Subject: ESTABLISHING 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. 
AC 21.101-1 4/28/03 

TABLE OF CONTENTS 

Paragraph Page 

2. Applicability................................................................................................ 2 

3. Related Code of Federal Regulations Paragraphs. ...................................... 3 

4. Explanation of Terminology........................................................................ 3 

5. General Overview of 14 CFR § 21.101....................................................... 4 

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. Documentation .......................................................................................... 16 

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. Introduction 

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. Introduction 

2. Guidelines

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 
4/28/03 AC 21.101-1 

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. 

Page 3 
AC 21.101-1 4/28/03 

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 
4/28/03 AC 21.101-1 

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. 

Page 5 
AC 21.101-1 4/28/03 

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

requirements 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

CERTIFICATION 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. 

Page 6 
4/28/03 AC 21.101-1 

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 
AC 21.101-1 4/28/03 

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 
4/28/03 AC 21.101-1 

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. 

Page 9 
AC 21.101-1 4/28/03 

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 
4/28/03 AC 21.101-1 

(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 
AC 21.101-1 4/28/03 

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 
4/28/03 AC 21.101-1 

materially to the level of safety, and as such the use of earlier requirements may be appropriate. 

Appendix 3, The Use of Service Experience in the Certification Process, provides additional 

guidance on the use of service experience, along with examples. 

(bb)There may be cases for rotorcraft and small airplanes where sufficient

and relevant data may not be available 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. 

(cc) The service experience levels 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 Administrator. 

3 Other exceptions. Compliance with later requirements would not be required 

where the amendment is of an administrative nature and has been made only to correct errors or 

omissions, consolidate text, or clarify an existing requirement. 

(b) Impractical. Compliance with the latest requirements may be considered

impractical if the applicant can substantiate that it would result in additional resource requirements 

that are not commensurate with the safety benefits. The additional resource requirements could 

include those arising from design changes required for compliance and the effort required to

demonstrate compliance, but would not include resource expenditures for prior product changes. 

1 Substantiating data and analyses must support an applicant’s position that 

compliance is impractical, and the Administrator must agree with this position. In evaluating an 

applicant’s position and substantiating data regarding impracticality, the Administrator 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

comply with the latest amendments. In these cases the applicant successfully demonstrated that full 

compliance would require a substantial increase in the outlay of resources with a very small increase

in the level of safety. These cases reflect an appropriate application of “impracticality” to a changed 

product. 

3 A proposal that a product design change would be impractical would be used, 

in most cases, where compliance with the latest requirements would contribute materially to the

level of safety, but this contribution may not be commensurate with the associated resource 

expenditures. 

4 Appendix 2, Procedures for Evaluating Impracticality of Applying Latest 

Requirements to a Changed Product, provides additional guidance and examples for determining

proceures for evaluating impracticality of applying latest requirements to a changed product rule. 

Page 13 
AC 21.101-1 4/28/03 

(c) This completes the step-by-step process used in determining the certification basis 

for the changed product. 

9. EXCEPTED PRODUCTS UNDER 14 CFR § 21.101(c).

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. Beginning with the existing

certification basis, the Administrator will step through each progressive regulation to determine the 

amendment appropriate for the change. However, if the Administrator also finds that compliance

with the amendment or regulation would not contribute materially to the level of safety of the

changed product or would be impractical, the Administrator may allow compliance with an earlier 

amendment to that requirement initially designated or with the existing certification basis, 

depending on the proposed design change. 

b. For a change that contains new design features that are novel and unusual, the 

Administrator will designate the applicable special conditions 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 features, which are not covered in 

the existing certification basis, the Administrator will designate the applicable airworthiness 

requirements at the appropriate amendment level, beginning with the existing certification basis and 

progressing to the most appropriate later amendment level for the change. 

c. The exception for products under 14 CFR § 21.101(c) applies at the aircraft level only. 

Design changes to engines and propellers installed on these excepted aircraft are assessed as 

separate products using 14 CFR §§ 21.101(a) and (b). 

