AC 21.101-1

Subject: ESTA BLISHING THE

CERTIFICATION BASIS OF

CHANGED AERONAUTICAL

PRODUCTS Date: 4/28/03 Initiated By: AIR-110 AC No: 21.101-1 Change: 1

1. PURPOSE.

a. This advisory circular (AC) provides guidance for establishing the certification basis for changed aeronautical products and identifying the conditions under which it will be necessary to apply for a new type certificate. Title 14, Code of Federal Regulations (14 CFR) § 21.101 requires an applicant for a change to a type certificate to meet the latest requirements, except where the change is not significant, where areas of the product are not affected, where it would be impractical, or where it would not contribute materially to the level of safety of the changed product. Title 14 CFR § 21.19 identifies the conditions under which an applicant for a design change is required to make application for a new type certificate. This AC discusses and explains the criteria of 14 CFR §§ 21.19 and 21.101, and their application. It provides guidance as to the assessment of significant vs. not significant changes to the type certificated product. This document also provides guidance for the determination of substantial vs. significant changes.

b. The intent of 14 CFR § 21.101 is to enhance safety through the incorporation of the latest requirements in the certification basis for changed products. This AC describes the application of the latest airworthiness requirements for the certification of significant design changes to aircraft, aircraft engines, and propellers. Significant changes are generally distinct from the vast majority of major changes. In the assessment of whether a change is significant, all previous relevant design changes need to be taken into consideration along with any previous updates to the certification basis. All changes must be FAA approved, however, an applicant may comply with earlier amendments to the regulations based upon a finding by the Administrator that the change is not significant, an area is not affected by a change, or compliance with the latest regulation is impractical or does not contribute materially to the level of safety. Each change must be judged on its own merit when making the final determination of the certification basis.

c. This AC is not mandatory and is not a regulation. It outlines one method of compliance with 14 CFR § 21.101. The applicant may elect to follow an alternate method, provided the alternate method is acceptable to the Administrator. Because the method of compliance presented in this AC is not mandatory, the term “must” used herein applies only to an applicant who chooses to follow this particular method without deviation.

TABLE OF CONTENTS Paragraph Page

2. Applicability2

3. Related Code of Federal Regulations Paragraphs3

4. Explanation of Terminology3

5. General Overview of 14 CFR § 21.1014

Figure 1: Establishing the Certification Basis for Changed Products. 6

6. Establishing the Certification Basis for Changed Products,

14 CFR § 21.101(b)(1)7

7. Using the Criteria To Determine Significance at the Product Level,

14 CFR §§ 21.101(b)(1)(I) and (II) (Step 4)…9

8. Showing Compliance with an Earlier Requirement, 14 CFR §§ 21.101(b)(2)

and (3)11

9. Excepted Products under 14 CFR § 21.101(c)14

10. Special Conditions, 14 CFR 21.101(d)14

11. Effective Period for an Application To Change a Type Certificate,

14 CFR § 21.101(e)14

12. Other Category Aircraft, 14 CFR § 21.101(f)15

13. Documentation16

Appendix 1: Classification of Changes Figure 1. Examples of Changes for Small Airplanes (Part 23) Figure 2. Examples of Changes for Transport Airplanes (Part 25) Figure 3. Examples of Changes for Rotorcraft (Parts 27 and 29) Figure 4. Examples of Changes for Engines and Propellers (Parts 33 and 35) Appendix 2. Procedure for Evaluating Impracticality of Applying Latest Requirements to a Changed Product

1. I ntroduction

2. Example 1: 14 CFR § 25.963 Fuel Tank Access Covers

3. Example 2: 14 CFR § 25.365 Pressurized Compartment Loads Appendix 3. The Use of Service Experience in the Certification Process

1. I ntroduction

2. Guide lines

3. Example: 14 CFR § 25.1441(f) Transport Airplanes

2. APPLICABILITY.

a. This document supersedes AC 21.101-1, dated August 8, 2001. This AC is applicable to changes to a type certificate under 14 CFR § 21.101. Minor design changes are approved under 14 CFR § 21.95, and are considered to be not significant under 14 CFR § 21.101. This AC applies equally to applications made for type certificate amendments, supplemental type certificates (STCs), or amended STCs.

b. This AC is also applicable to all significant design changes to aircraft (other than rotorcraft) of 6,000 lbs. or less maximum weight, or to a non-turbine rotorcraft of 3,000 lbs. or less maximum Page 2

weight (excepted aircraft per 14 CFR § 21.101 (c)). Unless the Administrator finds the change significant in an area, an applicant may show that the changed product complies with the requirements incorporated in the type certificate.

c. This AC is also applicable for aircraft certificated under 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27.

