidance for implementation of 14 CFR § 21.101(b) to establish the type-certification basis for changed aeronautical products. Chapter 3 describes in detail the various steps of the “top-down” certification basis development approach. Chapter 3 also addresses 14 CFR § 21.19 considerations to identify conditions under which an applicant for a type design change is required to submit application for a new TC and provides guidance at which stage of the process this assessment is to be performed. (4) Chapter 4 contains considerations for design related operating requirements, approach for changes on certain small airplanes and rotorcraft under specified maximum weight (“excepted products”), use of special conditions under 14 CFR § 21A.101 (d), the effective period of an application, 14 CFR part 26 requirements, documentation of revisions to the typecertification basis and approach to ‘other category aircraft’ under 14 CFR § 21.101 (f). (5) Appendix A contains examples of typical type design changes for small airplanes, transport category airplanes, rotorcraft, engines, and propellers which are categorized by the FAA into individual tables according to the classifications to the level of design change - substantial, significant, and not significant. (6) Appendix B provides detailed guidance with examples for evaluating when compliance to the rule would be impractical under the “impracticality” exception in the rule. (7) Appendix C provides guidance with examples on use of relevant service experience in the certification process as one way to show that a later amendment may not ### Page 5 2 contribute materially to the level of safety, allowing the use of earlier requirements. (8) Appendix D lists the applicable definitions, and terminology for application of the rule. (9) Appendix E contains cross references to the requirements of 14 CFR §§ 21.19 and 21.101. c. This AC is not mandatory and is not a regulation. This AC describes an acceptable means, but not the only means, to comply with 14 CFR § 21.101. However, if you use the means described in this AC, you must follow it entirely. 2. Audience. This AC is for applicants applying for STCs, amended type certificates (ATCs), or amended STCs. 3. Applicability. a. This document supersedes AC 21.101-1 Change 1, dated April 28, 2003. This document was revised to clarify current policy for complying with 14 CFR § 21.101. b. This AC applies to major type design changes under 14 CFR § 21.101 for aeronautical products certificated under 14 CFR §§ 21, 23, 25, 27, 29, 31, 33, and 35. c. Minor type design changes are automatically considered not significant under 14 CFR § 21.101(b) and the existing type certification basis is considered adequate for their approval under 14 CFR § 21.95. d. This AC also applies to aircraft certificated under 14 CFR §§ 21.17(b), 21.19, 21.24, 21.25, and 21.27. e. The term aeronautical products, or products, means type certificated aircraft, aircraft engines, and propellers. f. This AC is not intended to be used to determine the applicable aircraft noise, fuel venting, and exhaust emission requirements for changed products. 4. Definitions. See appendix D in this AC for a list of definitions. ### Page 6 Chapter 2. Overview of 14 CFR §§ 21.19 and 21.101 1. 14 CFR § 21.19. a. Title 14 CFR § 21.19 requires that as an applicant, you apply for a new TC for a changed product if the change in design, power, thrust, or weight is found by the FAA to be so extensive that a substantially complete investigation of compliance with the applicable regulations is required. b. Changes that require a substantial re-evaluation of the product’s compliance findings are referred to as “substantial changes”. For guidance see paragraph 3 of chapter 3. Appendix A in this AC provides examples of type design changes that will require application for a new TC. c. If the FAA has determined through 14 CFR § 21.19 that your proposed design change does not require a new TC, see 14 CFR § 21.101 for the applicable requirements to develop the certification basis for your proposed design change. For guidance, see chapter 3 and the examples in appendix A in this AC. 2. 14 CFR § 21.101. a. Title 14 CFR § 21.101(a) requires a change to a TC to comply with the latest requirements, unless the change meets the criteria for the exceptions identified in 14 CFR §§ 21.101(b) and (c). The intent of 14 CFR § 21.101 is to enhance safety through the incorporation of the latest regulatory standards in the type certification basis for changed products to the greatest extent practicable. b. You can comply with the earlier requirements consistent with the requirements of 14 CFR § 21.101(b), when:  A change is not significant (see 14 CFR § 21.101 (b) (1)), or  An area, system, component, equipment or appliance are not affected by the change (see 14 CFR § 21.101 (b) (2)), or  Compliance with a later amendment for a significant change does not contribute materially to the level of safety (see 14 CFR § 21.101 (b) (3)) , or  Compliance with a latest amendment would be impractical (see 14 CFR § 21.101(b)(3)). c. Note that earlier amendments may not precede either the corresponding amendment of the regulation incorporated by reference in the type certification basis and any requirement found in 14 CFR §§ 23.2, 25.2, 27.2, and 29.2. For transport category airplanes only, the provisions of ### Page 7 4 part 26 that is related to the change is also required. d. Title 14 CFR § 21.101(b) allows a changed product to comply with an earlier amendment of a regulation, provided the earlier amendment is considered adequate and meets the criteria in 14 CFR §§ 21.101(b)(1), (2) or (3). However, when a proposed design change involves features or characteristics considered novel and unusual and the proposed airworthiness standards do not contain adequate or appropriate safety standards for this feature, later amendments and/or special conditions will be applied. e. Title 14 CFR §§ 21.101(b)(1)(i) and (ii) describe the automatic criteria establishing that a change is significant. f. Title 14 CFR § 21.101(c) provides an exception from the requirements of 14 CFR § 21.101(a) for a change to certain aircraft with less than a specified maximum weight. If you apply for a type design change to an aircraft (other than rotorcraft) with a maximum weight of 6,000 pounds or less maximum weight, or to a non-turbine powered rotorcraft with a maximum of 3,000 pounds or less maximum weight, you can show that the changed product complies with the regulations incorporated by reference in the TC. You can also elect to comply, or may be required to comply, with the later regulations. See chapter 4, section 2 in this AC for specific guidance on this provision. g. Title 14 CFR § 21.101(d) provides for the use of special conditions, under 14 CFR § 21.16, when the proposed certification basis and any later regulations do not provide adequate standards to the proposed change because of a novel or unusual design feature. h. Title 14 CFR § 21.101(e) prescribes the effective period an application will remain valid for a change. This section is consistent with the requirements of 14 CFR § 21.17 for a new TC. i. Title 14 CFR § 21.101(f) pertains to aircraft certificated in certain categories and special classes (e.g. gliders, airships, and other nonconventional aircraft), including the engines and propellers installed on them, under the requirements of 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27 airworthiness requirements. j. Title 14 CFR § 21.101(g) pertains to regulatory compliance for transport category airplanes with applicable provisions of 14 CFR part 26 and/or corresponding later amendments to 14 CFR part 25. For transport category airplanes, you must comply with each applicable provision of 14 CFR part 26 for the change, unless you have elected or are required to comply with a corresponding amendment to 14 CFR part 25 that was issued on or after the date of the applicable 14 CFR part 26 provisions. ### Page 8 Chapter 3. The Process for Establishi ng the Certification Basis for Changed Products 14 CFR § 21.101(a) and (b) 1. Overview. a. The applicant and the FAA each have a responsibility under 14 CFR § 21.101(a) and (b). As an applicant for the certification of a type design change, you must show that the change complies with the latest applicable airworthiness requirements unless you propose an exception per 14 CFR § 21.101(b). If you are proposing exception, you should make a preliminary classification whether the change is significant or not significant, and propose an appropriate certification basis. The FAA determines whether your classification of the change and proposal for the certification basis are consistent with the applicable rules and their interpretation, but should not be dependent on whether the TC holder or applicant for a STC is originating the change. The certification basis can vary depending on the magnitude and scope of the change. The steps below present a streamlined approach for making this determination. In addition to assisting in the determination of significance and establishing the certification basis, this guidance will help to establish the appropriate amount of coordination required between the applicant and the FAA. b. Classifications of typical type design changes are in appendix A, Classification of Changes. See paragraph 6 (c), of this chapter for instructions on how to use appendix A. c. In cases where the examples in appendix A are not applicable for the proposed change, use the following steps in conjunction with figure 1, on the next page, to develop the appropriate certification basis for the type design change. ### Page 9 6 Figure 1. Establishing the Certification Basis for Changed Product Step 2. Is the change substantial? (§ 21.19) Step 1. Propose major type design change - Identify type design to be changed - Identify proposed change - Use high level descriptors Step 4. Arrange changes into related & unrelated groups Step 3. § 21.101(a) Will the latest requirements be used? **NEW TYPE** CERTIFICATE § 21.19 Step 5. Is the proposed change/grouping significant? § 21.101(b)(1) Will the latest requirements be used? Step 6. For every area, is the area affected by the proposed change? Step 7. Are the latest requirements practical or do they contribute materially to the level of safety? § 21.101(b)(3) Latest airworthiness requirements Earlier airworthiness requirements but not earlier than the existing certification basis Unaffected areas Impractical or not contributing materially to the level of safety Not significant Yes Yes No No Yes Yes No No Yes No Yes No **PROPOSED CERTIFICATION BASIS FOR THE CHANGE** **CONTINUED** **COMPLIANCE** **WITH THE** **EXISTING TC BASIS STOPA** ### Page 10 7 A **PROPOSED** **CERTIFICATION** BASIS FOR THE CHANGE Step 8. Is the proposed TC basis adequate? Proposed certification basis requires later airworthiness requirement(s) and/or special conditions Proposed certification basis completed YesNo 2. Step 1 of Figure 1. Identify The Proposed Type Design Change To An Aeronautical Product. a. Prior to describing the proposed change(s), it is important to clearly identify the type design configuration to be changed. A series of derivative aircraft, engines, or propellers (for example, x -100, x-200, x-300) may evolve based on predecessor type designs, each with its own design change that make it distinct from the other series. You should identify which model or series within that model the specific configuration that will be modified. Note: An STC is not a product; it is a change to a product. When changing or amending an STC the starting point is the existing modified product (TC with existing STC installed). For example, if you were amending an STC for an external cargo locker and you proposed changing the configuration of the locker, then your starting point would be the existing TC with the existing cargo locker installed by STC. You would then compare that configuration (TC with existing STC installed) to the changed product (TC with proposed amended STC installed). ### Page 11 b. Changes to a product can include physical design changes, changes to an operating envelope, and/or performance changes. The change can be a single change or a collection of changes. The purpose of this process step is to identify and describe the change to the aeronautical product. As an applicant for a type design change you must consider all previous related design changes and the amendment level of the certification basis used for these changes. Note 1: By definition all previously incorporated changes will have been approved. The purpose of step 1 is to consider the net cumulative effect of the changes since the last time the certification basis for the changed/affected area was upgraded from that of the original type design. Note 2: For the purpose of classifying the proposed design change, previously approved design data are still considered part of the proposed design and must be considered part of the proposed design change classification. c. When identifying the changes being proposed as part of a modification, consider previous relevant changes that create a cumulative effect, as these may influence the decisions regarding substantial and significant changes later in the process. By previous relevant changes we mean design changes whose effects accumulate, such as successive thrust increases, incremental weight increases, or sectional increases in fuselage length. Any previous relevant design changes in the area affected by the current change that did not involve an upgrade of the existing certification basis must be taken into account in the next design change proposal. (1) Example 1: A 5% weight increase is currently being proposed, but a previous 10% and another 15% weight increase has been incorporated into this aircraft without upgrading the existing certification basis. In the current proposal for a 5% weight increase, the cumulative effects of the two previous weight increases that did not involve upgrade of the certification basis will now be accounted for as an approximately 30% increase in weight, for the purpose of making the substantial and/or significant decisions. Note that the cumulative effects to be considered are those incremental increases since the last time the airworthiness requirements, in the type certification basis applicable to the area affected by the current change, were upgraded. (2) Example 2: The TC for airplane model X lists three series, namely X-300, X-200, and X-100. The X-300 is a derivative of the X-200 which is a derivative of the original X-100 series. An applicant proposes a design change to the X-300 series airplane. During the review of the X-300 certification basis and the regulations affected by the proposed change, it was identified that one regulation, 14 CFR § 25.571 (damage tolerance requirements), remained at the same amendment level as the X-100 original certification basis (exception was granted). Since the amendment level for this particular regulation was not changed for the two subsequent airplane series (X-200 and X-300), the cumulative effects of these two previous design changes that are related to the proposed change and the damage tolerance requirements must now be addressed. ### Page 12 d. To identify and describe the proposed changes to any aeronautical product, use a high level description of the design change that characterizes the intent of, or the reason for, the change. No complex technical details are necessary at this stage. For example, a proposal to increase maximum passenger-carrying capacity may require an addition of a fuselage plug, and as such, a “fuselage plug” becomes one possible high level description of this design change. Similarly, a thrust increase, a new or complete interior, an avionics system upgrade, or a passenger-to-cargo conversion are all high level descriptions that characterize typical changes to the aircraft, each driven by a specific goal, objective or purpose. e. Evolutionary Changes. Evolutionary changes that occur during the course of a certification program may require re-evaluation of the certification basis and may result in reclassification of the change. That is, any evolution in the proposed design change after the certification basis has been agreed to (or established) will necessitate a revisit of the certification basis to ensure that “evolved” aspects of the design change are still covered by the agreed upon certification basis. 3. Step 2 of Figure 1. Is the Change Substantial? (14 CFR § 21.19) a. Title 14 CFR § 21.19 requires that you apply for a new TC for a changed product if the proposed change in design, power, thrust, or weight is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. A new TC could be required for either a single extensive change to a previously type certificated product or for a changed design derived through the cumulative effect of a series of design changes from a previously type certificated product. b. A “substantially complete investigation” of compliance is required when most of the existing substantiation is not applicable to the changed product. A substantial change proposal will require the need to comply with all requirements applicable to a particular category of product. The number of requirements to which compliance must be re-established for the changed product may not necessarily be the sole determination as to whether the change is substantial, but rather it is the extent of effort to establish compliance, or the depth of investigation required to be done. In other words, the proposed design change may be considered substantial if it is so extensive (making the product sufficiently different from its predecessor) that the design models, methodologies, and approaches used to demonstrate a previous compliance finding could not be used. c. To address if a change is substantial at the beginning of the process, you must evaluate the total or combined effect of all the proposed changes identified in step 1, including the cumulative effects of previous relevant design changes since the last update of the certification basis (as explained in step 1). ### Page 13 d. If it is not initially clear if a new TC is required, appendix A provides some examples of substantial changes to aid in this classification. A substantial change requires application for a new TC under 14 CFR §§ 21.17 and 21.19. If the change is not substantial, then follow the 14 CFR § 21.101 process. 