10. SPECIAL CONDITIONS, 14 CFR § 21.101(d). Title 14 CFR 21.101(d) allows for the 

application of special conditions, or for changes to existing special conditions, to address the 

changed design. The objective is to achieve, for the significant change, a level of safety consistent 

with that provided by the requirements in effect on the date of application for the design change. 

The application of special conditions to a design change is not, in itself, a reason for it to be 

classified as either a substantial change or a significant change. When the change is not significant,

the special conditions must be consistent with the agreed certification basis. 

11. EFFECTIVE PERIOD FOR AN APPLICATION TO CHANGE A TYPE

CERTIFICATE, 14 CFR § 21.101(e). Title 14 CFR 21.101(e) is intended to ensure that, at the 

time the changed product is certificated, no longer than three or five years, as appropriate to the 

Page 14 
4/28/03 AC 21.101-1 

product, had elapsed from the date of application. This is to ensure that the certification basis for 

the changed product is as current as practical. This is consistent with the requirements of 

14 CFR § 21.17 for a new type certificate and prescribes the process of updating the certification 

basis if these time limits are exceeded. 

12. OTHER CATEGORY AIRCRAFT, 14 CFR § 21.101(f). For aircraft type certificated 

under 14 CFR §§ 21.17(b), 21.24, 21.25, and 21.27, the certification basis for the changed product 

will consist of the amendment levels of the applicable regulations that the Administrator finds 

appropriate for the change in effect on the date of application for the change. When selecting a 

certification basis for a change, an applicant may elect to propose compliance to an earlier 

amendment using the provisions of 14 CFR § 21.101(b). The exceptions in 14 CFR § 21.101(c) do 

not apply to categories of products defined in 14 CFR § 21.101(f). 

a. Special Classes Aircraft. For special classes of aircraft, including the engines and 

propellers installed thereon (for example, gliders, airships), certificated in accordance with

14 CFR § 21.17(b), the applicable requirements will be portions of those other airworthiness 

requirements in 14 CFR Parts 23, 25, 27, 29, 31, 33, and 35 found by the Administrator to be 

appropriate for the aircraft and applicable to the specific type design, or such airworthiness criteria 

that the Administrator may find provide an equivalent level of safety to those Parts. 

b. Primary Category Aircraft. For primary category aircraft certificated under 

14 CFR § 21.24, the applicable airworthiness requirements are in 14 CFR Parts 23, 27, 31, 33, and 

35, or such other requirements as the Administrator may find 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 Administrator. 

c. Restricted Category Aircraft. For aircraft certificated in the restricted category under 

14 CFR § 21.25(a)(1), the application of the latest regulations would not normally be considered to 

contribute materially to the level of safety or be practical for its intended use. However, if the 

regulations incorporated by reference in the type certificate do not provide an appropriate level of 

safety for its intended use, the application of a later regulation would be considered. 

(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 requirements in effect at the time of the existing restricted category

type certificate 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. 

(2) For the installation of turbo propeller engines instead of reciprocating engines, either 

in a restricted category aircraft that was originally 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 must be used to provide an 

appropriate level of safety for its intended operation. 

Page 15 
AC 21.101-1 4/28/03 

d. Military aircraft designs. Aircraft type certificated in the restricted category under 

14 CFR § 21.25(a)(2) are accepted on the basis of the U.S. military use instead of showing

compliance with airworthiness standards in 14 CFR Chapter 1. Many of these aircraft were not 

certificated to a specific set of airworthiness standards, therefore, an appropriate equivalent civilian 

certification basis could be determined using the table in 14 CFR § 21.27 for surplus military

aircraft. Title 14 CFR § 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 may not predate the equivalent

certification basis. If these regulations do not include design standards applicable to the change, 

later regulations appropriate to the product category will be applied. The goal is to maintain a level 

of safety at least equivalent to the original design and appropriate for the aircraft’s intended use. 

e. Surplus military aircraft. Aircraft type certificated under 14 CFR § 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. 

f. Limited category aircraft. Limited category aircraft are surplus military aircraft, mostly

from World War II, that were type certificated under Part 9 of the Civil Air Regulations for use in 

other than air transport. These aircraft were not intended to carry persons or property for hire, and 

normally were accepted on the basis of their previous military qualifications acceptance and service 

record. However, a change to these aircraft not supported by the military service history must 

comply with appropriate airworthiness standards. The appropriate standard should be determined 

with recognition that the aircraft has not been type certificated to a civil aircraft airworthiness 

standard. Therefore, a change to an aircraft of this type may not realize a safety benefit by

complying with later airworthiness standards. 