3. RELATED CODE OF FEDERAL REGULATIONS PARAGRAPHS.

a. 14 CFR § 21.16, Special conditions.

b. 14 CFR § 21.17, Designation of applicable regulations.

c. 14 CFR § 21.19, Changes requiring a new type certificate.

d. 14 CFR § 21.21, Issue of type certificate: normal, utility, acrobatic, commuter, and transport category aircraft; manned free balloons; special classes of aircraft; aircraft engines;

propellers.

e. 14 CFR § 21.93, Classification of changes in type design.

f. 14 CFR § 21.101, Designation of applicable regulations.

g. 14 CFR § 21.115, Applicable requirements.

4. EXPLANATION OF TERMINOLOGY. The following is a summary of the terminology used throughout this advisory material. Further explanation of some of these terms is found in paragraphs 6, 7, and 8.

a. Certification basis – The applicable airworthiness requirements as established in 14 CFR §§ 21.17 and 21.101, as appropriate; special conditions; equivalent level of safety findings;

and exemptions applicable to the product to be certificated.

NOTE: This AC is not intended to be used to determine the applicable aircraft noise, fuel venting, and exhaust emission requirements for changed products.

b. Earlier requirements – The requirements in effect prior to the date of application for the change, but not prior to the existing certification basis.

c. Existing certification basis – The requirements incorporated by reference in the type certificate of the product to be changed.

d. Latest requirements – The requirements in effect on the date of application for the change.

e. Previous relevant design changes – Previous design changes, the cumulative effect of which could result in a product significantly or substantially different from the original product or model, when considered from the last time the latest regulations were applied.

f. Product level change – A change or combination of changes that makes the product distinct from other models of the product (for example, range, payload, speed). Product level change is defined at the aircraft, aircraft engine, or propeller level of change.

g. Significant change – A change to the type certificate is significant to the extent that it changes one or more of the following: general configuration, principles of construction, or the assumptions used for certification, but not to the extent to be considered a substantial change. The significance of the change must be considered in the context of all previous relevant design changes and all related revisions to the applicable regulations. Not all changes or product level changes are significant.

h. Substantial change – A change which is so extensive that a substantially complete investigation of compliance with the applicable regulations is required, and consequently a new type certificate, in accordance with 14 CFR § 21.19.

5. GENERAL OVERVIEW OF 14 CFR § 21.101.

a. Title 14 CFR § 21.19 specifies changes that require a new type certificate. When a new type certificate is required, 14 CFR § 21.17 specifies the applicable requirements for the changed product.

b. When an application for a new type certificate is not required by 14 CFR § 21.19, 14 CFR § 21.101 defines the designation of applicable requirements for determining the certification basis for the changed product.

c. Title 14 CFR § 21.101(a) requires a change to a type certificated product to comply with the latest requirements, unless the change meets the criteria for the exceptions identified in 14 CFR §§ 21.101(b) and (c). The certification basis should not be dependent on whether the type certificate holder or an applicant for a supplemental type certificate is originating the change.

Where compliance with a later amendment for a significant change does not contribute materially to the level of safety, would be impractical, or is in an area not affected by the change, the applicant may comply with earlier requirements. However, the applicant may not use requirements prior to those specified by the existing certification basis.

d. Title 14 CFR § 21.101(b) pertains to changes for which earlier requirements provide adequate standards. Earlier requirements may be used when the change is not significant. In cases where design changes involve features that have no associated regulatory standard in the existing certification basis, the Administrator will review the proposed certification plan to ensure the adequacy of the requirements against the proposed design change.

e. Title 14 CFR § 21.101(b)(1) allows the applicant to show compliance with an earlier amendment when the Administrator determines the change is not significant. Title 14 CFR Page 4

21.101(b)1(i) and (ii) pertains to changes that meet the automatic criteria where the change is significant. Title 14 CFR §§ 21.101(b)(2) and (b)(3) allows the use of earlier requirements for significant changes for areas of the product not affected by the change and for cases where compliance to the latest requirements would not contribute materially to the level of safety or would be impractical. Note that earlier amendments may not precede either the corresponding regulation incorporated in the type certificate, or any requirement found in 14 CFR §§ 23.2, 25.2, 27.2, or 29.2.

f. Title 14 CFR § 21.101(c) provides an exception to the requirements of 14 CFR § 21.101(a). An applicant for a change to an 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, may show that the changed product complies with the regulations incorporated by reference in the type certificate. The applicant may elect to comply with the later regulations. If the Administrator finds that the change is significant in an area, the Administrator may designate compliance with a later amendment to the regulations incorporated by reference in the type certificate that applies to the change and any regulation the Administrator finds is directly related. See paragraph 9 of the AC for specific guidance on this provision.

