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3.2 Step 1. Identify the Proposed Type Design Changes to an Aeronautical Product.
• Identify the type design you are changing (the baseline product).
• Identify the proposed change.
• Use high level descriptors.
3.2.1 Identify the Type Design You are Changing (the Baseline Product).
Prior to describing the proposed change(s), it is important to clearly identify the specific type design configuration you are changing.
Note: For additional guidance on the baseline product, see paragraph 5.3 of this AC.
3.2.2 Identify the Proposed Change.
3.2.2.1 The purpose of this process step is to identify and describe the change to the aeronautical product. Changes to a product can include physical design changes and functional changes (e.g., operating envelope or performance changes). You must identify all changes and areas affected by the change, including those where you plan to use previously approved data. The FAA considers all of these changes and areas affected by the change part of the entire proposed type design and are considered as a whole in the classification of whether the proposed design change is substantial, significant, or not significant. The change can be a single change or a collection of changes. In addition to the proposed changes, consider the cumulative effect of previous relevant design changes incorporated since the last time the certification basis was upgraded. An applicant for a type design change must consider all previous relevant design changes and the amendment level of the certification basis used for these changes.
3.2.2.2 When you identify the proposed changes, consider previous relevant design changes that create a cumulative effect, as these may influence the decisions regarding the type of design change later in the process. By “previous relevant design changes,” the FAA means changes where effects accumulate, such as successive thrust increases, incremental weight increases, or sectional increases in fuselage length. You must account for any previous relevant design changes in the area affected by the proposed change that did not involve an upgrade of the certification basis in the proposed design change.
3.2.2.3 Example:
An applicant proposes a 5 percent weight increase, but a previous 4 percent and another 3 percent weight increase was incorporated into this aircraft without upgrading the existing certification basis. In the current proposal for a 5 percent weight increase, the cumulative effects of the two previous weight increases that did not involve an upgrade of the certification basis will now be accounted for as an approximate 12 percent increase in weight. Note that the cumulative effects the applicant accounts
3-4 for are only those incremental increases since the last time the airworthiness requirements in the type certification basis applicable to the area affected by the proposed change were upgraded.
3.2.3 Use High Level Descriptors.
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.
3.2.4 Evolutionary changes that occur during the course of a certification program may require re-evaluation of the certification basis, and those changes that have influence at the product level may result in re-classification of the change.
3.3 Step 2. Verify the Proposed Type Design Change is Not Substantial.
3.3.1 Section 21.19 requires that you apply for a new TC for a changed product if the 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.
3.3.2 A “substantially complete investigation” of compliance is required when most of the existing substantiation is not applicable to the changed product. In other words, you may consider the design change 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 in a similarity argument. The FAA considers a change substantial when these approaches, models, or methodologies of how compliance was shown are not valid for the changed product.
3.3.3 If it is not initially clear that a new TC is required, appendix A of this AC provides some examples of substantial changes to aid in this classification. A substantial change requires application for a new TC. See §§ 21.17 and 21.19. If the change is not substantial, proceed to Step 3.
3.4 Step 3. Will you Use the Latest Standards?
You can use the latest requirements for your proposed type design change and the area affected by the change. If you use the latest requirements, you will have met the intent
3-5 of § 21.101 and no further classification (significant or not significant) and justification is needed. Even though an applicant elects to use the latest certification requirements, the applicant will still be able to apply § 21.101 for future similar changes, and use the exceptions under § 21.101(b). However, the decision to comply with the latest requirements sets a new regulatory basis for all future related changes in the same affected area for that amended TC.
• If you are using the latest requirements, proceed to Step 6 (in paragraph 3.9 of this AC).
• If you are not using the latest requirements, proceed to Step 4 below.
3.5 Step 4. Arrange Changes into Related and Unrelated Groups.
3.5.1 You should now determine if any of the changes identified in Step 1 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 needed to achieve the goal of carrying more passengers. A decision to upgrade the flightdeck 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 would then be considered an unrelated (or a stand-alone) change. However, the simultaneous introduction of a new cabin interior is considered related 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. Figure 3-2 below illustrates the grouping of related and unrelated changes using the example of increasing the maximum number of passengers.
Note: If you plan changes in sequence over time, refer to the discussion on “sequential design changes” in paragraph 5.13 of this AC.