13. DOCUMENTATION. All changes that result in a revision to the product’s certification 

basis must be reflected on the type certification data sheet. Similarly, the certification basis must be

noted on all STCs. 

Original signed by 

David W. Hempe 

Manager, Aircraft Engineering Division, 

Aircraft Certification Service

Page 16 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

Appendix 1 includes tables of typical changes for small aircraft (Figure 1), transport aircraft

(Figure 2), rotorcraft (Figure 3), and engines/propellers (Figure 4) that meet the definition of a 

significant change or substantial change for each product line. The appendix also includes 

typical changes that do not achieve the significant level.

(a) The examples in the tables were developed using data from Administrator files and 

included industry review and input. They clearly are changes that we have seen in the past and 

will likely continue to see in the future. The Administrator has made the determination, based on 

applying the automatic criteria, that these changes are significant or not significant. 

NOTE: The small airplane table (Figure 1) was developed working

with industry representatives who identified 500+ distinct, typical 

changes for these airplanes. These examples were validated by a 

comprehensive review of Part 23 Certification Project Notifications 

(CPNs) for 1998, 1999, 2000, and 2001 (approximately 1,800 cases). 

The examples in appendix 1 reflect many of the significant, product 

level changes we would expect to see. The transport airplane table 

(Figure 2) was developed, in part, by reviewing all STC/ATC 

Certification Project Notifications for 2000 and 2001. The 3,000+ 

change projects for these two years represented 324 distinct major 

change projects the Transport Airplane Directorate reviewed over the 

two-year period. Part 25 industry representatives further reviewed and 

validated the examples. 

(1) The Figure 3 rotorcraft table was developed by a working group of Administrator 

and industry representatives. They were validated by the Rotorcraft Directorate review of recent 

applications for product changes and reflect changes the rotorcraft community typically makes. 

(2) The Engine and Propeller Directorate developed the Figure 4 engine and propeller 

table, and representatives of the engine manufacturers reviewed and validated it. They reflect the 

types of changes the engine and propeller industry would typically make to their products. 

(b) The columns “Change to General Configuration,” “Change to Principles of Construction,” 

and “Assumptions of Certification” reflect the automatic criteria of 14 CFR §§ 21.101(b)(1)(i) and 

(ii). The “Notes” column provides typical rationales that are considered in evaluating the designation 

of the criteria. 

(c) The tables can be used: 

(1) To classify a proposed change that is listed in the table, or 

(2) With the three automatic criteria to understand the table’s logic and to help classify

a proposed change not in the table. 

Page 1 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES (CONTINUED)

(d) The classification may change due to cumulative effects and/or combinations of 

individual changes.

Page 2 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23)

The following tables of substantial and significant changes are based on 10 years of international 

experience, and were added/revised by revisitation of the Directorates & U.S. Industry.

The following are examples of SUBSTANTIAL changes: 

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Change in wing location 

(tandem, forward, 

canard, high/low)

NA NA NA Proposed change in design is

so extensive that a 

substantially complete 

investigation of compliance 

with the applicable 

regulations is required. 

Fixed wing to tilt wing NA NA NA Proposed change in design is

so extensive that a 

substantially complete 

investigation of compliance 

with the applicable 

regulations is required. 

Increase in the number 

of engines from one to

two

NA NA NA Proposed change in design is

so extensive that a 

substantially complete 

investigation of compliance 

with the applicable 

regulations is required. 

Replacement of piston or 

turbo-prop engines with

turbojet or turbofan

engines 

NA NA NA Proposed change in design is

so extensive that a 

substantially complete 

investigation of compliance 

with the applicable 

regulations is required. 

Change in engine

configuration 

(tractor/pusher) 

NA NA NA Proposed change in design is

so extensive that a 

substantially complete 

investigation of compliance 

with the applicable 

regulations is required. 

