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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.
4-1 CHAPTER 4. EXCEPTED PRODUCTS UNDER § 21.101(C)
4.1 Excepted Products.
For excepted products as defined in paragraph 4.1.1 below, the starting point for regulatory analysis is the existing certification basis for the baseline product.
4.1.1 Section 21.101(c) provides an exception to § 21.101(a) compliance with the latest requirements for aircraft (other than rotorcraft) of 6,000 pounds or less maximum weight, or to a non-turbine rotorcraft of 3,000 pounds or less maximum weight. In these cases, the applicant may elect to comply with the existing certification basis. However, the applicant has the option of applying later appropriate regulations. Special classes of aircraft—including gliders, airships, and primary category—are addressed in § 21.101(f), and not in § 21.101(c).
4.1.2 If the FAA finds that the change is significant in an area, the FAA may require the applicant to comply with a later regulation and any regulation the FAA finds is directly related. Starting with the existing certification basis, the FAA will progress through each later regulation to determine the amendment appropriate for the change. However, if an applicant proposes, and the FAA finds, that complying with the later amendment or regulation would not contribute materially to the level of safety of the changed product or would be impractical, the FAA may allow the applicant to comply with an earlier amendment appropriate for the proposed design change. The amendment may not be earlier than the existing certification basis. For excepted products, changes that meet one of the following criteria, in the area of change, are automatically considered significant if—
4.1.2.1 The general configuration or the principles of construction are not retained.
4.1.2.2 The assumptions used for certification of the area to be changed do not remain valid.
4.1.2.3 The change contains new features (not foreseen in the existing certification basis and for which appropriate later regulations exist). In this case, the FAA will designate the applicable airworthiness requirements, starting with the existing certification basis and progressing to the most appropriate later amendment level for the change.
4.1.2.4 The change contains a novel or unusual design feature. In this case, the FAA will designate the applicable special conditions appropriate for the change, pursuant to § 21.101(d).
4.1.3 The exception for products under § 21.101(c) applies to the aircraft only. Design changes to engines and propellers installed on these excepted aircraft are assessed as separate type certificated products using § 21.101(a) and (b).
5-1 CHAPTER 5. OTHER CONSIDERATIONS
5.1 Design-Related Operating Requirements.
Some rules in other 14 CFR parts (e.g., parts 91, 121, 125, and 135) impose airworthiness standards that are not required for issuance of a TC or STC. If not already included in the certification basis, any such applicable airworthiness standards may be added to the type certification basis by mutual agreement between the applicant and the FAA. The benefit of adding these airworthiness standards to the type certification basis is to increase awareness of these standards, imposed by other 14 CFR parts, during design certification and future modifications to the airplane. The use of exceptions under § 21.101 is not intended to alleviate or preclude compliance with operating regulations.
5.2 FAA Policy.
Once the certification basis has been established, the exceptions of § 21.101 are not applicable in determining which policy (e.g., ACs and policy statements) applies to the design change. Guidance on the use of policy is found in FAA Order 8110.4C, Type Certification, and Order 8110.48A, How to Establish the Certification Basis for Changed Aeronautical Products. In general, you should use the latest FAA policy in effect at the date of application. However, there might be cases where policy may differ depending on the amendment level of the rule (i.e., the intent of the regulation may be different). It is acceptable to use another means of compliance, provided it is agreed upon by the FAA. This is typically documented via an issue paper.
5.3 A Baseline Product Consists of One Unique Type Design Configuration.
As mentioned in paragraph 3.2.1 of this AC, it is important to clearly identify the type design configuration to be changed. The baseline for a changed product consists of one unique type design configuration. This unique type design configuration is an approved product configuration that can be manufactured at one time or may consist of multiple approvals over time (e.g., airplane model with an approved STC incorporated). The applicant should identify the specific product configuration that will be modified. The FAA does not require an applicant to assign a new model name for a changed product.