g. Title 14 CFR § 21.101(d) provides for the use of special conditions, as prescribed under 14 CFR § 21.16, when the existing certification basis or the latest regulations do not provide adequate standards with respect to the proposed change because of a novel or unusual design feature.

h. Title 14 CFR § 21.101(e) prescribes the effective period an application will remain valid for a change to a type certificate. This section is consistent with the requirements of 14 CFR § 21.17 for a new type certificate.

i. Title 14 CFR § 21.101(f) pertains to aircraft certificated under the requirements of 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27 airworthiness requirements.

j. Figure 1 provides a flowchart of the process to determine the applicable certification basis for a proposed design change under 14 CFR § 21.101, following a determination that the proposed design change is not substantial under 14 CFR § 21.19.

FIGURE 1: ESTABLISHING THE CERTIFICATION BASIS FOR CHANGED PRODUCT

Step 2.

Is the change substantial?

(21.19) Step 1. Identify the proposed change to an aeronautical product.

21.101(a) Step 3.

Will the latest requirem ents be used?

NEW TYPE

CERTIFICATE

21.17 Step 4.

Is the proposed change significant?

21.101(b)(1) Will the latest requirements be used?

Step 5.

For every area, is the area affected by the proposed change?

Step 6.

Are the new requirements practical and do they contribute materially to the level of safety?

21.101(b)(3) LATEST

REQUIREMENTS

EARLIER REQUIREMENTS BUT NOT

PRIOR TO THE EXISTING

CERT IFICATION BASIS Unaffected areas Impractical or not contributing materially to the level of safety Not significant Yes

CERTIFICATION BASIS No Yes No No Yes Yes No No Yes No Yes

NOTE 1: In the vast majority of cases, the applicant will proceed to Step 4 as the initial step in the process. See paragraph 6 for guidance.

NOTE 2: For excepted products under 14 CFR § 21.101(c), see paragraph 9.

For conditions under 14 CFR § 21.101(d), see paragraph 10.

6. ESTABLISHING THE CERTIFICATION BASIS FOR CHANGED PRODUCTS, 14 CFR §§ 21.101(b)(1).

a. The administrative burden for the applicant is to demonstrate, and for the Administrator to find, that a changed product is significant or not significant, and to determine the resulting certification basis. The certification basis can vary depending on the magnitude and scope of the change. The steps below present a streamlined approach of making this determination. In addition to assisting in the determination of significance, this guidance will help establish the appropriate amount of coordination required between the applicant and the Administrator.

b. Classifications of typical changes are in the tables of Appendix 1, Classification of Changes. For instructions on how to use the appendix 1 tables, proceed to step 4 below. In cases where the classification in appendix 1 is not applicable or immediately obvious for the proposed change, the following steps should be used in conjunction with Figure 1 to determine the appropriate certification basis for the changed product. All other areas of the aircraft are considered to be unchanged or not affected by the change and continue to comply with the existing certification basis.

c. Step 1 of Figure 1. Identify the proposed change to an aeronautical product.

(1) The applicant must, as a first step, identify the proposed change to the aeronautical product. An applicant for a change to a type certificate must consider all previous relevant design changes to the aeronautical product. Changes to a product can include physical design changes, changes to an operating envelope, and/or performance changes. The change may be a single change or a collection of changes.

(2) For each change, it is important that the effects of the change on other systems, components, equipment, or appliances of the product are properly assessed. The characteristics affected by the change are not only physical changes. 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. All other areas of the aircraft are considered to be unchanged or not affected by the change and can continue to comply with the existing certification basis.

d. Step 2 of Figure 1. Is the change substantial?

(1) Title 14 CFR § 21.19 requires that an applicant obtain a new type certificate 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 type certificate could be required for either an extensive change to a previously type certificated product or for a new design derived through a series of design changes from a previously type certificated product. The need for a new type certificate may be obvious when the change is first considered or may need a more extensive evaluation through application of 14 CFR § 21.101.

(2) A “substantially complete investigation” of compliance is required when most of the existing substantiation is not applicable to the changed product. The question of whether a change Page 7

is substantial must be addressed at the beginning of the process. However, if at any point while developing the certification basis, it becomes clear that the proposed change is a substantial change, the process ceases to be an amendment process under 14 CFR part 21, Subpart D and becomes a new type certificate process under 14 CFR Part 21, Subpart B.

(3) If it is not initially clear that a new type certificate is required, appendix 1 provides some examples of substantial changes to aid in this classification.