Page 3 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of SUBSTANTIAL changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Change from an all 

metal airplane to all 

composite primary

structure (fuselage, 

wing, empennage) 

NA NA NA

Increase from

subsonic to supersonic 

flight regime

NA NA NA

Page 4 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

The following are examples of SIGNIFICANT changes:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Conventional tail to

T-tail or Y-tail, or vice 

versa

Yes No Yes Change in general

configuration. Requires

extensive structural, flying

qualities and performance 

reinvestigation. Requires 

new AFM to address

performance and flight

characteristics.

Changes in wing 

configuration, addition

of tail strakes or change

in dihedral, or changes in

wing span, flap or 

aileron span, angle of

incidence of the tail, 

addition of winglets, or

wing sweep of more than

10 percent

Yes No Yes Change in general

configuration. Likely

requires extensive changes to 

wing structure. Requires new

AFM to address performance 

and flight characteristics.

NOTE: Small changes to

wingtip are not significant

changes. See table for not 

significant changes. 

Tricycle/tailwheel 

undercarriage change or 

addition of floats 

Yes No No Change in general

configuration. Likely, at

airplane level, general

configuration and

certification assumptions

remain valid.

Increase in seating

capacity resulting in a 

different certification

category (e.g., from

normal to commuter 

category) where 

configuration or

principles of

construction changes or

assumptions do not 

remain valid. 

Yes Yes Yes Change in general

configuration. Change in

principles of construction. 

Requires extensive 

construction reassessment. 

Change in certification

assumptions. Requires new

AFM and pilot type rating.

Page 5 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of SIGNIFICANT changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Passenger to freighter

configuration conversion 

which involves the 

introduction of a cargo

door or an increase in

floor loading of more 

than 20 percent, or 

provision for carriage of

passengers and freight

together 

Yes No Yes Change in general

configuration affecting load

paths, aeroelastic 

characteristics, aircraft

related systems, etc. Change

in design assumptions.

A fuselage stretch would

be considered significant 

if it would invalidate the 

existing substantiation,

or would change the

primary structure, 

aerodynamics, or 

operating envelope

sufficiently to invalidate 

the assumptions of

certification.

Yes No Yes Likely extensive changes to

fuselage structure,

aerodynamics, aircraft

systems performance, and 

operating envelope.

Requires new AFM to

address performance and 

flight characteristics. 

Replace reciprocating

engines with the same

number of turbopropeller engines where 

the operating envelope is 

expanded. 

No No Yes Invalidates certification

assumptions. Requires new

AFM to address performance 

and flight characteristics.

Page 6 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Addition of a turbocharger that changes the 

power envelope, 

operating range, or

limitations appreciably

No No Yes Invalidates certification

assumptions due to changes

in operating envelope and 

limitations. 

Requires new AFM to

address performance and 

flight characteristics.

The replacement of an

engine of higher rated

power or increase thrust

would be considered 

significant if it would

invalidate the existing

substantiation, or would

change the primary

structure, aerodynamics, 

or operating envelope 

sufficiently to invalidate 

the assumptions of

certification.

No Yes Yes Invalidates certification

assumptions. Requires new

AFM to address performance 

and flight characteristics. 

Likely changes to primary

structure. Requires extensive 

construction reinvestigation.

A change in the type of

material, such as

composites in place of

metal, or one composite

fiber material system

with another (e.g., 

carbon for fiberglass), 

for primary structure 

would normally be

assessed as a significant 

change. 

No Yes Yes Change in principles of

construction and design from

conventional practices. 

Likely change in

design/certification

assumptions.

Page 7 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of SIGNIFICANT changes, continued:

Description of 

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Change involving

appreciable increase 

in design speeds Vd, 

Vmo, Vc, or Va

No No Yes Certification assumptions

invalidated. Requires new 

AFM to address 

performance and flight 

characteristics.

STOL kit No No Yes Certification assumptions

invalidated. Requires new 

AFM to address 

performance and flight 

characteristics.