Therefore, there are vastly different changed products with the same airplane model name, and there are changed products with minimal differences that have different model names. Since the assignment of a model name is based solely on an applicant business decision, the identification of the baseline product, for the purposes of § 21.101, is one unique type design configuration, as mentioned above.
Note: The type design configuration, for this purpose, could be based on a currently approved configuration or based on a proposed future configuration that is expected to be approved at a later date but prior to the proposed changed product.
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5.4 Predecessor Regulations.
The airworthiness requirements in effect on the date of application are in parts 21, 23, 25, 27, 29, 31, 33, and 35. Consequently, when electing to comply with later requirements, the predecessor regulations (i.e., Civil Air Regulations (CARs)) are not considered in effect on the date of application. Predecessor regulations are not recognized under § 21.101(a), but may be allowed under § 21.101(b), (c), and (f).When choosing the amendment level of a regulation, all related regulations associated with that amendment level would have to be included.
5.5 Special Conditions, § 21.101(d).
Section 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 to classify it as either a substantial change or a significant change. When the change is significant with earlier requirements allowed through exceptions, or not significant, the level of safety intended by the special conditions must be consistent with the agreed certification basis.
5.6 Effective Period for an Application to Change a Type Certificate, § 21.101(e).
According to § 21.101(e), an application for, or a change to, a TC for transport category aircraft is effective for five years from the date of application, and an application for a change to any other type certificate is effective for three years. The FAA intended this to ensure that the certification basis for the changed product is as current as practicable.
5.6.1 If the FAA has not approved a design change, or if it is clear that the FAA will not approve the change, within the time limit, the applicant may do either of the following:
(1) file for a new application, or (2) file for an extension to the original application.
5.6.2 When filing an extension, the applicant must choose a completion date, then apply the applicable effectivity from § 21.101(e) to determine an effective application date. The effective application date must not precede the original date of application for the proposed design change and must not be later than the filing date for an extension.
5.6.3 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, the applicant 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.
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5.6.4 If the product change is not significant, you may continue to use the existing certification basis for product certification. However, if you make additional design changes to the product, and the FAA finds the existing certification basis for the change inadequate, the new certification basis will require later appropriate standards.
5.7 Other Category Aircraft, § 21.101(f).
For aircraft type certificated under §§ 21.17(b), 21.24, 21.25, and 21.27, the certification basis for the changed product consists of the amendment levels of the applicable regulations that the FAA finds 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 § 21.101(b).
The exceptions in § 21.101(c) do not apply to categories of products defined in § 21.101(f).
5.7.1 Special Classes Aircraft.
For special classes of aircraft (e.g., gliders, airships, etc.) including any installed engines and propellers certificated pursuant to § 21.17(b), the applicable requirements are portions of those other airworthiness requirements in parts 23, 25, 27, 29, 31, 33, and 35 that the FAA finds appropriate for the aircraft and applicable to the specific type design, or such airworthiness criteria that the FAA finds an equivalent level of safety to those parts.
5.7.2 Primary Category Aircraft.
For primary category aircraft certificated under § 21.24, the applicable airworthiness requirements are in parts 23, 27, 31, 33, and 35, or such other requirements the FAA finds appropriate. These requirements must be applicable to the specific design and intended use of the aircraft and provide a level of safety acceptable to the FAA.
5.7.3 Restricted Category Aircraft.
For aircraft certificated in the restricted category under § 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 airworthiness regulations applicable to the aircraft at the time the TC was issued do not provide an adequate level of safety for the design change, the application of later regulations will be required.
5.7.3.1 Features of the changed product that are “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 applicable airworthiness 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.
5.7.3.2 For the installation of turbo-propeller engines instead of reciprocating engines (either in a restricted category aircraft that was originally
5-4 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.
In addition, any change to the aircraft must be shown to be “safe for its intended use” as required by § 21.25. See Order 8110.56A, Restricted Category Type Certification, for additional details.