4. Step 3 of Figure 1. Will the Latest Requirements be Used? (14 CFR § 21.101(a)) You can use the latest requirements for your proposed type design change. If you use the latest requirements you will meet the intent of 14 CFR § 21.101 and no further classification (significant or not-significant) and justification is needed. However, the decision to voluntarily comply with the latest certification standards for a design change sets a new regulatory baseline for all future related changes in the same affected area. Even though one applicant elects to use the latest certification requirements, another applicant could apply 14 CFR § 21.101 for a similar design change proposal, and use the exceptions in accordance with 14 CFR § 21.101(b). If you are not using the latest requirements then proceed as follows. 5. Step 4 of Figure 1. Relation of Changes. a. Once the proposed changes are identified usi ng high level descriptions, the next step is to determine if any of these changes are related to each other. Related changes are those that cannot exist without another, are co-dependent, or a pre-requisite of another. For example, a need to carry more passengers could require the addition of a fuselage plug, which will result in a weight increase, and may necessitate a thrust increase. Thus the fuselage plug, weight increase and thrust increase are all related high level changes that will be needed to achieve the goal of carrying more passengers. A decision to upgrade the cockpit to more modern avionics at the same time as these other design changes may be considered unrelated, as the avionics upgrade is not necessarily needed to carry more passengers (it has a separate purpose, likely just modernization). The proposed avionics upgrade could then be considered an unrelated (or a stand alone) change. However, the simultaneous introduction of a new cabin interior would be considered related since a cabin length change will have an impact on occupant safety considerations. Even if a new cabin interior is not included in the product level change, the functional effect of the fuselage plug has implications on occupant safety (e.g., the dynamic environment in an emergency landing, emergency evacuation, etc.), and thus the cabin interior becomes an affected area. ### Page 14 b. Once the change(s) are organized into groupings of those that are related and those that are unrelated (or stand alone), you are ready for step 5 of figure 1. The grouping of related and unrelated changes is particularly relevant to the significant Yes/No decision, (14 CFR § 21.101(b)(1)), described in step 5 of figure 1. Each group of related changes and each unrelated (stand alone) change is evaluated on its own merit for significance. c. After describing the design change groupings and the associated or supporting technical details for each change, you must identify areas, systems, components, equipment, or appliances of the product that are affected by the design change and the corresponding regulatory standards associated with these areas. For each group, you must assess the physical and/or functional effects of the change on other areas, systems, components, equipment, or appliances of the product. The characteristics affected by the change are not only physical changes, but also functional changes brought about by the physical changes. Examples of physical aspects are: structures, systems, equipment, component and appliances, and software in combination with the affected hardware. Examples of functional characteristics are performance, handling qualities, aeroelastic characteristics, and emergency egress. The intent is to encompass all aspects where there is a need for re-evaluation, that is, where the substantiation presented for the product being changed should be updated or rewritten. 6. Step 5 of Figure 1. Is the Proposed Change/Grouping Significant? (14 CFR § 21.101(b)(1)) a. In step 5 it is your responsibility to justify that a grouping of related changes or an unrelated change does not qualify as a significant change. Significant changes are product level changes which are distinct from the vast majority of major changes. In general, these changes are either the result of an accumulation of changes or occur through an isolated extensive change that makes the changed product distinct from its predecessors. Step 1 explains the accumulation of changes that must be considered. Title 14 CFR § 21.101(b)(1) defines a significant change as existing when one or more of three automatic criteria apply: (1) Changes where the general configuration is not retained (significant change to general configuration). A change to the general configuration at the product level that distinguishes the resulting product from other product models, for example, performance or interchangeability of major components. Typically, for these changes, an applicant will designate a new aircraft model or series number, although this is not required. For examples see appendix A. (2) Changes where the principles of construction are not retained (significant change to principles of construction). A change at the product level to the materials and/or construction ### Page 15 12 methods that affect the overall products’ operating characteristics or inherent strength and would require extensive reinvestigation to show compliance. For examples see appendix A in this AC. (3) Changes that invalidate the assumptions used for certification (significant change to the assumptions used for certification). A change to the assumptions at the product level associated with the compliance demonstration, performance, or operating envelope that by itself is so different that the original assumptions or methodologies of demonstrating compliance are invalidated. For examples see appendix A in this AC. b. The above criteria are used to determine if each change grouping and each stand-alone change is significant. These three criteria are assessed at the product level. In applying the automatic criteria you should focus on the design change itself. Consideration of only the regulatory importance or safety benefit of the latest certification requirements is not a justification by itself to cause a design change to be classified or re-classified as a significant change. c. Appendix A includes tables of typical change s for transport airplanes, small airplanes, rotorcraft, engines, and propellers that meet the definition of significant. In these tables, one or more of the three automatic criteria in 14 CFR § 21.101(b)(1) apply for each case where the changes are identified as significant. Experience has shown the concept of having only the three automatic criteria seems to fit most projects. The examples also include typical changes that do not achieve the significant level. The tables can be used in one of two ways: (1) To classify a proposed change that is listed in the tables of the appendix, or (2) In conjunction with the three automatic cr iteria, to help classify a proposed change not listed in the tables of the appendix by comparing the proposed change to changes which are similar in type and/or magnitude. d. Design changes can trigger one or more of the automatic criteria listed in 14 CFR §§ 21.101(b)(1)(i) and (ii) for the proposed design change. When assessing the design change grouping, consider the cumulative effect of previous relevant design changes. Design changes may have been incorporated over time with no change in the certification basis and the final product may be significantly different than would be represented by the existing certification basis. e. Each grouping of related changes and each unrelated (stand alone) change, identified using high level descriptions, will be evaluated on its own merit to determine if it is a significant or not significant change. Use the tables in appendix A as guidance to make the classification of significant, or not significant. Only when one or more of the three criteria is met can the type design change be considered significant for that grouping or unrelated change. The starting point for assessing the cumulative effects of previous relevant design changes is from the last time the applicable requirements in the certification basis for the affected area, system, component, equipment, or appliance was upgraded. ### Page 16 f. Typically, a change to a single area, system , component, or appliance may not result in a product level change. However, there may be distinct cases where the change to a single system or component may, in fact, result in a significant change due to its effect on the product overall. Examples may include addition of winglets, leading edge slats or change in primary flight controls to fly-by-wire system. g. A secondary change is a physical change (which may include physical aspects as referenced in paragraph 9 in this chapter) that is part of and consequential to an overall significant change. A secondary change is a physical change that does not change the system, structural capacity, or functionality, but is necessary to support a significant change. Based on this description, a secondary change is not required to comply with the latest requirements because it is considered “not contributing materially to the level of safety”, and therefore eligible for an exception under14 CFR § 21.101. Determining whether a change meets the description for a secondary change, and thus eligible for an exception, should be straightforward. Hence the substantiation or justification need only be minimal. If this determination is not straightforward, then your proposed change is very likely not a secondary change. (1) In some cases, however, the change which restores functionality may in fact contribute materially to the level of safety by meeting a later amendment. If this is the case it would not be considered a secondary change. (2) An example of secondary change is lengthening existing control cables passing through the new fuselage plug, to restore existing functions to systems that could be situated within or beyond the new plug. The lengthening of these cables can be accepted as not adding system capacity or capability, so these changes can be identified as secondary changes and not be required to meet the latest amendment. h. A new model number designation to a change d product is not necessarily indicative that the design change is significant under14 CFR § 21.101. Conversely, retaining the existing model designation does not mean that the design change is not significant. All changes are considered in light of the magnitude of the type design change. i. Making the determination. The final dete rmination of whether a design change is significant or not significant is retained by the FAA. To assist you in your assessment, the FAA has predetermined the classification of several typical design changes that can be used for reference. These examples are listed in appendix A in this AC. j. At this point, the determination of significant or not significant for each of the groupings of related changes and each stand alone change has been made. For significant changes, if you propose to comply with an earlier requirement, use the procedure outlined in paragraph 7 below. 7. Proposing an Amendment Level for a Significant Change. a. If an unrelated (stand alone) change or a grouping of related changes is classified as significant (14 CFR § 21.101(a)), you will comply with the latest regulatory requirements for ### Page 17 14 certification of the changed product, unless you justify use of one of the exceptions provided in 14 CFR §§ 21.101(b)(2) and/or (3) to show compliance with earlier amendment(s). The final certification basis may consist of a combination of the regulatory requirements ranging from the original aircraft certification basis to the most current regulatory amendments. b. If the classification of the change is significant, all areas, systems, components, parts or appliances affected by the change must comply with the regulatory requirements at the amendment level in effect on the date of application for the change. You can justify use of one of the exceptions in 14 CFR §§ 21.101(b)(2) and (3) to comply with an earlier amendment, but no earlier than the existing certification basis. You must comply with any retroactive requirement found in 14 CFR §§ 23.2, 25.2, 27.2, and 29.2 applicable on the date of the application for the change. c. For transport category airplanes only, you must also comply with any applicable provision of 14 CFR part 26 (related to the change) which is applicable on the date of the application for the change, unless you elected or were required to comply with later corresponding 14 CFR part 25 requirements. d. Title 14 CFR § 21.101(b)(3) provides two more exceptions applicable to areas, systems, parts or appliances which are affected by the significant change. For a group of related design changes or an unrelated design change that has been determined to be significant, 14 CFR §§ 21.101(b)(2) and (3) provide exceptions from the requirement of 14 CFR § 21.101(a). You can comply with an earlier amendment level or with the existing certification basis for areas not affected by the change, and any areas affected by the change for which compliance with the latest requirements would not contribute materially to the level of safety or would be impractical. e. The earlier amendments may not precede the corresponding requirements in the existing certification basis or any requirement found in 14 CFR §§ 23.2, 25.2, 27.2, 29.2, or for transport category airplanes only, 14 CFR part 26, that is related to the change. It is important when seeking to use earlier amendments that you demonstrate that compliance with the latest requirements does not contribute materially to the level of safety, or is impractical. f. You must provide acceptable justification for the application of earlier amendments for areas affected by a significant change. Your justification must show that compliance with later requirements in these areas would not contribute materially to the level of safety or would be impractical. Such justification should address all the aspects of the area, system, component, equipment, or appliance affected by the significant change. g. The final certification basis may combine latest, earlier (intermediate), and existing regulations, but cannot contain regulations preceding the existing certification basis. h. Note that should you decide to use the latest airworthiness standards without any exceptions, no further evaluations and justifications are needed. In such a case, proceed to step 8 (section 11). ### Page 18 8. Proposing an Amendment Level for a Not Significant Change. a. When a change is classified not significant, the rule (14 CFR § 21.101(b)(1)) allows the use of the earlier regulatory requirements, but not dated prior to the existing certification basis. Within this limit, you are allowed to propose an amendment level for each certification standard for the affected area. However, you should be aware that your proposal for the type certification basis will be reviewed by the FAA to ensure that the certification basis is adequate for the proposed change (see paragraph 8.d). You must also comply with the retroactive requirements found in 14 CFR §§ 23.2, 25.2, 27.2, 29.2 applicable on the date of the application for the change. b. For transport category airplanes only, you must comply with any 14 CFR part 26 requirements related to the change applicable on the date of the application for the change, unless you elected, or were required, to comply with later corresponding 14 CFR part 25 requirements. c. When choosing the above option of the existing type certification basis, you can elect to comply with a specific airworthiness requirement or a subset of airworthiness requirements at later amendments. In such a case, you should consult with the FAA to ensure the certification basis includes other airworthiness requirements that are directly related. Some later regulatory requirements may be less restrictive; therefore, you may see advantage in using them on the elect to comply basis. However, it is recommended you do not make a final decision until you learn from the FAA which other airworthiness requirements are found directly related. d. For a design change that contains features which are not covered in the existing certification basis, the FAA will designate the applicable airworthiness standard at the appropriate amendment level, beginning with the existing certification basis and progressing to the most appropriate later amendment level for the change. For a change that contains new design features that are novel and unusual, for which there is no later applicable airworthiness requirement, the FAA will designate special conditions. For new design features or characteristics which may pose a potential unsafe condition for which there are not later applicable airworthiness requirements, new requirements may be imposed per 14 CFR § 21.21(b)(2). e. Adequacy of Certification Basis: The certification basis for a changed product under 14 CFR § 21.101 is considered adequate when the FAA determines that the prescribed airworthiness requirements (existing, later, or latest amendments, including special conditions, or new requirements per 14 CFR § 21.21(b)(2)) provide an appropriate level of safety for the changed product and do not result in any unsafe design features or characteristics for the intended use of the product. ### Page 19 9. Step 6 of Figure 1. Is the Area Affected By the Proposed Change? (14 CFR § 21.101(b)(2)) a. A not affected area is any area, system, component, equipment, or appliance that is not affected by the proposed type design change. For a type design change, it is important that the effects of such change on other areas, systems, components, equipment, or appliances of the product are properly assessed because areas that have not been physically changed may still be considered part of the affected area. If a new compliance finding is