3-6 Figure 3-2. Related and Unrelated Changes for Example of Increasing the Maximum Number of Passengers The Aeronautical Product Affected Area Affected Area Grouping of Related Changes (significant change) Thrust Increase (physical and performance changes) MTOW Increase (physical and performance changes) Fuselage Stretch (physical change) Changed Area Comprehensive Flightdeck Upgrade (typically, a stand-alone significant physical change) Not Affected Area
3.5.2 Once you organize the change(s) into groupings of those that are related and those that are unrelated (or stand-alone), proceed to Step 5 below.
3.6 Step 5. Is Each Related or Unrelated Group a Significant Change?
3.6.1 The applicant is responsible for proposing the classification of groups of related design changes or unrelated design changes as significant or not significant. Significant changes are product level changes that could result from an accumulation of changes, or occur through a single significant change that makes the changed product distinct from its baseline product. The grouping of related and unrelated changes is particularly relevant to the FAA’s significant Yes/No decision (§ 21.101(b)(1)) described in Step 5 of figure 3-1. The FAA evaluates each group of related changes and each unrelated (stand-alone) change on its own merit for significance. Thus, there may be as many evaluations for significance as there are groupings of related and unrelated changes.
Step 1 of figure 3-1 explains the accumulation of changes that you must consider.
3-7 Additionally, § 21.101(b)(1) defines a design change as significant when at least one of three automatic criteria applies:
3.6.1.1 Changes where the General Configuration is Not Retained (Significant Change to General Configuration).
A change to the general configuration at the product level is one 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 product model, although this is not required. For examples, see appendix A of this AC.
3.6.1.2 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 methods that affects the overall product’s operating characteristics or inherent strength and would require extensive reinvestigation to show compliance.
For examples, see appendix A of this AC.
3.6.1.3 Product Level Changes that Invalidate the Assumptions used for Certification of the Baseline Product.
Examples include—
• Change of an aircraft from an unpressurized to pressurized fuselage,
• Change of operation of a fixed wing aircraft from land-based to water-based, and
• Operating envelope expansions that are outside the approved design parameters and capabilities.
For additional examples, see appendix A of this AC.
3.6.2 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 and the examples in appendix A of this AC, you should focus on the design change and how it impacts the existing product (including performance, operating envelope, etc.). A design change cannot be classified or re-classified as a significant change on the basis of the importance of a later amendment level.
3.6.3 Appendix A of this AC includes tables of typical changes (examples) for small airplanes, transport airplanes, rotorcraft, engines, and propellers that meet the criteria for a significant design change. The appendix also includes tables of typical design changes that the FAA classifies as not significant. The tables can be used in one of two ways—
3.6.3.1 To identify the classification of a proposed design change listed in the table, or
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3.6.3.2 In conjunction with the three automatic criteria, to help classify a proposed design change not listed in the table by comparison to determinations made for changes with similar type and magnitude.
3.6.4 In many cases, a significant change may involve more than one of these criteria and will be obvious and distinct from other product improvements or production changes. There could be cases where a change to a single area, system, component, or appliance may not result in a product level change. There could also be other cases where the change to a single system or component might result in a significant change due to its effect on the product overall. Examples may include the addition of winglets or leading edge slats, or a change in primary flight controls to a fly-by-wire system.
3.6.5 If an unrelated (stand-alone) change or a grouping of related changes is classified as—
3.6.5.1 Significant (§ 21.101(a)):
You must comply with the latest airworthiness standards for certification of the design change and areas affected by change, unless you justify use of one of the exceptions provided in § 21.101(b)(2) or (3) to show compliance with earlier amendment(s). The final certification basis may consist of a combination of the requirements recorded in the certification basis ranging from the original aircraft certification basis to the most current regulatory amendments.
3.6.5.2 Not Significant (§ 21.101(b)(1)):
You may comply with the existing certification basis unless the standards in the proposed certification basis are deemed inadequate. In cases where the existing certification basis is inadequate or no regulatory standards exist, later requirements and/or special conditions will be required. See paragraph 3.11 of this AC for a detailed discussion.
3.6.6 A new model designation to a changed product is not necessarily indicative that the design change is significant under § 21.101. Conversely, retaining the existing model designation does not mean that the design change is not significant. Significance is determined by the magnitude of the type design change.