(4) In considering the above, a substantial change will require a new type certificate, therefore, 14 CFR § 21.19 applies; if the change is not substantial, 14 CFR § 21.101 applies.

e. Step 3 of Figure 1. Will the latest requirements be used?

(1) Where the latest requirements are used, the intent of 14 CFR § 21.101 has been met, including the case where the applicable requirements have not changed since the previous update of the certification basis, or where the applicant elects to comply with the latest amendments.

f. Step 4 of Figure 1. Is the proposed change significant? 14 CFR § 21.101(b)(1)

(1) Significant changes are typically product level changes and, by their very nature, 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 major change rising to the product level that makes the changed product distinct from others. Additionally, 14 CFR § 21.101(b)(1) defines a significant change as existing when one or more of three automatic criteria apply: (1) the general configuration is not retained; (2) principles of construction are not retained; and (3) the assumptions used for certification of the product do not remain valid. 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.

(2) Previous relevant design changes of the product 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, either singularly or collectively, the cumulative effect of previous relevant design changes must be considered. These design changes may have been incorporated through earlier changes in the type certificate on areas related to the current proposed change and the associated areas, systems, components, equipment, or appliances. The collective result may be a product considerably different from the latest updated certification basis for the product or model. For example, previous relevant aircraft design changes may address incremental increases in weight or thrust that, while individually not significant (for example, 2 percent, 4 percent, 5 percent discrete increases), can, through a series of changes, achieve a significant product level change.

(3) The applicant may use the tables in appendix 1 and the criteria described in paragraph 7 as guidance to make the classification of significant. The examples of significant and not significant changes in appendix 1 are predicated upon more than 10 years of international certification experience. One or more of the three automatic criteria in 14 CFR § 21.101(b)(1) were found in all cases where the changes were identified as significant. Other criteria reflecting Page 8

significance were not found. The concept of having only the three automatic criteria seems to fit most projects. Therefore, typically only when one or more of the three criteria is affected is the design change considered significant. The starting point to begin accumulating previous relevant design changes is the time the latest applicable requirements were applied in the affected area, system, component, equipment, or appliance.

(4) Typically, a change to a single area, system, or component will 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.

7. USING THE CRITERIA TO DETERMINE SIGNIFICANCE AT THE PRODUCT LEVEL, 14 CFR §§ 21.101(b)(1)(i) and (ii) (Step 4).

a. Typically, significant product level changes result in a model change necessitating an amendment to the type certificate or an STC that rises to the level of an amended type certificate.

Applications for a new model not associated with hardware changes, that is, commercial considerations, are not an indication of a significant change under 14 CFR § 21.101. All changes are considered in light of the change itself and its classification.

b. The following definitions build upon the criteria identified in the rule and provide additional guidance on how to apply the criteria when classifying product level changes. In cases of doubt, and to ensure a consistent outcome, the applicant is encouraged to seek the advice of the Aircraft Certification Office.

(1) Changes Where the General Configuration is Not Retained (Significant Change to General Configuration). A change to the general configuration of the product level that is likely to require a new model designation because of the need to distinguish the different product from other product models, for example, performance or interchangeability of major components.

(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 affect the overall products’ operating characteristics or inherent strength and would require extensive reinvestigation to show compliance.

(3) Changes that Invalidate the Assumptions used for Certification (Significant Change to the Assumptions used for Certification). A change to the product level assumptions associated with the compliance demonstration, performance, or operating envelope that by itself is so different that the original assumptions are invalidated. Examples may include:

(a) Change of an aircraft from an unpressurized to pressurized fuselage;

(b) Change of operation of a fixed wing aircraft from land-based to water-based; and

(c) Operation envelope expansions that are outside the existing design parameters and capabilities.

NOTE: Merely operating a product to an expanded envelope for which it was originally designed is generally not a significant change. In this case, the assumptions used for certification of the basic product remain valid and the results can be applied to cover the changed product with predictable effects or can be demonstrated without significant physical changes to the product.

c. The above criteria are used to determine if a change is significant. In applying the automatic criteria and the examples in appendix 1, the applicant should focus on the change itself.

Consideration of only the latest certification requirements is not reason enough to cause a classification of significance under 14 CFR § 21.101.

d. Appendix 1 includes tables of typical changes for transport aircraft, small aircraft, rotorcraft, and engines/propellers that meet the definition of a significant change for each product line. The appendix also includes 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 table, or

(2) In conjunction with the three automatic criteria, to help classify a proposed change not listed in the table.

e. If the change is classified as:

(1) Significant (14 CFR §§ 21.101(b)(1) and (2)). The applicant will comply with the latest amendments of the applicable requirements for certification of the changed product. The applicant can use the exceptions in 14 CFR §§ 21.101(b)(2) and/or (3) to show compliance with earlier amendments. The final certification basis may consist of a combination of the latest, and earlier or existing requirements for the change.