required, regardless of its amendment level, it is an affected area. If the significant change does not affect the area, then the certification basis of that area need not be revisited, in other words, the unaffected area continues to comply with the existing amendment level without further substantiation. b. To determine whether an area is affected or not, consider the following aspects of a type design change: (1) Physical aspects. The physical aspects include direct changes to structures, systems, equipment, components, and appliances (physical aspects may include software/airborne electronic hardware changes and the resulting effect on systems functions). (2) Performance/functional characteristics. The less obvious aspect of the word “areas” covers general characteristics of the type certificated product, such as performance features, handling qualities, emergency egress, structural integrity, aeroelastic characteristics, or crashworthiness. These characteristics may be affected by a product level change. For example, adding a fuselage plug could affect performance and handling qualities, and thus regulations associated with these aspects would be considered part of the affected area. Another example is the addition of a fuel tank and new fuel conditioning unit. This change affects the fuel transfer and fuel quantity indication system resulting in the airplane’s unchanged fuel tanks being affected. Thus, the entire fuel system (changed and unchanged areas) becomes part of the affected area due to the change in functional characteristics. Note: Substantiating data for the affected area for your proposed type design change can include compliance findings from a previously approved design change, in supporting compliance findings for your proposal. However, your proposal to use previously approved compliance data must be considered part of the entire proposed type design change and should be approved as part of your proposed design change. ### Page 20 c. All areas affected by the proposed design change must comply with the latest requirements, unless you show that demonstrating compliance with the latest amendment of a requirement would not contribute to the level of safety or would be impractical. Step 7 provides further explanation. ### Page 21 10. Step 7 of Figure 1. Are the New Requirements Practical and Do They Contribute Materially to the Level of Safety? (14 CFR § 21.101(b)(3)) a. Compliance with the latest requirements could be considered to “not to contribute materially to the level of safety” if the existing type design and/or relevant experience demonstrates a level of safety comparable to that provided by the latest requirements. You must provide sufficient justification to allow us to make this determination. This exception could be applicable in the situations described in the paragraphs below: Note: Compliance with later requirements would not be required where the amendment is of an administrative nature and has been made only to correct inconsequential errors or omissions, consolidate text, or clarify an existing requirement. (1) Design features that exceed the ex isting certification basis requirements, but do not meet the latest requirements, can be used as a basis for granting an exception under the “does not contribute materially” exception. These design features, if accepted as a justification for an exception, must be incorporated in the amended type design configuration and recorded, where necessary, in the certification basis. The description of the design feature would be provided in the type certificate data sheet (TCDS) or STC at a level that allows the design feature to be maintained, but does not contain proprietary information. For example, an applicant proposes to install winglets on a 14 CFR part 25 airplane. Part of the design involves adding a small number of new wing fuel tank fasteners. The latest 14 CFR § 25.981 at amendment 25-102 requires structural lightning protection. The applicant proposes an exception from these latest structural lightning protection requirements because the design change uses new wing fuel tank fasteners with cap seals installed. The cap seal is a design feature that exceeds the requirement of 14 CFR § 25.981 at a previous amendment level, but does not meet the latest amendment 25- 102. If the applicant can successfully substantiate that compliance with amendment 25-102 would not materially increase the level of safety of the changed product, then this design feature can be accepted as an exception to compliance with the latest amendment. (2) Consistency of design should be considered when applying the latest requirements. Below, an airplane example is provided for describing how this provision may be used; however, the rationale in this example may be applied to any product covered by this AC. ### Page 22 19  For example, when a small fuselage plug is added, additional seats and overhead bins are likely to be installed, and the lower cargo hold extended. These components may be identical to the existing components. The level of safety may not materially increase by applying the latest requirements.  However, if a fuselage plug is large enough relative to the original certificated aircraft structure, seats, bins, doors, and cargo compartment, the change may require compliance with the latest requirements, comparable with what will be required for a new airplane. In these circumstances, the proposed certification basis should encompass the requirements in effect on the date of application for the change. (3) Service experience: Relevant service experience, such as fleet performance or utilization over time (relevant flight hours or cycles), is one way of showing that a later amendment may not contribute materially to the level of safety, so the use of earlier requirements could be appropriate. Appendix C provides additional guidance on the use of service experience, along with examples.  There may be cases for rotorcraft and small airplanes where relevant data may not be sufficient or not available at all because of the reduced utilization and the different amount and type of data available. In such cases, other service history information may provide sufficient data to justify the use of earlier requirements, such as: warranty, repair, and parts usage data; accident, incident, and service difficulty reports; service bulletins; airworthiness directives; or other pertinent and sufficient data collected by the manufacturers, authorities, or other entities.  The service experience necessary to demonstrate the appropriate level of safety as they relate to the proposed design change would have to be reviewed and agreed to by the FAA. b. Impractical. Compliance with the latest requirements may be considered impractical if you can justify that it would result in additional resource requirements that are not commensurate with the incremental safety benefit (difference between the latest and proposed certification basis). The additional resource requirements could include those arising from design changes required for compliance and the effort required to demonstrate compliance, but excludes resource expenditures for prior product changes. (1) Support your position that compliance is impractical with substantiating data and analyses. While evaluating your position and your substantiating data regarding impracticality, we may consider other factors (for example, the costs and safety benefits for a comparable new design). (2) A review of transport category projects showed that in certain cases, where an earlier amendment to applicable requirements was allowed, design changes were made to nearly ### Page 23 20 comply with the latest amendments. In these cases, the applicants were able to successfully demonstrate that full compliance would require a substantial increase in the outlay or expenditure of resources with a very small increase in the level of safety. These design features can be used as a basis for granting an exception under the “impracticality” exception. (3) Appendix B provides additional guidance and examples for determining procedures for evaluating impracticality of applying latest requirements to a changed product rule. (a) The exception of impracticality is a qualitative and/or quantitative cost/safety benefit assessment for which it is difficult to specify clear criteria. Experience to date with applicants has shown that justification of impracticality is more feasible when both applicant and FAA agree at an earlier discussion that the effort (in terms of cost, changes in manufacturing, etc.), required to comply would not be commensurate with a small incremental safety gain. This would be clear even without the need to perform any detailed cost/safety benefit analysis (although cost analysis could always be used to support an appropriate amendment level). However, there should be enough detail in the applicant’s rationale to justify the exception. Note: The impractical exception should not be based on the size of the applicant’s company or their financial resources. Costs to comply with a later amendment must be evaluated against the safety benefit of complying with the later amendment. Applicants that may not be able to afford the cost because of reasons such as fewer resources, will not be granted the impractical exception when the cost is comparable to the safety benefit achieved by complying with a later amendment. (b) For example, a complex redesign of an area of the baseline aircraft may be required to comply with a new requirement, and that redesign may make the changed product uncommon with respect to design and manufacturing processes from the existing family of derivatives. Relevant service experience of the existing fleet of the baseline aircraft family would be required to show that there has not been a history of problems associated with the hazard that the new amendment in question was meant to address. In this way, the incremental cost/impact to the applicant is onerous and the incremental safety benefit that would be realized by complying with the later amendment would be minimal. This would be justified with a demonstrated acceptable service experience in relation to the hazard that the new rule addresses. 11. Step 8 of Figure 1. Is the Proposed Type Certification Adequate? a. Regardless of whether the change is significant or not, your proposed type certification basis may be deemed inadequate – that is, the change includes features or characteristics that were ### Page 24 21 not foreseen during the initial (or previously approved) type certification. These features or characteristics, if not adequately addressed, may make the product unsafe for the uses for which certification is requested. This would obstruct issuance of the requested approval for the change. The change must comply with later regulations (such as, a later amendment or a special condition). An example is adding a flight critical system such as an electronic air data display on a 14 CFR part 25 airplane whose existing certification basis did not have lightning protection requirements. In this case, compliance with the regulations for lightning protection will be required, even though this is not a significant change. b. In cases where inadequate or no airworthiness standards exist for the change in the existing type certification basis, but adequate standards exist in a subsequent amendment of the applicable airworthiness code, the subsequent amendment will be made part of the type certification basis to assure its certification basis is adequate. c. In cases where no adequate standard exist in any subsequent amendment of the applicable airworthiness code because of novel or unusual design features, the FAA will prescribe special conditions under 14 CFR § 21.16. Title 14 CFR § 21.101(d) allows for the application of special conditions, or for changes to existing special conditions, to address the changed designs where the proposed certification basis does not provide adequate standards for an area, system, component, equipment or appliance related to the change. Reference section 3 of chapter 4 for additional information pertaining to special conditions. d. For new design features or characteristics which may pose a potential unsafe condition for which there are not later applicable airworthiness requirements, new requirements may be imposed per 14 CFR § 21.21(b)(2). e. The final type certification basis may consist of a combination of the certification standards, ranging from the original aircraft type certification basis to the most current regulatory amendments, and special conditions. ### Page 25 Chapter 4. Other Considerations 1. Design Related Operating Requirements. The use of exceptions under 14 CFR § 21.101 is not intended to alleviate or preclude compliance with operating regulations (such as 14 CFR part 121) that prescribes compliance with a specific or later amendment of the airworthiness (design-related) requirements. 2. Excepted Products under 14 CFR § 21.101(c). a. As an applicant for a design change to an excepted product, you may show that the changed product complies with the existing certification basis incorporated by reference in the TC. If we, the FAA, find that the change is significant “in an area”, we will require compliance with a later amendment to the existing certification basis in the TC that applies to that affected area and any regulation we find is directly related. For excepted products, changes that meet one of the following criteria, in the area of change, are considered significant if:  The general configuration or the principles of construction are not retained, or  The assumptions used for certification of the area to be changed do not remain valid. b. However, we may allow you to comply with an earlier amendment to the requirement initially designated or with the existing certification basis if we agree to your justification. c. For a design change to an excepted product that contains new features, which are not covered in the existing certification basis, we will designate the applicable airworthiness requirements at the appropriate amendment level, beginning with the existing certification basis and progressing to the most appropriate later amendment level for the change. For a change that contains new design features that are novel and unusual for which there are no later applicable requirements at a later amendment level, we will designate special conditions per 14 CFR § 21.101(d). d. The exception provided for excepted products under 14 CFR § 21.101(c) applies at the aircraft level only. Design changes to type certificated engines and propellers installed on these excepted aircraft are assessed as separate products using 14 CFR §§ 21.101(a) and (b). 3. Special Conditions, 14 CFR § 21.101(d). Title 14 CFR § 21.101(d) allows for the application of special conditions, or for changes to existing special conditions, to address the changed designs where the proposed certification basis does not provide adequate standards for an area, system, component, equipment, or appliance related to the change. The objective is to achieve a level of safety consistent with that provided for other areas, systems, components, equipment, or appliances affected by the change by the other requirements of the proposed certification basis. The application of special conditions to a design change is not, in itself, a reason for it to be classified as either a substantial change or a significant change. When the change is significant with earlier requirements allowed through exceptions, or not significant, the ### Page 26 23 level of safety intended by the special conditions must be consistent with the agreed certification basis. 4. Effective Period for an Application to Change a TC. (14 CFR § 21.101(e)) Per 14 CFR § 21.101(e), an application for, or a change to, a TC for transport category aircraft is effective for 5 years, and an application for a change to any other TC is effective for 3 years. This is intended to ensure that the certification basis for the changed product is as current as practical. This is consistent with the requirements of 14 CFR § 21.17 for a new TC and defines the process of updating the certification basis if these time limits are exceeded. a. If a design change has not been approved, or if it is clear that the change will not be approved, within the time limit, you may do either of the following: (1) File for a new application, or (2) File for an extension to the original application. b. If you request an extension to the application date, and the product change is significant, a new certification basis is required. The new certification basis requires the additional latest regulations effective through the new application extension date. However, you may use earlier regulations by documenting justification that the latest regulations for the change would not contribute materially to the level of safety or would be impractical. c. If the product change is not significant, the existing certification basis can continue to be used as the basis for product certification. However, if additional design changes are made to the product, and the existing certification basis for the change is found to be inadequate, the new certification basis will require later appropriate standards. 