3.6.7 The FAA determines the final classification of whether a design change is significant or not significant. To assist you in your assessment, the FAA has predetermined the classification of several typical design changes that you can use for reference, and these examples are listed in appendix A of this AC.
3.6.8 At this point, the determination of significant or not significant for each of the groupings of related changes and each stand-alone change is completed. For significant changes, if you propose to comply with an earlier requirement, use the procedure outlined in paragraph 3.7 below. For changes identified as not significant, see paragraph 3.8 below.
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3.7 Proposing an Amendment Level for a Significant Change.
3.7.1 If the classification of the group of related changes is significant, all areas, systems, components, parts, or appliances affected by the change must comply with the airworthiness standards at the amendment level in effect on the date of application for the change. You can justify use of one of the exceptions in § 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 §§ 23.2, 25.2, 27.2, 29.2 applicable on the date of the application for the change. See paragraphs 3.9 and 3.10 of this AC.
3.7.2 For transport category airplanes only, § 21.101(g) requires that you comply with any applicable provision of 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 part 25 requirements.
3.7.3 The final certification basis may combine latest, earlier (intermediate), and existing regulations, but cannot contain regulations preceding the existing certification basis.
3.8 Proposing an Amendment Level for a Not Significant Change.
3.8.1 When the FAA classifies the type design change as not significant, the rule allows compliance with earlier amendments but not prior to the existing certification basis.
Within this limit, the applicant may propose an amendment level for each certification standard for the affected area. However, you should be aware that the FAA will review your proposal for the type certification basis to ensure that the certification basis is adequate for the proposed change under Step 8. (See paragraph 3.11 of this AC.) You must also comply with the retroactive requirements found in §§ 23.2, 25.2, 27.2, 29.2, applicable on the date of the application for the change.
3.8.2 For transport category airplanes only, § 21.101(g) also requires that you comply with any applicable provision of 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 part 25 requirements. You may comply with a specific airworthiness requirement or a subset of airworthiness requirements at later amendments. In such a case, any other airworthiness requirements that are directly related should be included in the certification basis for the change.
3.9 Step 6. Prepare your Proposed Certification Basis List.
As part of preparing your proposed certification basis list, you must identify any 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 any areas, systems, components, equipment, or appliances of the product. The characteristics affected by the change are not only physical changes, but also functional
3-10 changes brought about by the physical changes. Examples of physical aspects are structures, systems, components, equipment, 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 you are changing should be updated or rewritten. Appendix H of this AC contains two examples of how to document a proposed certification basis list.
3.9.1 An area affected by the change is any system, component, part, or appliance of the aeronautical product that you physically and/or functionally change. Figure 3-3 of this AC illustrates concepts of physical and functional changes of an affected area.
Appendix C of this AC contains a method used to define the change and areas affected by the change. This appendix is meant to assist you when you propose a large, complex design change. For a type design change, it is important that you properly assess the effects of such change on any areas, systems, components, equipment, or appliances of the product 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.
Figure 3-3. Affected Areas versus Not Affected Areas Physical Change Functional Change Not Affected Area Significant Change and Area Affected by the Significant Change The Aeronautical Product
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3.9.2 An area not affected by a change can remain at the existing certification basis, provided you present to the FAA acceptable justification that the area is not affected.
3.9.3 For sample questions to assist in determining affected areas, see paragraph D.1 of appendix D of this AC.
3.9.4 Consider the following aspects of a type design change:
3.9.4.1 Physical Aspects.
The physical aspects include direct changes to structures, systems, equipment, components, and appliances, and may include software/airborne electronic hardware changes and the resulting effect on systems functions.
3.9.4.2 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 (including load carrying), aeroelastic characteristics, or crashworthiness. A product level change may affect these characteristics. 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) may become part of the affected area due to the change in functional characteristics. Another example is changing turbine engine ratings and operating limitations affecting the engine rotors’ life limits.
3.9.5 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 materially to the level of safety or would be impractical. Step 7 below provides further explanation.
3.9.6 The applicant should document the change and area affected by the change using high level descriptors along with the applicable regulations and their associated amendment levels. The applicant proposes this change in certification basis that the FAA will consider for documentation in the type certificate data sheet (TCDS) or STC, if they are different from that recorded for the baseline product in the TCDS.