(2) Not Significant (14 CFR § 21.101(b)(1)). The applicable requirements are those contained in the existing certification basis. The applicant may elect to comply with later amendments.

NOTE: In cases where no regulatory standards are defined in the existing certification basis for the design change but applicable regulatory standards exist in a subsequent amendment to the regulations, the subsequent amendment will be made part of the certification basis.

f. Making the Classification. A classification of significant or not significant can be made (the application of 14 CFR § 21.101(b)(1)) in one of two ways:

(1) By delegation, where appropriate guidelines are in place to support a classification of not significant by the applicant. The Administrator may accept the not significant determination without further evaluation and rely on the applicant’s design control system and the Administrator’s oversight system to monitor and validate decisions; or Page 10

(2) By the Administrator accepting the determination of significance relevant to a major modification based on the data submitted by the applicant.

g. At this point the determination of significant or not significant has been made. For significant changes, if the applicant proposes to show compliance with an earlier requirement, the procedure outlined in paragraph 8 should be used.

8. SHOWING COMPLIANCE WITH AN EARLIER REQUIREMENT, 14 CFR §§ 21.101(b)(2) AND (3).

a. For a design change that has been determined to be significant, 14 CFR §§ 21.101(b)(2) and (3) provide the exceptions from the requirement of 14 CFR § 21.101(a) to meet the latest requirements for design changes. An applicant may elect to show compliance with an earlier amendment level or with the existing certification basis for areas not affected by the change, and 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.

b. The earlier amendment level with which the applicant intends to show compliance may not precede the corresponding requirements in the existing certification basis. It is incumbent upon the applicant to demonstrate to the Administrator that compliance with the latest requirements does not contribute materially to the level of safety, or is impractical.

c. The following steps should be used in conjunction with figure 1, when an applicant wishes to comply with an earlier requirement for a significant change:

(1) Step 5 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 product level change. For a product level change, it is important that the effects of such change on other systems, components, equipment, or appliances of the product are properly assessed because areas that have not been changed may be affected. If the significant change does not affect the area, then the certification basis of that area need not be revisited.

(b) In assessing the affected areas, it may be necessary to identify secondary changes resulting from a product level change. The secondary changes may be changes in both physical aspects and/or performance characteristics that are part of, and consequential to, the overall product level change. Secondary changes may be evaluated to the existing certification basis for the product being changed; however, care should be taken to ensure that affected areas are not overlooked. The intent is to encompass all aspects where there is a need for re-evaluation.

(c) The following aspects of a product level change should be considered:

1 Physical aspects. The physical aspects include, but are not limited to, structures, systems, equipment, components, and appliances (physical aspects can cover both “hardware” and “software”). When evaluating the physical aspects, it is necessary to make a distinction between the product level change and the resulting secondary effects. An example of a Page 11

secondary effect may be the lengthening and re-routing of the various airplane circuits as a result of a fuselage plug.

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 provisions, fire protection, structural integrity, aeroelastic characteristics, or crashworthiness. These characteristics may be affected by a product level change.

For example, adding a fuselage plug could significantly affect performance and handling qualities.

(d) All areas affected by the proposed design change must comply with the latest requirements, unless the applicant shows that demonstrating compliance with an amendment of a requirement would not contribute materially to the level of safety or would be impractical. Step 6 provides further explanation.

(2) Step 6 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) Not contributing materially to the level of safety. Compliance with the latest requirements could be considered “not to contribute materially to the level of safety” if the change to type design and/or relevant experience demonstrates a level of safety comparable to that provided by the latest requirements, or if compliance may compromise the existing level of safety for that particular changed product. The applicant must provide sufficient justification to allow the Administrator to make this determination. This exception could be applicable in the situations described in the paragraphs below:

1 Design.

(aa) 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 be materially increased by applying the latest requirements only to the changed parts since the entire modified design may not be any safer than the original design. Similarly, there may be no safety benefit in applying later requirements to both the new and unaltered components. Compliance of the new areas with the existing certification basis may be acceptable.

(bb) However, a fuselage plug may be large relative to the original certificated structure, seats, bins, doors, and cargo compartment. The change may require a new compliance substantiation that is comparable with that required for a new model airplane. In these circumstances the proposed certification basis should encompass the requirements in effect on the date of application for the change.

2 Service experience.

(aa) This provision permits the use of relevant service experience, such as fleet hours, to demonstrate that compliance with the latest requirements would not contribute Page 12