5. Other Category Aircraft (14 CFR § 21.101(f)). For aircraft type certificated under 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27, the certification basis for the changed product will consist of the amendment levels of the applicable regulations that we find appropriate for the change in effect on the date of application for the change. When selecting a certification basis for a change, you can propose compliance to an earlier amendment using the provisions of 14 CFR § 21.101(b). The exceptions in 14 CFR § 21.101(c) do not apply to categories of products defined in 14 CFR § 21.101(f). a. Special Classes Aircraft. For special classes of aircraft (for example, gliders, airships, etc.) including any installed engines and propellers certificated in accordance with 14 CFR § 21.17(b), the applicable requirements will be portions of those other airworthiness requirements in 14 CFR parts 23, 25, 27, 29, 31, 33, and 35 we find appropriate for the aircraft and applicable to the specific type design, or such airworthiness criteria that provide an equivalent level of safety to those parts. b. Primary Category Aircraft. For primary category aircraft certificated under 14 CFR § 21.24, the applicable airworthiness requirements are in 14 CFR parts 23, 27, 31, 33, and 35, or such other requirements as we may find appropriate. These requirements must be ### Page 27 24 applicable to the specific design and intended use of the aircraft and provide a level of safety acceptable to us. c. Restricted Category Aircraft – Civil Derived. For aircraft certificated in the restricted category under 14 CFR § 21.25(a)(1), the application of the latest regulations typically would be considered not to contribute materially to the level of safety or be practical for its intended use. However, if the regulations incorporated by reference in the TC do not provide an adequate level of safety for its intended use, the application of a later regulation will be required. (1) Features of the changed product that ar e “novel” or “unusual” to the original certificated restricted category product may be assessed against a later requirement that addresses the feature. In this case, the requirements in effect at the time of the existing restricted category TC may be viewed as a starting point, with subsequent amendments being examined, if necessary, to arrive at a requirement that provides an appropriate level of safety. (2) For the installation of turbo propeller engines instead of reciprocating engines, either in a restricted category aircraft that was originally certificated based on satisfactory military service experience or in a restricted category aircraft for which the original certification basis did not contain regulations for turbine engine installations, later amendments will be used to provide an appropriate level of safety for its intended operation. d. Restricted Category Aircraft - Military Derived. Aircraft type certificated in the restricted category under 14 CFR § 21.25(a)(2) are accepted on the basis of U.S. Military use instead of showing compliance with airworthiness standards in 14 CFR Chapter 1. Many of these aircraft were not certificated to a specific set of airworthiness standards, therefore, an appropriate equivalent civilian certification basis could be determined using the table in 14 CFR § 21.27 for surplus military aircraft. Title 14 CFR § 21.101(f) requires the application of the latest amendments to significant changes to these products. However, since the latest amendments may not be appropriate for the aircraft’s intended use, earlier regulations are acceptable. They may not predate the equivalent certification basis. If these regulations do not include design standards applicable to the change, later regulations appropriate to the product category will be applied. The goal is to maintain a level of safety appropriate for the aircraft’s intended use. e. Surplus military aircraft. Aircraft type certificated under 14 CFR § 21.27 are entitled to a TC in the normal, utility, acrobatic, commuter, or transport category, except those certified under 21.27(b). These aircraft were designed and constructed in the United States, accepted for operational use, and declared surplus by the U.S. Armed Forces. These aircraft may be counterparts, and are considered equivalent, to the previously civil certificated aircraft. Product changes or modifications to these aircraft, including their engines, propellers, and related accessories, are certificated in the same manner as their civil counterparts. f. Limited category aircraft. Limited category aircraft are surplus military aircraft, mostly from World War II, that were type certificated under Civil Aviation Regulation (CAR) part 9 for use other than air transport. These aircraft were not intended to carry persons or property for hire, and normally were accepted based on their previous military qualifications and ### Page 28 25 service record. A change to aircraft not supported by the military service history must comply with appropriate airworthiness standards. The level of safety associated with earlier standards may be acceptable for limited category aircraft. 6. Clarification of 14 CFR 21.101 (g) (Part 26 Requirements). a. Title 14 CFR part 26 establishes requirements for support of continued airworthiness of and safety for transport category airplanes. If you are an applicant, you must comply with each applicable provision of 14 CFR part 26, unless you have elected or were required to comply with a corresponding amendment to 14 CFR part 25 that was issued on or after the date of the applicable part 26 provision. Title 14 CFR § 21.101(g) does not allow you to use an exception under 14 CFR § 21.101(b) for relief from complying with the applicable provisions of 14 CFR part 26. b. The language in 14 CFR § 21.101(g) also recognizes that future 14 CFR part 25 amendments will be issued after the requirements in 14 CFR part 26 are established. Note that you could be required to comply with a later amendment to 14 CFR part 25 per 14 CFR § 21.101(a). However, earlier part 25 requirements, but not earlier than those established in the existing certification basis that include part 26 requirements, could be justified using the exceptions in 14 CFR § 21.101(b). 7. Documentation. All changes that result in a revision to the product’s certification basis must be reflected on the amended TCDS or STC. The resulting certification basis must be retained as it forms part of the compliance record required by FAA Order 8110.4, Type Certification. ### Page 29 Appendix A A-1 Appendix A. Classification of Changes The following examples of substantial, significant and not significant changes are adopted by the FAA, European Aviation Safety Agency (EASA) and Transport Canada Civil Aviation (TCCA) through an international collaboration. The classification may change due to cumulative effects and/or combinations of individual changes. The “NA” in the substantial example tables indicates that the automatic classification criteria in the heading are “Not Applicable” at the “21.19 Substantial Evaluation” phase. Table 1. Examples of Changes for Small Airplanes (Part 23) The following examples are for SUBSTANTIAL changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in wing location (tandem, forward, canard, high/low) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Fixed wing to tilt wing NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Increase or decrease in the number of engines NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 30 Appendix A A-2 The following examples are for SUBSTANTIAL changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Replacement of piston or turbo-prop engines with turbojet or turbofan engines NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change in engine configuration (tractor/pusher) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Increase from subsonic to supersonic flight regime NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change from an all metal airplane to all composite primary structure (fuselage, wing, empennage) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 31 Appendix A A-3 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Conventional tail to T-tail or V-tail, or vice versa Yes No Yes Change in general configuration. Requires extensive structural, flying qualities and performance reinvestigation. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Changes in wing configuration such as change in dihedral, changes in wing span, flap or aileron span, addition of winglets, or increase of more than 10 percent of the original wing sweep at the quarter chord Yes No Yes Change in general configuration. Likely requires extensive changes to wing structure. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Note: Small changes to wingtip are not significant changes. See table for not significant changes. Changes to tail configuration such as the addition of tail strakes or angle of incidence of the tail Yes No Yes Change in general configuration. Likely requires extensive changes to tail structure. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Note: Small changes to tail are not significant changes. ### Page 32 Appendix A A-4 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Tricycle/tail wheel undercarriage change or addition of floats Yes No No Change in general configuration at product level. Principles of construction and certification assumptions remain valid. Passenger to freighter configuration conversion which involves the introduction of a cargo door or an increase in floor loading of more than 20 percent, or provision for carriage of passengers and freight together. Yes No Yes Change in general configuration affecting load paths, aeroelastic characteristics, aircraft related systems, etc. Change in design assumptions. Replace reciprocating engines with the same number of turbo-propeller engines where the operating envelope is expanded. No No Yes Invalidates certification assumptions. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Addition of a turbocharger that changes the power envelope, operating range, or limitations. No No Yes Invalidates certification assumptions due to changes in operating envelope and limitations. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. ### Page 33 Appendix A A-5 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes The replacement of an engine of higher rated power or increase thrust would be considered significant if it would invalidate the existing substantiation, or would change the primary structure, aerodynamics, or operating envelope sufficiently to invalidate the assumptions of certification. No Yes Yes Invalidates certification assumptions. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Likely changes to primary structure. Requires extensive construction reinvestigation. A change in the type of material, such as composites in place of metal, or one composite fiber material system with another (e.g., carbon for fiberglass), for primary structure would normally be assessed as a significant change. No Yes Yes Change in principles of construction and design from conventional practices. Likely change in design/certification assumptions. Change involving appreciable increase in design speeds Vd, Vmo, Vc, or Va No No Yes Certification assumptions invalidated. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. ### Page 34 Appendix A A-6 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Short take off and landing (STOL) kit. No No Yes Certification assumptions invalidated. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. A change in the rated power or thrust is likely to be regarded as significant if the design speeds are thereby changed so that compliance needs to be rejustified with a majority of requirements. No No Yes Certification assumptions invalidated. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. Fuel state: such as compressed gaseous fuels, or fuel cells. This could completely alter the fuel storage and handling systems and possibly affect the airplane structure. No No Yes Changes in design/certification assumptions. Extensive alteration of fuel storage and handling systems. ### Page 35 Appendix A A-7 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes A design change that alters the aircraft flight characteristics or performance from the type design would normally be significant if it appreciably changes the kinematics or dynamics of the airplane. No No Yes Certification assumptions invalidated. Requires a new aircraft flight manual (AFM) to address performance and flight characteristics. A change in the flight control concept for an aircraft, for example, to fly by wire (FBW) and sidestick control, or a change from hydraulic to electronically actuated flight controls, would in isolation normally be regarded as a significant change. No No Yes Changes in design and certification assumptions. Requires extensive systems architecture and integration reinvestigation. Requires a new aircraft flight manual (AFM). Change to airplane’s cabin operating altitude, or operating pressure No No Yes An increase greater than 10% in maximum cabin pressure differential invalidates certification assumptions and the fundamental approach used in decompression, structural strength, and fatigue. ### Page 36 Appendix A A-8 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Increase in cabin pressurization. No No Yes Typically, a change greater than 10% in operational cabin pressure differential. May require extensive airframe changes affecting load paths, fatigue evaluation, aeroelastic characteristics, etc. Invalidates design assumptions. Addition of cabin pressurization system. No Yes Yes Extensive airframe changes affecting load paths, fatigue evaluation, aeroelastic characteristics, etc. Invalidates design assumptions. Changes in types and number of emergency exits or an increase in maximum certificated passenger capacity. Yes No Yes Emergency egress requirements exceed those previously substantiated. Invalidates assumptions of certification. ### Page 37 Appendix A A-9 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes A change in the required number of flight crew, which necessitates a complete cockpit rearrangement, and/or an increase in pilot workload would be a significant change. No No Yes Extensive changes to avionics and aircraft systems. Invalidates certification assumptions. Requires a new aircraft flight manual (AFM). Expansion of an aircraft’s operating envelope. No No Yes An appreciable expansion of operating capability would normally be a significant change (e.g., an increase in maximum altitude limitation, approval for flight in known icing conditions, or an increase in airspeed limitations). Merely operating a product to an expanded envelope for which it was originally designed is generally not a significant change. In this case, the assumptions used for certification of the basic product remain valid and the results can be applied to cover the changed product with predictable effects or can be demonstrated without significant. ### Page 38 Appendix A A-10 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Replacement of an aviation gasoline engine with an engine of approximately the same horsepower utilizing diesel fuel. No No Yes A major change to the airplane. The general configuration and principles of constructions will usually remain valid; however, the assumptions for certification are invalidated. Comprehensive flight deck upgrade, such as conversion from entirely federated, independent electromechanical flight instruments to highly integrated and combined electronic display systems with extensive use of software and/or complex electronic hardware. No No Yes Affects avionics and electrical systems integration and architecture concepts and philosophies. Introduction of autoland No No Yes Invalidates original design assumptions ### Page 39 Appendix A A-11 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Airframe life extension No No Yes This modification pertains to fuselage and/or wing limits, and aging airplane concerns. An increase from the original life limit which constitutes a reevaluation of certification design assumptions. Extensive structural airframe modification, such as a large opening in fuselage Yes No No Requires extensive changes to fuselage structure, affects aircraft systems, and requires a new airplane flight manual to address performance and flight characteristics. ### Page 40 Appendix A A-12 The following examples are for SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Fuselage stretch or shortening in the cabin or pressure vessel Yes No Yes Cabin interior changes are related changes since occupant safety considerations are impacted by a cabin length change. Even if a new cabin interior is not included in the product level change, the functional effect of the fuselage plug has implications on occupant safety (e.g., the dynamic environment in an emergency landing, emergency evacuation, etc.), and thus the existing cabin interior becomes an affected area. Conversion from normal category to commuter category airplane Yes No Yes In many cases this change could be considered to be a substantial change to the type design. Therefore, a proposed change of this nature would be subject to an FAA determination under 14 CFR § 21.19. ### Page 41 Appendix A A-13 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Addition of wingtip modifications (not winglets) No No No A major change to the airplane. Likely the original general configuration, principles of construction, and certification assumptions remain valid. Installation of skis or wheel skis No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. FLIR or surveillance camera installation No No No Additional flight or structural evaluation may be necessary, but the change does not alter basic airplane certification. Litter, berth and cargo tie down device installation No No No Increased tire size, including tundra tires No No No Replacement of one propeller type with another (irrespective of increase in number of blades) No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. ### Page 42 Appendix A A-14 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Addition of a turbocharger that does not change the power envelope, operating range, or limitations (e.g., a turbonormalized engine, where the additional power is used to enhance high altitude or hot day performance) No No No Substitution of one method of bonding for another (e.g., change in type of adhesive) No No No Substitution of one type of metal for another No No No Any change in construction or fastening not involving primary structure No No No A new fabric type for fabric skinned aircraft No No No ### Page 43 Appendix A A-15 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Increase in flap speed or undercarriage limit speed No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Structural strength increases No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Instrument flight rules (IFR) upgrades involving installation of components (where the original certification does not indicate that the airplane