5.7.3.3 If the design change includes a new special purpose, it may require a re-evaluation of the regulations for certification. See Order 8110.56A, Chapter 3.
5.7.4 Military Aircraft Designs.
Aircraft type certificated in the restricted category under § 21.25(a)(2) are accepted on the basis of the U.S. military use and other eligibility factors, instead of showing compliance with airworthiness standards in 14 CFR Chapter 1. (See Order 8110.56, Chapter 4, for additional details.) Many of these aircraft were not certificated to airworthiness standards; therefore, any modifications made to the military configuration must meet an equivalent civil certification basis derived from the airworthiness regulations contained in 14 CFR. This baseline certification basis is the airworthiness regulations (i.e., parts 23, 25, 27, 29, 33, or 35, or CARs, as appropriate) that were in effect on the date that the first military model was accepted for operational use by the U.S. Armed Forces. Section 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 cannot predate the equivalent certification basis. If these regulations do not include airworthiness standards applicable to the change, later regulations appropriate to the product category will be applied. In addition, any design change to the aircraft must be shown to be “safe for its intended use” as required by § 21.25. See Order 8110.56A for additional details.
5.7.5 Surplus Military Aircraft.
Aircraft type certificated under § 21.27 are entitled to a TC in the normal, utility, acrobatic, commuter, or transport category. 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 are certificated in the same manner as their civil counterparts.
5.7.6 Limited Category Aircraft.
Limited category aircraft are surplus military aircraft, mostly from World War II, that were type certificated under CAR part 9 for use other than air transport. These aircraft are not permitted to carry persons or property for hire, and were accepted based on their previous military qualifications. 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.
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5.8 Clarification of § 21.101(g), Part 26 Requirements.
5.8.1 Part 26 establishes requirements for support of continued airworthiness of and safety for transport category airplanes. The applicant must show compliance with each applicable provision of part 26, unless the applicant has elected or was required to comply with a corresponding amendment to part 25 that was issued on or after the date of the applicable part 26 provision. Section 21.101(g) does not allow an applicant to use an exception under § 21.101(b) for relief from complying with the applicable provisions of part 26.
5.8.2 The language in § 21.101(g) also recognizes that future part 25 amendments will be issued after the requirements in part 26 are established. Consequently, an applicant may be required to comply with a later part 25 amendment. Section 21.101(g) does not contain exception provisions for reverting to earlier part 26 requirements. However, under § 21.101(b) instead of complying with a later part 25 requirement, an applicant may be allowed to comply with an earlier part 26 or earlier part 25 requirement if justification is provided.
5.9 Documentation.
5.9.1 Documenting the Proposal.
In order to efficiently determine and agree upon a certification basis with the FAA, the following information is useful to understand your position:
• The current certification basis of the product you are changing, including amendment level.
• The amendment level of all the applicable airworthiness requirements at the date of application.
• Your proposed certification basis, including amendment levels.
• Description of the affected area.
• If you propose a certification basis that includes amendment levels earlier than what was in effect at the date of application, include the exception as outlined in § 21.101(b) and your justification if needed.
Please see appendix H for examples of optional tools you can use to document your proposed certification basis.
5.9.2 Documenting the Significant/Not Significant Decision.
5.9.2.1 The FAA determines whether the design changes are significant or not significant, and this decision is documented on the Certification Project Notification according to FAA Order 8110.115, Certification Project Initiation and Certification Project Notification. However, the FAA provides an optional decision record for the applicant to make a predetermination to facilitate the FAA decision. This form is provided in
5-6 appendix G of this AC and follows the flowchart in figure 3-1 of this AC.
If used, you should submit it along with the certification plan.
5.9.2.2 Design changes that are determined to be significant changes under § 21.101, the exceptions, and agreement of affected and unaffected areas is typically documented through the G-1 issue paper process. An example tool is provided in appendix H of this AC.
