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.
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3.10.1.4.3 In assessing the areas affected by the design change, it may be helpful to identify secondary changes. A secondary change is a change in physical and/or functional aspects that is part of but consequential to a significant physical design change, whose only purpose is to restore, and not add or increase, existing functionality or capacity. The term “consequential” is intended to refer to—
• A change that would not have been made by itself; it achieves no purpose on its own.
• A change that has no effect on the existing functionality or capacity of areas, systems, structures, components, parts, or appliances affected by the design change.
• A change that would not create the need for: (1) new limitations or affect existing limitations; (2) a new airplane flight manual (AFM) or instructions for continued airworthiness (ICA) or a change to the AFM or ICA; or (3) special conditions, equivalent safety findings, or exemptions.
3.10.1.4.4 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 under § 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 not a secondary change.
3.10.1.4.5 In some cases, a secondary area of change that restores functionality may in fact contribute materially to the level of safety by meeting a later amendment. If this is the case, it is not considered a secondary change.
3.10.2 Are the Latest Requirements Practical?
The intent of § 21.101 is to enhance safety by applying the latest airworthiness standards to the greatest extent practicable. The concepts of contributing materially and practicality are linked. If compliance with the latest airworthiness standards does contribute materially to the level of safety, then the applicant may assess incremental costs to see if it is commensurate with the increase in safety. 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. The cost of changing compliance documentation and/or drawings is not an acceptable reason for an exception.
3.10.2.1 Support your position that compliance is impractical with substantiating data and analyses. While evaluating your position and your substantiating data regarding impracticality, the FAA may consider other factors
(e.g., the costs and safety benefits for a comparable new design).
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3.10.2.2 A review of transport category projects showed that, in certain cases where the FAA allowed an earlier amendment of applicable requirements, the applicants made design changes that nearly complied with the latest amendments. In these cases, the applicants successfully demonstrated 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 § 21.101(b)(3) on the basis of “impracticality.”
3.10.2.3 Appendix E of this AC provides additional guidance and examples for evaluating the impracticality of applying the latest requirements to a changed product for which compliance with the latest requirements would contribute materially to the level of safety of the product.
3.10.2.3.1 The exception of impracticality is a qualitative and 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 the 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 an applicant could always use cost analysis to support an appropriate amendment level). However, there should be enough detail in the applicant’s rationale to justify the exception.
Note: An applicant should not base the impractical exception on the size of the applicant’s company or their financial resources. The applicant must evaluate the costs to comply with a later amendment against the safety benefit of complying with the later amendment.
3.10.2.3.2 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 models. 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 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.
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3.11 Step 8. Ensure Proposed Certification Basis is Adequate.
The FAA considers a proposed certification basis for any design change (whether it is significant or not significant) to be adequate when—
• The airworthiness standards provide an appropriate level of safety for the intended change, and
• The change and the areas affected by the change do not result in unsafe design features or characteristics for the intended use.
3.11.1 For a change that contains new design features that are novel and unusual for which there are no later applicable airworthiness requirements at a later amendment level, the FAA will designate special conditions pursuant to § 21.16. The FAA will impose later airworthiness standards that contain adequate or appropriate safety standards for this feature, if they exist, in lieu of special conditions. An example is adding a flight critical system such as an electronic air data display on a part 25 airplane whose existing certification basis does not have lightning and high intensity radiated fields (HIRF) protection requirements. In this case, the FAA will require compliance with the regulations for lightning and HIRF protection, even though the FAA determined the change is not significant.
3.11.2 For new design features or characteristics that may pose a potential unsafe condition for which there are no later applicable airworthiness requirements, new requirements may be required to address § 21.21(b)(2).
3.11.3 In cases where inadequate or no airworthiness standards exist for the change in the existing type certification basis but adequate standards exist in a later amendment of the applicable airworthiness code, the later amendment will be made part of the type certification basis to ensure adequacy of its certification basis.
3.11.4 The FAA determines the final certification basis for a product design change. This may consist of a combination of those airworthiness standards ranging from the existing certification basis of the baseline product to the latest amendments and special conditions.