is not suitable as an IFR platform, e.g., special handling concerns) No No No ### Page 44 Appendix A A-16 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Fuel lines, where engine horsepower is increased but fuel flow is not increased beyond the certificated maximum amount No No No Fuel tanks, where fuel is changed from gasoline to diesel fuel and tank support loads are small enough that an extrapolation from the previous analysis would be valid. Chemical compatibility would have to be substantiated. No No No Limited changes in a pressurization system, e.g., number of outflow valves, type of controller, or size of pressurized compartment, but the system must be resubstantiated if the original test data are invalidated. No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. ### Page 45 Appendix A A-17 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Install a quieter exhaust system No No No Changes in engine cooling or cowling No No No Changing fuels of substantially the same type: such as AvGas to AutoGas, AvGas (80/87) to AvGas (100LL), ethanol to isopropyl alcohol, Jet B to Jet A (although Jet A to Jet B may be considered significant due to the fact that Jet B is considered potentially more explosive). No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Fuels that specify different levels of “conventional” fuel additives that do not change the primary fuel type. Different additives (MTBE, ETBE, ethanol, amines, etc.) in AvGas would not be considered a significant change. No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. ### Page 46 Appendix A A-18 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes A change to the maximum take-off weight of less than 5 percent unless assumptions made in justification of the design are thereby invalidated. No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. (Unless this weight increase would result in a shift to commuter category.) An additional aileron tab (e.g., on the other wing) No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Larger diameter flight control cables with no change in routing, or other system design No No No Autopilot installation (for instrument flight rules (IFR) use, unless the original certification indicates that the airplane is not suitable as an IFR platform) No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Increased battery capacity or relocate battery No No No ### Page 47 Appendix A A-19 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Replace generator with alternator No No No Additional lighting (e.g., navigation lights, strobes) No No No Higher capacity brake assemblies No No No Increase in fuel tank capacity No No No Not a product level change, unless it is tied with an increase in gross weight. Addition of an oxygen system No No No Relocation of a galley No No No Passenger to freight (only) conversion with no change to basic fuselage structure No No No Although a major change to the airplane, likely the original general configuration, principles of construction, and certification assumptions remain valid. Requires certification substantiation applicable to freighter requirements. New cabin interior with no fuselage length change No No No ### Page 48 Appendix A A-20 The following examples are for NOT SIGNIFICANT changes for Small Airplanes (Part 23): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Installation of new seat belt or shoulder harness No No No A small increase in c.g. range No No No At a product level, no change in general configuration, principles of construction, and certification assumptions. APU installation that is not flight essential No No No Although a major change to the airplane level, likely the original general configuration, principles of construction, and certification assumptions remain valid. An alternative autopilot No No No Addition of Class B terrain awareness and warning systems (TAWS) No No No ### Page 49 Appendix A A-21 Table 2. Examples of Changes for Transport Airplanes (Part 25) The following examples are for SUBSTANTIAL changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in the number or location of engines, e.g., four to two wing-mounted engines or two wingmounted to two body-mounted engines NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change from a highwing to low-wing configuration NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change from an all metal airplane to all composite primary structure (fuselage, wing, empennage) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change of empennage configuration for larger airplanes (cruciform vs. ‘T’ or ‘V’ tail) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 50 Appendix A A-22 The following examples are for SUBSTANTIAL changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Increase from subsonic to supersonic flight regime NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 51 Appendix A A-23 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Reduction in the number of flight crew (In conjunction with flight deck update) No No Yes Extensive changes to avionics and aircraft systems. Impact to crew workload and human factors, pilot type rating. Modify an airplane for flight in known icing conditions by adding systems for ice detection and elimination Yes No Yes New aircraft operating envelope. Requires major new systems installation and aircraft evaluation. Operating envelope changed. Conversion – passenger or combination freighter/passenger to all freighter, including cargo door, redesign floor structure and 9g net or rigid barrier Yes No Yes Extensive airframe changes affecting load paths, aeroelastic characteristics, aircraft related systems for fire protection, etc. Design assumptions changed from passenger to freighter. Increase in cabin pressurization No No Yes Typically, a change greater than 10% in operational cabin pressure differential. May require extensive airframe changes affecting load paths, fatigue evaluation, aeroelastic characteristics, etc. Invalidates design assumptions. ### Page 52 Appendix A A-24 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Addition of leading edge slats Yes No No Requires extensive changes to wing structure, adds aircraft systems, and requires a new airplane flight manual to address performance and flight characteristics. Fuselage stretch or shortening in the cabin or pressure vessel Yes No Yes Cabin interior changes are related changes since occupant safety considerations are impacted by a cabin length change. Even if a new cabin interior is not included in the product level change, the functional effect of the fuselage plug has implications on occupant safety (e.g., the dynamic environment in an emergency landing, emergency evacuation, etc.), and thus the cabin interior becomes an affected area. ### Page 53 Appendix A A-25 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Extensive structural airframe modification, such as installation of a large telescope with large opening in fuselage Yes No No Requires extensive changes to fuselage structure, affects aircraft systems, and requires a new airplane flight manual to address performance and flight characteristics. Changing the number of axles or number of landing gear done in context with a product change that involves changing the airplane gross weight Yes No No Requires extensive changes to aircraft structure, affects aircraft systems, and requires new aircraft flight manual changes. Primary structure changes from metallic material to composite material No Yes No Change in principles of construction and design from conventional practices. Airframe life extension No No Yes This modification pertains to fuselage and/or wing limits, and aging airplane concerns. It results in an increase from the original life limit which constitutes a reevaluation of certification design assumptions. ### Page 54 Appendix A A-26 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Typically, an increase in design weight of more than 10 percent No No Yes Requires extensive resubstantiation of aircraft structure, aircraft performance and flying qualities and associated systems. Installation of winglets Yes No Yes Wing changes in span, sweep, tip designs or wing chord Yes No Yes When it requires extensive changes to wing structure, adds aircraft systems, and requires a new airplane flight manual to address performance and flight characteristics. (Note: Potentially substantial if it is a change from a high wing to a low wing, or a new wing.) Change in type or number of emergency exits or an increase in the maximum certificated number of passengers Yes No Yes The new emergency egress requirements exceed those previously substantiated. ### Page 55 Appendix A A-27 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Comprehensive flight deck upgrade, such as conversion from entirely federated, independent electromechanical flight instruments to highly integrated and combined electronic display systems with extensive use of software and/or complex electronic hardware. No No Yes Affects avionics and electrical systems integration and architecture concepts and philosophies. Change in primary flight controls to fly by wire (FBW) system. (Some airplanes have some degree of FBW. Achieving full FBW may be a not significant change on some airplanes.) No No Yes When the degree of change is so extensive that it affects basic aircraft systems integration and architecture concepts and philosophies. This drives a complete reassessment of flight crew workload, handling qualities, and performance evaluation, which are different from the original design assumptions. ### Page 56 Appendix A A-28 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Replace reciprocating with turbo-propeller engines Yes No No Requires extensive changes to airframe structure, addition of aircraft systems, and new airplane flight manual to address performance and flight characteristics. Typically a thrust increase of more than 10 percent No No Yes Requires resubstantiation of powerplant installation, and has a marked affect on aircraft performance and flying qualities. Initial installation of an autoland system No No Yes Baseline airplane not designed for autoland operation, potential crew workload and systems compatibility issues. Installation of a new fuel tank, (horizontal stabilizer tank or auxiliary fuel tank in the fuselage outside the wing in conjunction with increased maximum takeoff weight and takeoff thrust) No No Yes Requires changes to airframe, systems and Requires a new aircraft flight manual (AFM). Results in performance changes. ### Page 57 Appendix A A-29 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Main deck cargo door installation Yes No No Redistribution of internal loads, change in aeroelastic characteristics, system changes. Expansion of an aircraft’s operating envelope No No Yes An appreciable expansion of operating capability would normally be a significant change (e.g., an increase in maximum altitude limitation, approval for flight in known icing conditions, or an increase in airspeed limitations). Merely operating a product to an expanded envelope for which it was originally designed is generally not a significant change. In this case, the assumptions used for certification of the basic product remain valid and the results can be applied to cover the changed product with predictable effects or can be demonstrated without significant physical changes to the product. ### Page 58 Appendix A A-30 The following examples are for SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Conversion from a passenger floor to a cargo floor and installation of a cargo handling system No No Yes Completely new floor loading and design. Redistribution of internal loads, change in cabin safety requirements, system changes. Initial installation of an auxiliary power unit (APU) essential for aircraft flight operation No No Yes Changes emergency electrical power requirements, change in flight manual and operating characteristics. Conversion from hydraulically actuated brakes to electrically actuated brakes No No Yes Assumptions of certification for airplane performance are changed. Change to airplane’s cabin operating altitude, or operating pressure No No Yes An increase greater than 10% in maximum cabin pressure differential invalidates certification assumptions and the fundamental approach used in decompression, structural strength, and fatigue. Installation of engine thrust reversers Yes No Yes ### Page 59 Appendix A A-31 The following examples are for NOT SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Alternate engine installation or hush kit at same position No No No Typically it is not significant so long as there is not more than a 10 percent increase in thrust or a change in the principles of propulsion. A small change in fuselage length due to refairing the aft body or radome No No No For cruise performance reasons, where such changes do not require extensive structural, systems, aerodynamic, or aircraft flight manual (AFM) changes. Refairing of wing tip caps (for lights, fuel dump pipes) and addition of splitter plates to the trailing edge thickness of the cruise airfoil No No No Does not require extensive structural, AFM, or systems changes. Additional power used to enhance high altitude or hot day performance No No No Usually no change in basic operating envelope. Existing certification data can be extrapolated. Could be significant product change if the additional power is provided by installation of a rocket motor or additional, on demand engine due to changes in certification assumptions. ### Page 60 Appendix A A-32 The following examples are for NOT SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Installation of an autopilot system No No See note It may be possible that the modification is adaptive in nature, with no change to original certification assumptions. However, in certain cases the installation of an autopilot may include extensive changes and design features which change both the general configuration and the assumptions for certification (i.e., installation of the autopilot may introduce a number of additional mechanical and electronic failure modes and change the hazard classification of given aircraft level failures). Change from assembled primary structure to monolithic or integrally machined structure No No No Method of construction must be well understood. Modification to ice protection systems No No No Recertification required, but certification basis is adequate. Brakes: design or material change, e.g., steel to carbon No No No Recertification required, but certification basis is adequate. ### Page 61 Appendix A A-33 The following examples are for NOT SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Redesign floor structure No No No By itself, not a significant product change. It is significant if part of a cargo conversion of a passenger airplane. New cabin interior with no fuselage length change No No No A new cabin interior includes new ceiling and sidewall panels, stowage, galleys, lavatories, and seats. New and novel features in the cabin interior may require special conditions. Many interior related requirements are incorporated in operational rules. Even though the design approval holder may not be required to comply with these requirements, the operator may be required to comply. A rearrangement of an interior (e.g. seats, galleys, lavatories, closets, etc) No No No Rearrangement requires the use of the existing floor structure. Novel or unusual method of construction of a component No No No The component change does not rise to the product level. Special conditions could be required if there are no existing regulations that adequately address these features. ### Page 62 Appendix A A-34 The following examples are for NOT SIGNIFICANT changes for Transport Airplanes (Part 25): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Initial installation of a non-essential auxiliary power unit (APU) No No No A stand-alone initial APU installation on an airplane originally designed to use ground/airport supplied electricity, and airconditioning. In this case, the APU would be an option to be independent of airport power. A rearrangement of an interior (e.g. seats, galleys, lavatories, closets, etc) No No No Requires the use of the existing mounting structure. ### Page 63 Appendix A A-35 Table 3. Examples of Changes for Rotorcraft (Parts 27 and 29) The following examples are for SUBSTANTIAL changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change from the number and or configuration of rotors (e.g., main & tail rotor system to two main rotors) NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Change from an all metal rotorcraft to all composite rotorcraft NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 64 Appendix A A-36 The following examples are for SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Comprehensive flight deck upgrade, such as conversion from entirely federated, independent electromechanical flight instruments to highly integrated and combined electronic display systems with extensive use of software and/or complex electronic hardware. No No Yes Affects avionics and electrical systems integration and architecture concepts and philosophies. Certification for flight into known icing conditions No No Yes (Fixed) flying controls from mechanical to fly by wire No No Yes This drives a complete reassessment of the rotorcraft controllability and flight control failure. ### Page 65 Appendix A A-37 The following examples are for SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Addition of an engine; e.g., from single to twin or reduction of the number of engines; e.g., from twin to single Yes Yes Yes May be a substantial change depending upon project details. A change of rotor drive system primary gearbox splash type lubrication system to a pressure lubricated system due to an increase in horsepower of an engine or changing a piston engine to a turbine engine No Yes Yes A fuselage or tail boom modification that changes the primary structure, aerodynamics, and operating envelope sufficiently to invalidate the certification assumptions Yes No Yes ### Page 66 Appendix A A-38 The following examples are for SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Application of an approved primary structure to a different approved model (e.g., installation on a former model of the main rotor approved on a new model that results in increase performance) No Yes Yes Extensive primary structure changes from metallic material to composite material No Yes Yes Change in principles of construction and assumptions used for certification for the product level change. Changes of a few individual elements from metal to composite are not typically considered a significant change. Emergency medical service (EMS) configuration with primary structural changes sufficient to invalidate the certification assumptions. No No Yes Many EMS configurations will not be classified as significant. Modifications made for EMS are typically internal, and the general external configuration is normally not affected. These changes should not automatically be classified as significant. Skid landing gear to wheel landing gear or wheel landing to skid Yes No Yes ### Page 67 Appendix A A-39 The following examples are for SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change of the number of rotor blades Yes No Yes Change tail anti-torque device (e.g., tail rotor, ducted fan or other technology) Yes Yes No Passenger configured helicopter to a Firefighting equipment configured helicopter Yes No Yes Depends on the firefighting configuration. Passenger configured helicopter to a agricultural configured helicopter Yes No Yes Depends on the agricultural configuration. A new Category A certification approval to an existing configuration No No Yes Instrument flight rules (IFR) upgrades involving installation of upgraded components for new IFR configuration No No Yes ### Page 68 Appendix A A-40 The following examples are for SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Human external cargo (HEC) certification approval No No Yes Must comply with the latest HEC certification requirements in order to obtain operational approval. HEC include fatigue, quick release systems, high intensity radio frequency (HIRF), one engine inoperative (OEI) performance and OEI procedures. Reducing the number of pilots for instrument flight rules (IFR) from 2 to 1 No No Yes ### Page 69 Appendix A A-41 The following examples are for NOT SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Emergency floats No No No Must comply with the specific applicable requirements for emergency floats. This installation, in itself, does not change the rotorcraft configuration, overall performance, or operational capability. Expanding an operating envelope (such as operating altitude and temperature) and mission profile (such as passenger carrying operations to external load operations, or flight over water, or operations in snow conditions) are not by themselves so different that the original certification assumptions are no longer valid at the type certificated product level. FLIR or surveillance camera installation No No No Additional flight or structural evaluation may be necessary but the change does not alter the basic rotorcraft certification. Helicopter terrain awareness warning system (HTAWS) for operational credit No No No Certificated per rotorcraft HTAWS AC guidance material and FAA TSO-C194. Health usage monitoring system (HUMS) for maintenance credit. No No No Certificated per rotorcraft HUMS AC guidance material. ### Page 70 Appendix A A-42 The following examples are for NOT SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Expanded limitations with minimal or no design changes, following further tests/justifications or different mix of limitations (center of gravity (CG) limits, oil temperatures, altitude, minimum/maximum weight, minimum/max external temperatures, speed, ratings structure). No No No Expanding an operating envelope (such as operating altitude and temperature) and mission profile (such as passenger carrying operations to external load operations, or flight over water, or operations in snow conditions) are not by themselves so different that the original certification assumptions are no longer valid at the type certificated product level. Installation of a new engine type, equivalent to the former one; leaving aircraft installation and limitations substantially unchanged No No No Refer to AC 27-1 or AC 29-2 for guidance Windscreen installation No No No Does not change the rotorcraft overall product configuration. Snow skis, “Bear Paws” No No No Must comply with specific requirements associated with the change. Expanding an operating envelope (such as operating altitude and temperature) and mission profile (such as passenger carrying operations to external load operations, or flight over water, or operations in snow conditions) are not by themselves so different that the original certification assumptions are no longer valid at the type certificated product level. ### Page 71 Appendix A A-43 The following examples are for NOT SIGNIFICANT changes for Rotorcraft (Parts 27 and 29): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes External cargo hoist No No No Must comply with the specific applicable requirements for external loads. This installation, in itself, does not change the rotorcraft configuration, overall performance, or operational capability. Expanding an operating envelope (such as operating altitude and temperature) and mission profile (such as passenger carrying operations to external load operations, excluding human external cargo (HEC), or flight over water, or operations in snow conditions) are not by themselves so different that the original certification assumptions are no longer valid at the type certificated product level. Instrument flight rules (IFR) upgrades involving installation of upgraded components to replace existing components No No No Not a rotorcraft level change. ### Page 72 Appendix A A-44 Table 4. Examples of Changes for Engines (Parts 33) The following are examples of SUBSTANTIAL changes for Engines (Parts 33 ) Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Turbine Engines Traditional turbofan to geared-fan engine NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required Note: There may be certain circumstances where this change would be significant. Low bypass ratio engine to high bypass ratio engine with an increased inlet area NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Note: There may be certain circumstances where this change would be significant. Turbojet to Turbofan NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Note: There may be certain circumstances where this change would be significant. ### Page 73 Appendix A A-45 The following are examples of SUBSTANTIAL changes for Engines (Parts 33 ) Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Turbo-shaft to turbopropeller NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Note: There may be certain circumstances where this change would be significant. Conventional ducted fan to unducted fan NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. Turbine engine for subsonic operation to afterburning engine for supersonic operation NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 74 Appendix A A-46 The following are examples of SIGNIFICANT changes for Engines (Parts 33 ) Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Increase/decrease in the number of compressor/turbine stages with resultant change in approved operational limitations* (* exclude life limits) Yes No Yes Change is associated with other changes to the ratings and operating limitations; engine dynamic behavior, in terms of backbone bending, torque spike effects on casing, surge and stall characteristics, etc. New design fan blade and fan hub, or a bladed fan disk to a blisk, or a fan diameter change, that could not be retrofitted Yes No Yes Change is associated with other changes to the engine thrust, ratings, and operating limitations; engine dynamic behavior in terms of backbone bending; torque spike effects on casing; foreign object ingestion behavior; blade-out test and containment; burst model protection for the aircraft. If there is a diameter change, installation will be also affected. Hydro-mechanical control to electronic engine controls (FADEC/EEC) without hydro mechanical backup Yes No No Change in engine control configuration. Not interchangeable. Likely fundamental change to engine operation. ### Page 75 Appendix A A-47 The following are examples of SIGNIFICANT changes for Engines (Parts 33 ) Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes A change in the containment case from hard-wall to composite construction or vice versa, that could not be retrofitted without additional major changes to the engine or restricting the initial limitations or restrictions in the initial installation manual. No Yes No Change in methods of construction that have affected inherent strength, backbone bending, blade to case clearance retention, containment wave effect on installation, effect on burst model, torque spike effects. Replace gas generator (core, turbine/ compressor/combustor) with a different one associated with changes in approved operational limitations* (*exclude life limits) No No Yes Change is associated with other changes that would affect engine thrust/power and have affected the dynamic behavior of the engine. Assumptions used for certification may no longer be valid. Piston Engines Convert from mechanical to electronic control system Yes Yes No Change in engine configuration: Installation interface of engine changed. Changes to principles of construction: Digital controllers and sensors require new construction techniques and environmental testing. ### Page 76 Appendix A A-48 The following are examples of SIGNIFICANT changes for Engines (Parts 33 ) Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Add turbocharger that increases performance and changes in overall product Yes No Yes Change in general configuration: Installation interface of engine changed (exhaust system). Certification assumptions invalidated: Change in operating envelope and performance. Convert from aircooled cylinders to liquid cooled cylinders Yes No Yes Change to general configuration: Installation interface of engine changed (cooling lines from radiator, change to cooling baffles). Certification assumptions invalidated: Change in operating envelope and engine temperature requirements. Convert from sparkignition to compressionignition Yes No Yes Change in general configuration: Installation interface of engine changed (no mixture lever). Certification assumptions invalidated: Change in operating envelope and performance. ### Page 77 Appendix A A-49 The following are examples of NOT SIGNIFICANT changes for Engines (Parts 33 ): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Turbine Engines Change in the material from one type of metal to another type of metal of a compressor drum No No No No change in performance. Assumptions are still valid. Increase/decrease in the number of compressor/turbine stages without resultant change in operational performance envelope No No No No change in performance. Assumptions are still valid. New components internal to the electronic engine controls (FADEC/EEC) the introduction of which does not change the function of the system No No No No change in configuration. Retrofitable Assumptions used for certification are still valid. Possible changes in principles of construction are insignificant. Software changes No No No Rub-strip design changes No No No A new combustor that does not change the approved limitations, or dynamic behavior* (*exclude life limits) No No No Bearing changes No No No New blade designs with similar material that can be retrofitted No No No Fan blade redesign that can be retrofitted No No No ### Page 78 Appendix A A-50 The following are examples of NOT SIGNIFICANT changes for Engines (Parts 33 ): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Oil tank redesign No No No Change from one hydro-mechanical control to another hydro-mechanical control No No No Change to limits on life limited components No No No Changes to limits on exhaust gas temperature No No No Changes in certification maintenance requirements (CMR) with no configuration changes No No No Bump ratings within the product’s physical capabilities that may be enhanced with gas path changes such as blade restaggered, cooling hole patterns, blade coating changes, etc No No No ### Page 79 Appendix A A-51 The following are examples of NOT SIGNIFICANT changes for Engines (Parts 33 ): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes A change in principal physical properties and mechanics of load transfer of a material of primary structure or highly loaded components. For example, change from traditional metal to either an exotic alloy or a composite material on a highly loaded component. No No No Piston Engine A change in principal physical properties and mechanics of load transfer of a material of primary structure or highly loaded components. For example, change from traditional metal to either an exotic alloy or a composite material on a highly loaded component. No No No New or redesigned cylinder head, or valves, or pistons No No No Changes in crankshaft No No No Changes in crankcase No No No Changes in carburetor No No No ### Page 80 Appendix A A-52 The following are examples of NOT SIGNIFICANT changes for Engines (Parts 33 ): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Changes in mechanical fuel injection system No No No Changes in mechanical fuel injection pump No No No Engine model change to accommodate new airplane installation. No change in principles of operation of major subsystems; no significant expansion in power or operating envelopes or in limitations. No No No No change in basic principles of operation, or a simple mechanical change. For example, change from dual magneto to two single magnetos on a model. No No No Subsystem change produces no changes in base engine input parameters, and previous analysis can be reliably extended. For example, a change in turbocharger where induction system inlet conditions remain unchanged, or if changed, the effects can be reliably extrapolated. No No No ### Page 81 Appendix A A-53 The following are examples of NOT SIGNIFICANT changes for Engines (Parts 33 ): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in material of secondary structure or not highly loaded component. For example, a change from metal to composite material in a non-highly loaded component, such as an oil pan that is not used as a mount pad. No No No Change in material that retains the physical properties and mechanics of load transfer. For example, a change in trace elements in a metal casting for ease of pouring or to update to a newer or more readily available alloy with similar mechanical properties. No No No ### Page 82 Appendix A A-54 Table 4. Examples of Changes for Propellers (Parts 35) The following are examples of SUBSTANTIAL changes for Propellers (Parts 35): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in the number of blades NA NA NA Proposed change in design is so extensive that a substantially complete investigation of compliance with the applicable regulations is required. ### Page 83 Appendix A A-55 The following are examples of SIGNIFICANT changes for Propellers (Part 35): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Principle of pitch change such as a change from single acting to dual acting Yes Yes Yes Requires extensive modification of the pitch change system with the introduction of back-up systems. The inherent control system requires re-evaluation. Introduction of a different principle of blade retention such as a single row to a dual row bearing Yes Yes No Requires extensive modification of the propeller hub and blade structure. The inherent strength requires re-evaluation. A hub configuration change such as a split hub to a one piece hub Yes Yes No Requires extensive modification of the propeller hub structure. The inherent strength requires re-evaluation. Changing the method of mounting the propeller to the engine such as a spline to a flange mount Yes Yes No Requires extensive modification of the propeller hub structure. Note: Such a change could be considered notsignificant if implemented without a change in general configuration or principals of construction. ### Page 84 Appendix A A-56 The following are examples of SIGNIFICANT changes for Propellers (Part 35): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in hub material from steel to aluminum Yes Yes No Requires extensive modification of the propeller hub structure and change to method of blade retention. The inherent strength requires re-evaluation. Change in blade material from metal to composite Yes Yes Yes Requires extensive modification of the propeller blade structure and change to method of blade retention. Composite construction methods required. The inherent strength requires re-evaluation. Change from hydromechanical to electronic control Yes Yes Yes Electronic manufacturing and design methods required. Assumptions used for certification are no longer valid or were not addressed in the original certification, i.e., high intensity radio frequency (HIRF) and lightning protection, fault tolerance, software certification and other aspects. The propeller will require special conditions under 14 CFR § 21.16. ### Page 85 Appendix A A-57 The following are examples of NOT SIGNIFICANT changes for Propellers (Parts 35): Description of change Is there a change to the general configuration? 14 CFR § 21.101(b)(1)(i) Is there a change to the principles of construction? 14 CFR § 21.101(b)(1)(i) Have the assumptions used for certification been invalidated? 14 CFR § 21.101(b)(1)(ii) Notes Change in the material of a blade bearing No No No Change to a component in the control system No No No Change to a propeller de-icer boot No No No Changes to the operational design envelope such as an increase in power No No No Propeller's operating characteristics and inherent strength require re-evaluation. Change to the intended usage such as normal to acrobatic category No No No Propeller's operating characteristics and inherent strength require re-evaluation. ### Page 86 Appendix B B-1 Appendix B. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product 1. Introduction. a. The basic principal of enhanc ing the level of safety of changed aeronautical products is to apply the latest regulations for significant design changes, to the greatest extent practical. In certain cases, the cost of complying fully with a later regulation may not be commensurate with the small safety benefit achieved. It is recognized that the existing fleet and newly produced airplanes, engines and propellers are safe, and any unsafe condition is immediately addressed through the airworthiness directive process. These factors form the basis where compliance with the latest standard may be considered impractical, thereby allowing compliance with an earlier regulation. This appendix gives one method of determining if compliance with a later regulation is impractical, however, this does not preclude the use of other methods for improving the safety of aeronautical products. b. This AC recognizes that other procedures can be used and have historically been accepted on a case-by-case basis. The acceptance of results through the use of these procedures may vary from state to state. Consequently, they may not be accepted through all bilateral certification processes. Regardless of which method is used, the process must show that a proposed certification basis is able to achieve a positive safety benefit for the overall product. c. In this regard, any method used must encourage incorporating safety enhancements that will have the most dramatic impact on the level of safety of the aircraft while considering effective use of resources. This important point is illustrated graphically in the accompanying figure. This figure notionally shows the interrelation between the total resources required for incorporating each potential safety enhancement with the corresponding net increase in safety benefit. Figure 2. Safety Benefits vs. Resources Safety benefit of the regulatory requirement Resources to implement the regulatory requirement Potential safety enhancements ### Page 87 Appendix B B-2 d. Typically, one will find that th ere are proposals that can achieve a positive safety benefit that are resource effective. Conversely, there are proposals that may achieve a small safety benefit at the expense of a large amount of resources to implement. Clearly, there will be a point where a large percentage of the potential safety benefit can be achieved with a reasonable expenditure of resources. The focus of the methods used should be to determine the most appropriate regulatory standards relative to the respective cost to reach this point. e. This appendix provides procedural guidance for determining the practicality of applying a requirement at a particular amendment level to a changed product. This guidance can be used to evaluate the safety benefit and resource impact of implementing the latest airworthiness requirements in the certification basis of a changed product. The procedure is generic in nature and describes the steps and necessary inputs that any applicant can use on any project to develop a position. f. The procedure is intended to be used , along with good engineering judgment, to evaluate the relative merits of a changed product complying with the latest regulations. It provides a means, but not the only means, for an applicant to present its position in regard to impracticality. g. The certification basis for a change to a product will not be at an amendment level earlier than the existing certification basis or any requirement found in 14 CFR §§ 23.2, 25.2, 27.2, 29.2 or part 26 that is related to the change. Therefore, when determining the impracticality of applying a requirement at the latest amendment level, only the increase in safety benefits and costs beyond compliance with the existing certification basis should be considered. 2. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product. The following are steps to determine the impracticality of applying a requirement at a particular amendment level. a. Step 1: Identify the Regulatory Change Being Evaluated. In this step, document: (1) The specific requirement (for example, 14 CFR § 25.365); (2) The amendment level of the existing certification basis for the requirement; and (3) The latest amendment level of the requirement. b. Step 2: Identify the Specific Ha zard that the Requirement Addresses. (1) Each requirement and subsequent amendments are intended to address a hazard or hazards. In this step, the specific hazard(s) is/are identified. This identification will allow for a comparison of the effectiveness of amendment levels of the regulation at addressing the hazard. (2) In many cases, the hazard and the cause of the hazard will be obvious. When the hazard and its related cause are not immediately obvious, it may be necessary to review the ### Page 88 Appendix B B-3 preamble of the regulation. It may also be helpful to discuss the hazard with the responsible FAA office. c. Step 3: Review the Consequences of the Hazard(s). (1) Once the hazard has been identified, it is possible to identify the types of consequences that may occur because of the presence of the hazard. More than one consequence can be attributed for the same hazard. Typical examples of consequences would include, but are not be limited to:  Incidents where only injuries occurred;  Accidents where less than 10 percent of the passengers died;  Accidents where 10 percent or more passengers died; and  Accidents where a total hull loss occurred. (2) The preamble to the regulation may pr ovide useful information regarding the consequences of the hazard the requirement is intended to address. d. Step 4: Identify the Historical and Predicted Frequency of Each Consequence. (1) Another source for determining impracti cality is the historical record of the consequences of the hazard that led to a requirement or an amendment to a requirement. From these data, a frequency of occurrence for the hazard can be determined. It is important to recognize that the frequency of occurrence may be higher or lower in the future. Therefore, it also is necessary to predict the frequency of future occurrences. (2) More than one consequence can be attr ibuted for the same hazard. Therefore, when applicable, the combination of consequences and frequencies of those consequences should be considered together. (3) The preamble of the regulation may provide useful information regarding the frequency of occurrence. e. Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Requirement Would Be at Addressing the Hazard. (1) When each amendment is promulgated, it is usually expected that compliance with the requirement would be completely effective at addressing the associated hazard. It is expected that the hazard would be eliminated, avoided, or mitigated. However, in a limited number of situations, this may not be the case. It is also possible that earlier amendment levels may have addressed the hazard, but were not completely effective. Therefore, in comparing the benefits of compliance with the existing certification basis to the latest amendment level, it is useful to estimate the effectiveness of both amendment levels in dealing with the hazard. ### Page 89 Appendix B B-4 (2) It is recognized that the determination of levels of effectiveness is normally of a subjective nature. These are relative assessments of a qualitative nature that should not be treated as absolute determinations. Therefore, exercise good judgment when making these determinations. In all cases, it is necessary to document the assumptions and data that support the determination. (3) The following five levels of effec tiveness are provided as a guideline: (a) Fully effective in all cases. Compliance with the requirement eliminates the hazard or provides a means to avoid the hazard completely. (b) Considerable potential for elimina ting or avoiding the hazard. Compliance with the requirement eliminates the hazard or provides a means to completely avoid the hazard for all probable or likely cases, but it does not cover all situations or scenarios. (c) Adequately deals with the hazar d. Compliance with the requirement eliminates the hazard or provides a means to avoid the hazard completely in many cases. However, the hazard is not eliminated or avoided in all probable or likely cases. Usually this action only addresses a significant part of a larger or broader hazard. (d) Hazard only partly addressed. In some cases compliance with the requirement partly eliminates the hazard or does not completely avoid the hazard. The hazard is not eliminated or avoided in all probable or likely cases. Usually this action only addresses part of a hazard. (e) Hazard only partly addressed, but action has negative side effect. Compliance with the requirement does not eliminate or avoid the hazard or may have negative safety side effects. The action is of questionable benefit. f. Step 6: Determine Resource Costs and Cost Avoidance. (1) There is always cost associated with complying with a requirement. This cost may range from minimal administrative efforts to the resource expenditures that support full scale testing or the redesign of a large portion of an aircraft. However, there are also potential cost savings from compliance with a requirement. For example, compliance with a requirement may avoid aircraft damage or accidents and the associated costs to the manufacturer for investigating accidents. Compliance with the latest amendment of a requirement may also help a foreign authority certificate a product. (2) When determining the impracticality of applying a requirement at the latest amendment level, only the incremental costs and safety benefits from complying with the existing certification basis should be considered. (3) When evaluating the incremental cost, it may be beneficial for the applicant to compare the increase in cost to comply with the latest requirements to the cost to incorporate the ### Page 90 Appendix B B-5 same design feature in a new airplane. In many cases an estimate for the cost of incorporation in a new airplane is provided in the regulatory evaluation by the FAA, which was presented when the corresponding regulation was first promulgated. Incremental costs of retrofit/incorporation on existing designs may be higher than that for production. Examples of costs may include, but are not limited to: (a) Costs: The accuracies of fleet si ze projections, utilization, etc. may be different than that experienced for derivative product designs and must be validated.  Labor: Work carried out in the design, fabrication, inspection, operation or maintenance of a product for the purpose of incorporating or demonstrating compliance with a proposed action. Non-recurring labor requirements, including training, should be considered.  Capital: Construction of new, modified or temporary facilities for design, production, tooling, training, or maintenance.  Material: Cost associated with product materials, product components, inventory, kits, and spares.  Operating Costs: Costs associated with fuel, oil, fees, and expendables.  Revenue/Utility Loss: Costs resulting from earning/usage capability reductions from departure delays, product downtime, capability reductions of performance loss due to seats, cargo, range, or airport restrictions. (b) Cost Avoidance:  Avoiding cost of accidents, including investigation of accidents, lawsuits, public relations activities, insurance, and lost revenue.  Foreign Certification: Achieve a singular effort that would demonstrate compliance to the requirements of most certifying agencies, thus minimizing certification costs. g. Step 7: Document Conclusion. Once the information from previous steps has been documented and reviewed, the applicant’s position and rationale regarding practicality can be documented. Examples of possible positions would include but are not limited to: (1) Compliance with the latest requirement is necessary. The applicant would pursue the change at the latest amendment level. (2) Compliance with an amendment level be tween the existing certification basis and the latest amendment would adequately address the hazard at an acceptable cost, while meeting ### Page 91 Appendix B B-6 the latest amendment level would be impractical. The applicant would then propose the intermediate amendment level of the requirement. (3) The increased level of safety is not commensurate with the increased costs associated with meeting the latest amendment instead of the existing certification basis. Therefore, the applicant would propose the existing certification basis. (4) The results of this analysis were inc onclusive. Further discussions with the FAA are warranted. Note: This process may result in a required certification basis that renders the proposed modification economically not viable. 3. Examples of How to Certify Changed Aircraft. The following examples are for transport airplanes and illustrate the typical process an applicant follows. The process will be the same for all product types. a. Example 1: 14 CFR § 25.963, Fuel Tank Access Covers. (1) This change is part of a significant transport airplane change that increases passenger payload and gross weight by extending the fuselage 20 feet. To accommodate the higher design weights and increased braking requirements, and to reduce runway loading, the applicant will change the landing gear from a two-wheel to four-wheel configuration; this changes the debris scatter on the wing from the landing gear. The new model airplane will be required to comply with the latest applicable regulations based on the date of application. (2) The wing will be strengthened locally at the side of the body and at the attachment of engines and landing gear, but the applicant would not like to alter wing access panels and the fuel tank access covers. Although the applicant recognizes that the scatter pattern and impact loading on the wing from debris being thrown from the landing gear will change, he proposes that it would be impractical to redesign the fuel tank access covers. (3) Step 1: Identify the Regulatory Change Being Evaluated. (a) The existing certification basis of the ai rplane that is being changed is part 25 prior to amendment 25-69 (25-40). (b) Amendment 25-69 added the requirement that fuel tank access covers on transport category airplanes be designed to minimize penetration by likely foreign objects, and be fire resistant. (4) Step 2: Identify the Specific Hazard th at the Regulation Addresses. Fuel tank access covers have failed in service due to impact with high-energy objects such as failed tire tread material and engine debris following engine failures. In one accident, debris from the runway impacted a fuel tank access cover, causing its failure and subsequent fire, which resulted ### Page 92 Appendix B B-7 in fatalities and loss of the airplane. Amendment 25-69 will ensure that all access covers on all fuel tanks are designed or located to minimize penetration by likely foreign objects, and are fire resistant. (5) Step 3: Review the History of the Consequences of the Hazard(s). Occurrences with injuries and with more than 10 percent deaths. (6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence. (a) In 200 million departures of large jets:  One occurrence with more than 10 percent deaths; and  One occurrence with injuries. (b) There is no reason to believe that the future rate of accidents will be significantly different than the historical record. (7) Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Regulation Would Be at Addressing the Hazard. (a) Considerable potential for eliminating or avoiding the hazard. (b) Compliance with amendment 25-69 eliminates the hazard or provides a means to avoid the hazard completely for all probable or likely cases. However, it does not cover all situations or scenarios. (8) Step 6: Determine Resource Costs and Cost Avoidance. (a) Costs:  For a newly developed airplane, there would be minor increases in labor resulting from design and fabrication.  