A change in the rated 

power or thrust is 

likely to be regarded 

as significant if the 

design speeds are 

thereby changed so 

that compliance needs 

to be rejustified with a 

majority of 

requirements. 

No No Yes Certification assumptions

invalidated. Requires new 

AFM to address 

performance and flight 

characteristics.

Fuel state: such as 

compressed gaseous 

fuels, or fuel cells. 

This could completely

alter the fuel storage 

and handling systems 

and possibly affect the 

airplane structure. 

No No Yes Changes in

design/certification 

assumptions. Extensive 

alteration of fuel storage 

and handling systems.

Page 8 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

A design change that 

alters the aircraft 

flight characteristics 

or performance from

the type design would 

normally be 

significant if it 

appreciably changes 

the kinematics or 

dynamics of the 

airplane. 

No No Yes Certification assumptions

invalidated. 

Requires new AFM to 

address performance and 

flight characteristics.

Weight increase that 

places the aircraft into 

the commuter 

category (i.e., above 

12,500 lbs) 

No No Yes Certification assumptions

invalidated. Requires new 

AFM.

A change in the flight 

control concept for an 

aircraft, for example, to 

fly by wire (FBW) and 

side-stick control, or a 

change from hydraulic 

to electronically

actuated flight controls, 

would in isolation 

normally be regarded as 

a significant change. 

No No Yes Changes in design and 

certification assumptions. 

Requires extensive 

systems architecture and 

integration reinvestigation. 

Requires new AFM.

Addition of cabin 

pressurization 

No Yes Yes Extensive airframe changes 

effecting load paths, fatigue 

evaluation, aero elastic 

characteristics, etc. 

Requires extensive 

construction reinvestigation. 

Invalidates design

assumptions.

Page 9 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of SIGNIFICANT changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Changes in types and 

number of emergency

exits or an increase in 

passenger capacity in 

excess of maximum 

passenger capacity

demonstrated for the 

aircraft type. 

No No Yes Emergency egress

requirements exceed those 

previously substantiated. 

Invalidates assumptions of 

certification.

A change in the 

required number of 

flight crew, which 

necessitates a complete 

cockpit rearrangement, 

and/or an increase in 

pilot workload would 

be a significant change. 

No No Yes Extensive changes to

avionics and aircraft 

systems. Invalidates 

certification assumptions. 

Requires new AFM.

An appreciable 

expansion of an 

aircraft’s operating

envelope or operating

capability would 

normally be a 

significant change, 

e.g., an increase in 

maximum altitude 

limitation, approval 

for flight in known 

icing conditions, an 

increase in airspeed 

limitations 

No No Yes Invalidates certification

assumptions. Requires 

new AFM to address 

performance and flight 

characteristics.

Page 10 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of

change 

Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

A major flight deck

upgrade 

No No Yes Extensive changes to

avionics and electrical 

systems design.

Invalidates certification 

assumptions. 

Extensive reassessments of 

systems integration, flight 

crew workload, and human 

factors evaluation are 

required. Requires new 

AFM.

Introduction of 

autoland 

No No Yes Invalidates original design

assumptions.

Page 11 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

The following are examples of NOT SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Addition of wingtip 

modifications (not 

winglets) 

No No No A major change to the

airplane. Likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Installation of skis or 

wheel skis 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

FLIR or surveillance 

camera installation 

No No No Additional flight or

structural evaluation may

be necessary, but the 

change does not alter basic 

airplane certification. 

Litter, berth and cargo

tie down device 

installation 

No No No Not an airplane level

change. 

Increased tire size, 

including tundra tires 

No No No Not an airplane level

change. 

Replacement of one 

propeller type with 

another (irrespective of 

increase in number of 

blades) 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Page 12 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Addition of a turbo-charger 

that does not change the 

power envelope, operating

range, or limitations (e.g., a 

turbo-normalized engine,

where the additional power 

is used to enhance high 

altitude or hot day

performance). 

No No No Not an airplane level change. 

Replacement of petrol 

engine with a diesel 

engine or approximately

the same horsepower. 

No No No Although a major change to

the airplane, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid.

Substitution of one 

method of bonding for 

another (e.g., change in

type of adhesive) 

No No No Not an airplane level change. 