3.10 Step 7. Do the Latest Requirements Contribute Materially to the Level of Safety and are They Practical?
Pursuant to § 21.101(a), compliance with the latest airworthiness standards is required.
However, exceptions may be allowed pursuant to § 21.101(b)(3). The applicant must provide justification to support the rationale for the application of earlier amendments
3-12 for areas affected by a significant change to document 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. See paragraphs 3.10.1 and 3.10.1.4 of this AC.
3.10.1 Do the Latest Requirements Contribute Materially to the Level of Safety?
You could consider compliance with the latest requirements to “not 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. In cases where design features provide a level of safety greater than the existing certification basis, you may use acceptable data, such as service experience to establish the effectiveness of those design features at mitigating the specific hazards by a later amendment. You must provide sufficient justification to allow the FAA to make this determination. An acceptable means of compliance is described in appendix E of this AC. Justification is sufficient when it provides a summary of the evaluation that supports the determination using an agreed evaluation method such as that in appendix E of this AC. This exception could be applicable in the situations described in the paragraphs below.
Note: Compliance with later requirements is not required where the amendment is of an administrative nature and made only to correct inconsequential errors or omissions, consolidate text, or clarify an existing requirement.
3.10.1.1 Improved Design Features.
Design features that exceed the existing certification basis requirements, but do not meet the latest requirements, can be used as a basis for granting an exception under § 21.101(b)(3) since complying with the latest amendment of the requirements would not contribute materially to the level of safety of the product. If the FAA accepts these design features as justification for an exception, you must incorporate them in the amended type design configuration and record them, where necessary, in the certification basis. The description of the design feature would be provided in the 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 part 25 airplane, and part of the design involves adding a small number of new wing fuel tank fasteners. Assuming that the latest applicable amendment of § 25.981 is Amendment 25-102, which requires structural lightning protection, the applicant could propose 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 § 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
3-13 design feature can be accepted as an exception to compliance with the latest amendment.
3.10.1.2 Consistency of Design.
This provision gives the opportunity to consider the consistency of design.
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 in the area of the fuselage plug. Compliance of the new areas with the existing certification basis may be acceptable.
3.10.1.3 Service Experience.
3.10.1.3.1 Relevant service experience, such as experience based on fleet performance or utilization over time (relevant flight hours or cycles), is one way of showing that the level of safety will not materially increase by applying the latest amendment, so the use of earlier requirements could be appropriate. Appendix F of this AC provides additional guidance on the use of service experience, along with examples.
3.10.1.3.2 When establishing the highest practicable level of safety for a changed product, the FAA has determined that it is appropriate to assess the service history of a product, as well as the later airworthiness standards. It makes little sense to mandate changes to well understood designs, whose service experience has been acceptable, merely to comply with new standards.
The clear exception to this premise is if the new standards were issued to address a deficiency in the design in question, or if the service experience is not applicable to the new standards.
3.10.1.3.3 There may be cases for rotorcraft and small airplanes where relevant data may not be sufficient or not available at all because of the low utilization and insufficient 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.
3.10.1.3.4 The FAA will determine if proposed service experience levels necessary to demonstrate the appropriate level of safety as they relate to the proposed design change is acceptable.
3.10.1.4 Secondary Changes.
3.10.1.4.1 The design change proposed by the applicant can consist of physical and/or functional changes to the product. See figure 3-4 below. There may
3-14 be aspects of the existing type design of the product that the applicant may not be proposing to change directly, but that are affected by the overall design change. For example, changing an airframe’s structure, such as adding a cargo door in one location, may affect the frame or floor loading in another area. Further, upgrading engines with new performance capabilities could require additional showing of compliance for minimum control speeds and airplane performance requirements. For many years, the FAA has required applicants to consider these effects, this practice is unchanged under the procedures of § 21.101.
Figure 3-4. Design Change Affected Areas with Secondary Changes Physical Change Functional Change Not Affected Area Significant Change and Area Affected by the Significant Change The Aeronautical Product Secondary Changes
3.10.1.4.2 For each design change, it is important that the effects of the change on other systems, components, equipment, or appliances of the product are properly identified and assessed. 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 reviewed, updated, or rewritten.