5.9.3 Documenting the Certification Basis.
5.9.3.1 The FAA will amend the certification basis for all design changes that result in a revision to the product’s certification basis on the amended TCDS or STC. The FAA will document the resulting certification basis because it is part of the compliance record required by FAA Order 8110.4C. For further information on documenting the certification basis, see FAA Order 8110.48A.
5.9.3.2 The FAA will document the certification basis of each product model on all STCs, including Approved Model List STCs.
5.10 Incorporation of STCs into the Type Design.
The incorporation of STCs into the product type design may generate an additional major change when that change is needed to account for incompatibility between several STCs that were initially not intended to be applied concurrently.
5.10.1 If the incorporation of the STC(s) does not generate an additional major change, the incorporation is not evaluated pursuant to § 21.101. The existing certification basis should be updated to include the later amendments of the STC(s) being incorporated.
5.10.2 If the incorporation of the STC(s) generates an additional major change, the change must be evaluated pursuant to § 21.101, and the existing certification basis should be updated to include the amendments resulting from the application of § 21.101.
5.11 Removing Design Changes.
Approved design changes may be removed after incorporation in an aeronautical product. These design changes will most commonly occur via an STC or service bulletin kit.
5.11.1 You should identify a product change that you intend at its inception to be removable as such, and you should develop instructions for its removal during the initial certification.
The FAA will document the certification basis for both the installed and removed configuration separately on the TCDS or STC.
5.11.2 If specific removal instructions and a certification basis corresponding to the removed condition are not established at the time of the initial product change certification, the removal of design changes or portions of those changes may constitute a significant
5-7 change to type design. A separate STC or amended TC may be required to remove modifications and the resulting certification basis established for the changed product.
5.12 The Certification Basis is Part of the Design Change.
A new design change may be installed in a product in production or via a service bulletin or STC. In terms of § 21.101, each of the approved design changes has its own basis of certification. If an applicant chooses to remove an approved installation
(e.g., interior installation, avionics equipment) and install a new installation, a new certification basis may be required for the new installation, depending on whether the change associated with the new installation is considered significant compared to the baseline configuration that the applicant chooses. If the new installation is a not significant change, the unmodified product’s certification basis may be used (not the previous installation certification basis), provided the certification basis is adequate. For example, a transport category airplane is certified as a “green” configuration. The airplane certification basis does not include § 25.562 if it was manufactured prior to October 28, 2009. An interior is installed under an STC, and the applicant elects to include § 25.562 (dynamic seats) in the certification basis to meet specific operational requirements. At a later date, the airplane is sold to a non-part 121 operator (i.e., does not have the same operating requirements). A new interior is installed; there will be no requirement for § 25.562 to be included in the new certification basis.
5.13 Sequential Design Changes—Cumulative Effects.
5.13.1 If you intend to accomplish a product change by incorporating several design changes in a sequential manner, you should identify to the FAA up front when the first application is made. In addition, the cumulative effects arising from the initial change, and from all of the follow-on changes should be included as part of the description of the change in the initial proposal. The classification of the intended product change will not be evaluated solely on the basis of the first application, but rather on the basis of all the required design changes needed to accomplish the intended product change. If the FAA determines that the current application is a part of a sequence of related changes, then the FAA will re-evaluate the determination of significance and the resulting certification basis as a group of related changes.
5.13.2 Example: Cumulative Effects—Advancing the Certification Basis.
The type certificate for airplane model X lists three models, namely X-300, X-200, and X-100. The X-300 is derived from the X-200, which is derived from the original X-100 model. An applicant proposes a design change to the X-300 airplane model. During the review of the X-300 certification basis and the regulations affected by the proposed change, it was identified that one regulation, § 25.571 (damage tolerance requirements), remained at the same amendment level as the X-100 original certification basis (exception granted on the X-200). Since the amendment level for this particular regulation was not changed for the two subsequent airplane models (X-200 and X-300), the applicant must now examine the cumulative effects of these two previous design
5-8 changes that are related to the proposed change and the damage tolerance requirements to determine whether the amendment level needs to advance.