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.
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
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.
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
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).
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.
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.
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.
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.
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
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
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.