Substitution of one type

of metal for another 

No No No Not an airplane level change. 

Any change in

construction or fastening 

not involving primary

structure 

No No No Not an airplane level change. 

A new fabric type for 

fabric skinned aircraft

No No No Not an airplane level change. 

Increase in flap speed or 

undercarriage limit speed 

No No No Although a major change to

the airplane, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid. 

Page 13 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Structural strength 

increases 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

IFR upgrades involving

installation of 

components (where the 

original certification 

does not indicate that 

the airplane is not 

suitable as an IFR

platform, e.g., special 

handling concerns). 

No No No Not an airplane level

change. 

Fuel lines, where 

engine horsepower is 

increased but fuel flow

is not increased beyond 

the certificated 

maximum amount. 

No No No Not an airplane level

change. 

Fuel tanks, where fuel 

is changed from

gasoline to diesel fuel 

and tank support loads 

are small enough that 

an extrapolation from

the previous analysis 

would be valid. 

Chemical compatibility

would have to be 

substantiated. 

No No No Not an airplane level

change. 

Page 14 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Limited changes in a 

pressurization system, 

e.g., number of outflow 

valves, type of 

controller, or size of 

pressurized 

compartment, but the 

system must be 

resubstantiated if the 

original test data are 

invalidated. 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Install a quieter exhaust 

system

No No No Not an airplane level

change. 

Changes in engine 

cooling or cowling

No No No Not an airplane level

change. 

Fuel type: AvGas to 

Diesel/Jet A, AvGas to

Ethanol/Methanol. 

Changing to multiple 

fuel systems containing

fuel types (other than 

systems used for 

starting): such as 

AvGas/Ethanol, or 

Jet A/AutoGas

(turbine). Unrestricted 

mixtures in one fuel 

system of different fuel 

types: such as 

AvGas/Diesel or Jet

A/Ethanol. 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Page 15 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Fuels of substantially

the same type: such as 

AvGas to AutoGas,

AvGas (80/87) to 

AvGas (100LL), 

ethanol to isopropyl 

alcohol, Jet B to Jet A 

(although Jet A to Jet B 

may be considered 

significant due to the 

fact that Jet B is 

considered potentially

more explosive). 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Fuels that specify

different levels of 

“conventional” fuel 

additives that do not 

change the primary fuel 

type. Different 

additive levels 

(controlled) of MTBE, 

ETBE, ethanol, amines, 

etc., in AvGas would 

not be considered a 

significant change. 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

A change to the 

maximum take-off

weight of less than 5 

percent, unless 

assumptions made in 

justification of the 

design are thereby

invalidated. 

No No No Although a major change

to the airplane, likely the 

original general 

configuration, principles 

of construction, and 

certification assumptions 

remain valid.

Page 16 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

An additional aileron tab

(e.g., on the other wing)

No No No Although a major change to

the airplane, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid.

Larger diameter flight

control cables with no

change in routing, or

other system design 

No No No Not an airplane level change. 

Autopilot installation (for

IFR use, where the 

original certification does 

not indicate that the 

airplane is not suitable as 

an IFR platform) 

No No No Although a major change to

the airplane, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid.

Increased battery capacity

or relocate battery

No No No Not an airplane level change. 

Replace generator with

alternator 

No No No Not an airplane level change. 

Additional lighting (e.g., 

navigation lights, strobes)

No No No Not an airplane level change. 

Higher capacity brake 

assemblies 

No No No Not an airplane level change. 

Increase in fuel tank

capacity

No No No Not an airplane level change. 

Addition of an oxygen

system 

No No No Not an airplane level change. 

Relocation of a galley No No No Not an airplane level change.

Page 17 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 23) 

(CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Passenger to freight

(only) conversion with no

change to basic fuselage

structure. 

No No No Although a major change to

the airplane, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid. 

Requires certification

substantiation applicable to

freighter requirements.

Installation of new seat 

belt or shoulder harness

No No No Not an airplane level change. 

A small increase in cg

range

No No No At airplane level, no change

in general configuration,

principles of construction,

and certification assumptions.