There would be a negligible increase in costs related to materials, operating costs, and revenue utility loss. (b) Cost Avoidance:  There were two accidents in 200 million departures. The applicant believes that it will manufacture more than 2,000 of these airplanes or derivatives of these airplanes. These airplanes would average five flights a day. Therefore, statistically there will be accidents in the future if the hazard is not alleviated. Compliance will provide cost benefits related to avoiding lawsuits, accident investigations, and public relation costs. ### Page 93 Appendix B B-8  There are cost savings associated with meeting a single certification basis for FAA and foreign regulations. (9) Conclusion. It is concluded that compliance with the latest regulation increases the level of safety at a minimal cost to the applicant. Based on the arguments and information presented by the applicant through the issue paper process, the FAA determined that meeting the latest amendment would be practical. b. Example 2: 14 CFR § 25.365 P ressurized Compartment Loads. (1) This example is a passenger to freighter conversion STC. (2) This change affects the floor loads on the airplane as well as the decompression venting. (3) Step 1: Identify the Regulatory Change Being Evaluated. (a) The existing certification basis of the airplane that is being changed includes 14 CFR § 25.365 at amendment 25-00. The initial release of 14 CFR § 25.365 required that the interior structure of passenger compartments be designed to withstand the effects of a sudden release of pressure through an opening resulting from the failure or penetration of an external door, window, or windshield panel, or from structural fatigue or penetration of the fuselage, unless shown to be extremely remote. (b) Amendment 25-54 revised 14 CFR § 25.365 to require that the interior structure be designed for an opening resulting from penetration by a portion of an engine, an opening in any compartment of a size defined by 14 CFR § 25.365(e)(2), or the maximum opening caused by a failure not shown to be extremely improbable. The most significant change is the “formula hole size” requirement introduced into 14 CFR § 25.365(e)(2) at amendment 25- 54. (c) Amendment 25-71/72 (amendments 25- 71 and 25-72 are identical) extended the regulation to all pressurized compartments, not just passenger compartments, and to the pressurization of unpressurized areas. Pressurization of unpressurized areas had previously been identified as an unsafe feature under 14 CFR § 21.21(b)(2). (d) Amendment 25-87 redefined the pressure differential load factor that applies above an altitude of 45,000 feet. Compliance with amendment 25-87 is not affected since the airplane does not operate above an altitude of 45,000 feet. The applicant proposes to meet the “pressurization into unpressurized areas” requirement introduced in amendment 25-71/72. The applicant does not propose to comply with the formula hole size requirement introduced in 14 CFR § 25.365(e)(2) at amendment 25-54. (4) Step 2: Identify the Specific Hazard that the Regulation Addresses. The hazard is a catastrophic structure and/or system failure produced by a sudden release of pressure through an opening in any compartment in flight. This opening could be caused by an uncontained engine ### Page 94 Appendix B B-9 failure, an opening of a prescribed size due to the inadvertent opening of an external door in flight, or an opening caused by a failure not shown to be extremely improbable. The opening could be produced by an event that has yet to be identified. (5) Step 3: Review the History of the Consequences of the Hazard(s). Occurrences with injuries, less than 10 percent deaths, and more than 10 percent deaths. (6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence. (a) In 200 million departures of large jets:  Two occurrences with more than 10 percent deaths;  One occurrence with less than 10 percent deaths; and  One occurrence with injuries. (b) There is no reason to believe that the future rate of accidents will be significantly different than the historical record. (7) Step 5: Determine How Effective Full Compliance with the Latest Amendment of the Regulation Would Be at Addressing the Hazard. (a) Compliance with the latest amendmen t eliminates the hazard or provides a means to avoid the hazard completely. (b) Design changes made to the proposed de rivative airplane bring it closer to full compliance with 14 CFR § 25.365 at amendment 25-54. The original airplane was shown to meet the requirements for a hole size of 1.1 square feet. Amendment 25-54 would require a hole size of 5.74 square feet, and the current reinforcements for the converted airplane can sustain a hole size of 3.65 square feet in the forward area and 2.65 at the aft area. This is 3.1 and 2.4 times, respectively, better than the original design condition of amendment 25-0 and is a significant improvement over the world wide passenger fleet in service. (8) Step 6: Determine Resource Costs and Cost Avoidance. (a) Costs: There would be savings in both labor and capital costs if compliance were shown to amendment 25-0 instead of amendment 25-54. Major modifications to the floor beams would be necessary to meet the formula hole size requirement in amendment 25-54. (b) Cost Avoidance:  There were four accidents in 200 million departures. The applicant believes that it will manufacture more than 2,000 of these airplanes or derivatives of these airplanes. These airplanes would average two flights a day. Therefore, statistically there will be accidents in the future if the ### Page 95 Appendix B B-10 hazard is not alleviated. Compliance will provide cost benefits related to avoiding lawsuits, accident investigations, and public relation costs.  There are cost savings associated with meeting a single certification basis for FAA and foreign regulations. (9) Step 7: Document Conclusion Regard ing Practicality. The design complies with 14 CFR § 25.365 at amendments 25-0, 25-71/72, and 25-87, and is nearly in full compliance with amendment 25-54 (and certain aspects of amendments 25-71/72 and 25-87). The design would adequately address the hazard at an acceptable cost. Therefore, based on arguments of impracticality discussed in an issue paper, the FAA accepts the applicant’s proposal to comply with 14 CFR § 25.365 at amendment 25-0. ### Page 96 09/03/2010 AC21.101A Appendix C C-1 Appendix C. The Use of Service Experience in the Certification Process 1. Introduction. Service experience may support the application of an earlier regulatory standard if, in conjunction with the applicable service experience and other compliance measures, the earlier standard provides a level of safety comparable to that provided by the latest requirements. The applicant must provide sufficient substantiation to allow the FAA to make this determination. A statistical approach may be used, subject to the availability and relevance of data, but sound engineering judgment must be used. For service history to be acceptable, the data must be both sufficient and pertinent. The essentials of the process involve: a. A clear understanding of the requirement ch ange and the purpose for the change and hazard addressed; b. A determination based on detailed k nowledge of the proposed design feature; c. The availability of pertinent and su fficient service experience data; and d. A comprehensive review of that service experience data. 2. Guidelines. The issue paper process (either a standalone issue paper or included in the G-1 issue paper) would be used, and the applicant should provide documentation to support the following: a. The identification of the differences between the requirement in the existing certification basis and the requirement as amended, and the effect of the change in the requirement. b. A description as to what aspect(s) of the latest requirements the proposed changed product would not meet. c. Evidence showing that the proposed certi fication basis for the changed product, together with applicable service experience, relative to the hazard, provides a level of safety consistent with complying with the latest requirements. d. A description of the design feat ure and its intended function. e. Data for the product pert inent to the requirement. (1) Service experience from such data sources as the following: (a) Accident reports; (b) Incident reports; ### Page 97 09/03/2010 AC21.101A Appendix C C-2 (c) Service bulletins; (d) Airworthiness directives; (e) Repairs; (f) Modifications; (g) Flight hours/cycles for fl eet leader and total fleet; (h) World airline accident summary data; (i) Service difficulty reports; (j) National Transportation Safety Board reports; and (k) Warranty, repair and parts usage data. (2) Show that the data presented represen t all relevant service experience for the product, including the results of any operator surveys, and is comprehensive enough to be representative. (3) Show that the service experience is relevant to the hazard. (4) Identification and evaluation of each of th e main areas of concern with regard to: (a) Recurring and/or common failure modes; (b) Cause; (c) Probability, by qualita tive reasoning; and (d) Measures already taken and their effects. (5) Relevant data pertaining to aircraft of similar design and construction may be included. (6) Evaluation of failure modes and conseque nces through analytical processes. The analytical processes should be supported by: (a) A review of previ ous test results; (b) Additional detailed testing as required, or ### Page 98 09/03/2010 AC21.101A Appendix C C-3 (c) A review aircraft functional hazard assessments (FHA) and any applicable system safety assessments (SSA) as required. f. A conclusion that draws together the data and the rationale. g. These guidelines are not intended to be limiting, either in setting required minimum elements or in precluding alternative forms of submission. Each case may be different, based on the particulars of the system being examined and the requirement to be addressed. 3. Example: 14 CFR § 25.1141(f) Powerplant Controls for Transport Airplanes. a. The following example, for transport airplanes (14 CFR § 25.1141(f) Auxiliary Power Unit (APU) Fuel Valve Position Indication System), illustrates the typical process an applicant follows. The process will be the same for all product types. b. This example comes from a derivative model transport airplane where significant changes were made to the main airframe components, engines and systems, and APU. The baseline airplane has an extensive service history. The example shows how the use of service experience supports a finding that compliance with the latest regulation would not contribute materially to the level of safety and that application of the existing certification basis (or earlier amendment) would be appropriate. The example is for significant derivatives of transport airplanes with extensive service history, and illustrates the process, following the guidelines in this appendix, but does not include the level of detail normally required. (1) Determine the differences between the re gulation in the existing certification basis and the regulation as amended, and the effect of the change in the regulation. The existing certification basis of the airplane that is being changed is the initial release of part 25. Amendment 25-40 added requirement 14 CFR § 25.1141(f), which mandates that power-assisted valves must have a means to indicate to the flight crew when the valve is in the fully open or closed position, or is moving between these positions. The addressed hazard would be risk of APU fire due to fuel accumulation caused by excessive unsuccessful APU start attempts. (2) What aspect of the proposed changed pr oduct would not meet the latest regulations? The proposed APU fuel valve position indication system does not provide the flight crew with fuel valve position or transition indication and, therefore, does not comply with the requirements of 14 CFR § 25.1141(f). (3) The applicant provides evidence that the proposed certification basis for the changed product, together with applicable service experience of the existing design, provide a level of safety comparable to that intended by the latest regulation. The APU fuel shut-off valve and actuator are unchanged from those used on the current family of airplanes, and have been found to comply with the earlier amendment 25-11 of 14 CFR § 25.1141(f). The existing fleet has achieved approximately (#) flights during which service experience of the existing design has been found to be acceptable. If one assumes a complete APU cycle, i.e., start-up and shutdown for each flight, the number of APU fuel shut-off valve operations would be over 108 ### Page 99 09/03/2010 AC21.101A Appendix C C-4 cycles, which demonstrates that the valve successfully meets its intended function and complies with the intent of the regulation. In addition, the system design for the changed product incorporates features that increase the level of functionality and safety. (4) The applicant provides a description of th e design feature and its intended function. The fuel shut-off valve, actuator design, and operation is essentially unchanged; with the system design ensuring that the valve is monitored for proper cycling from closed to open at start. If the valve is not in the appropriate position (i.e., closed), then the APU start is terminated, an indication is displayed on the flight deck, and any further APU starts are prevented. Design improvements using the capability of the APU electronic control unit (ECU) have been incorporated in this proposed product change. These design changes ensure that the fuel valve indication system will indicate failure of proper valve operation to the flight crew, but the system does not indicate valve position as required by 14 CFR § 25.1141(f). (5) The FAA and applicant record this in an issue paper. We can use the G-1 or a technical issue paper for this purpose. An issue paper was coordinated, included data, or referenced reports, documenting relevant service experience that has been compiled from incident reports, fleet flight hour/cycle data, and maintenance records. The issue paper also discussed existing and proposed design details, failure modes and analyses showing to what extent the proposed airplane complies with the latest amendment of 14 CFR § 25.1141. Information is presented to support the applicant’s argument that compliance with the latest amendment would not materially increase the level of safety. Comparative data pertaining to aircraft of similar design and construction are also presented. (6) The conclusion, drawing together the data and rationale, is documented in the G-1 issue paper. The additional features incorporated in the APU fuel shut-off valve will provide a significant increase in safety to an existing design with satisfactory service experience. The applicant proposes that compliance with the latest amendment would not materially increase the level of safety and that compliance with 14 CFR § 25.1141 at amendment 25-11 would provide an acceptable level of safety for the proposed product change. ### Page 100 09/03/2010 AC21.101A Appendix D D-1 Appendix D. Definitions and Terminology 1. Adequate Certification Basis - The type certification basis for a changed product under 14 CFR § 21.101 is considered adequate when the FAA determines that it provides adequate standards for the design change, i.e. when the certification specifications of the applicable airworthiness code and prescribed special conditions provide an appropriate level of safety for the changed product and do not result in any unsafe design features. 2. Aeronautical product – The terms aeronautical product or product(s) used in this guidance material includes type certificated aircraft, engines, and propellers. 3. Affected area, system, component, part or appliance – any system, component, part, or appliance which is either physically altered by a proposed design change or, even if not altered physically, its functional characteristics are altered due to the effects of the physical change. 4. Certification basis – The applicable airworthiness requirements as established in 14 CFR §§ 21.17 and 21.101, as appropriate; special conditions; equivalent level of safety findings; requirements per 14 CFR § 21.21(b)(2); and exemptions applicable to the product to be certificated. 5. Design Change – A change in the type design of an aeronautical product. In the context of this document the terms “change”, “design change” and “type design change” are synonymous. 6. Earlier requirements – The requirements in effect prior to the date of application for the change, but not prior to the existing certification basis. 7. Existing certification basis – The requirements incorporated by reference in the type certificate of the product to be changed. 8. Latest requirements – The requirements in effect on the date of application for the change. 9. Previous relevant design changes – Previous design changes, the cumulative effect of which could result in a product significantly or substantially different from the original product or model, when considered from the last time the latest regulations were applied. 10. Product level change – A change or combination of changes that makes the product distinct from other models of the product (for example, range, payload, speed, design philosophy). Product level change is defined at the aircraft, aircraft engine, or propeller level of change. 11. Secondary change – A change is a secondary change if compliance to the latest amendment would not contribute materially to the level of safety and where it is part of and consequential to an overall significant change. A secondary change is a physical change that restores without changing the system, structural capacity, or functionality, but is necessary to support a significant change. ### Page 101 09/03/2010 AC21.101A Appendix D D-2 12. Significant change – A change to the type certificate significant to the extent that it changes one or more of the following: general configuration, principles of construction, or the assumptions used for certification, but not to the extent to be considered a substantial change. The significance of the change is considered in the context of all previous relevant design changes and all related revisions to the applicable regulations. Not all product level changes are significant. 13. Significant change in an area (for excepted aircraft under 14 CFR § 21.101(c) only) – A change in an area is significant if the general configuration or the principals of construction in that area are not retained, or the assumptions used for certification of that area do not remain valid. 14. Substantial change – A change which is so extensive that a substantially complete investigation of compliance with the applicable regulations is required, and consequently a new type certificate, in accordance with 14 CFR § 21.19. ### Page 102 09/03/2010 AC21.101A Appendix E E-1 Appendix E. Related Code of Federal Regulations Sections 1. Related Code of Federal Regulations Paragraphs.  14 CFR § 21.16, Special conditions.  14 CFR § 21.17, Designation of applicable regulations.  14 CFR § 21.19, Changes requiring a new type certificate.  14 CFR § 21.21, Issue of type certificate: normal, utility, acrobatic, commuter, and transport category aircraft; manned free balloons; special classes of aircraft; aircraft engines; propellers.  14 CFR § 21.93, Classification of changes in type design.  14 CFR § 21.101, Designation of applicable regulations.  14 CFR § 21.115, Applicable requirements. 2. How to Get Publications.  Order copies of 14 CFR, parts from the Superintendent of Documents, Government Printing Office, P.O. 979050, St. Louis, MO 63197. For general information telephone (202) 512-1800 or fax (202) 512-2250. You can order copies online at www.access.gpo.gov. Select “Access” then “Online Bookstore.” Select “Aviation,” then “Code of Federal Regulations.”  Order copies of FAA advisory circulars from the U.S. Department of Transportation, Subsequent Distribution Office, M-30, Ardmore East Business Center, 3341 Q 75th Avenue, Landover, MD 20785. You can also get copies from our FAA website at http://www.airweb.faa.gov/. --- OpenAirDocs.org is an independent research project of [Contrail Labs](https://contraillabs.com/). It is not affiliated with the FAA or the NTSB. Source documents are U.S. government works in the public domain.