APU installation that is 

not flight essential 

No No No Although a major change to

the airplane level, likely the 

original general 

configuration, principles of

construction, and certification

assumptions remain valid. 

Requires certification

substantiation applic. to APU 

installation requirements.

An alternative autopilot No No No Not an airplane level change.

Addition of Class B

Terrain Awareness and 

Warning Systems 

(TAWS)

No No No Not an airplane level change. 

Page 18 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 2. EXAMPLES OF CHANGES FOR TRANSPORT AIRPLANES 

 (Part 25)

The following are examples of SUBSTANTIAL changes: 

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Change in the number 

or location of engines, 

e.g., four to two wingmounted engines or 

two wing-mounted to 

two body-mounted 

engines. 

NA NA NA Proposed change in design

is so extensive that a 

substantially complete 

investigation of 

compliance with the 

applicable regulations is 

required. 

Change from a highwing to low-wing

configuration. 

NA NA NA Proposed change in design

is so extensive that a 

substantially complete 

investigation of 

compliance with the 

applicable regulations is 

required. 

Change from an all 

metal airplane to all 

composite primary

structure (fuselage, 

wing, empennage). 

NA NA NA Proposed change in design

is so extensive that a 

substantially complete 

investigation of 

compliance with the 

applicable regulations is 

required. 

Page 19 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 25) 

(CONTINUED)

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Derivative model, e.g., 

increased passenger 

payload, freighter version 

or complete update of a 

certificated airplane. 

Yes Yes Yes Multiple changes packaged

into a new model. Increased 

payload or new freighter 

would change the general

configuration and

assumptions. Updated

airplane could change

principles of construction.

Reduction in the number 

of flight crew (In

conjunction with flight

deck update). 

Yes No No Extensive changes to avionics

and aircraft systems. Impact 

to crew workload and human

factors, pilot type rating.

Modify an airplane for 

flight in known icing 

conditions by adding

systems for ice detection

and elimination.

Yes No Yes New aircraft operating

envelope. Requires major new

systems installation and aircraft 

evaluation. Operating envelope

changed. 

Conversion – passenger

or combo to all freighter, 

including cargo door, 

redesign floor structure 

and 9g net or rigid barrier 

Yes No Yes Extensive airframe changes

affecting load paths, 

aeroelastic characteristics,

aircraft related systems for 

fire protection, etc. Design

assumptions changed from

passenger to freighter.

Change to pressurized 

cabin, including the 

introduction of a 

pressurization system.

No No Yes Essentially a recertification of

airframe and systems

associated with operating

envelope change.

Addition of leading edge

slats 

Yes No No Requires extensive changes to 

wing structure, adds aircraft

systems, and requires a new

airplane flight manual to

address performance and 

flight characteristics.

Page 20 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 2. EXAMPLES OF CHANGES FOR TRANSPORT AIRPLANES (Part 25) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Fuselage length change –

lengthen or shorten 

fuselage

Yes No No Only when it requires 

extensive changes to fuselage

structure, affects aircraft

systems, and requires a new

airplane flight manual to

address performance and 

flight characteristics.

Extensive structural 

airframe modification,

such as installation of a 

large telescope with large 

opening in fuselage.

Yes No No Requires extensive changes to 

fuselage structure, affects

aircraft systems, and requires

a new airplane flight manual

to address performance and 

flight characteristics.

Changing the number of

axles or number of

landing gear done in

context with a product 

change that involves

changing the airplane

gross weight. 

Yes No No Requires extensive changes to 

aircraft structure, affects

aircraft systems, and requires

AFM changes.

Primary structure changes

from metallic material to

composite material. 

No Yes No Change in principles of

construction and design from

conventional practices. 

Typically, an increase in

design weight of more

than 10 percent.

No No Yes Requires extensive

resubstantiation of aircraft

structure, aircraft

performance and flying

qualities and associated

systems.

Page 21 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 25) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Wing changes in span,

sweep, and tip designs or 

wing chord. 

(NOTE: Potentially

substantial if it is a 

change from a high wing 

to a low wing, or a new 

wing.)

Yes No No When it requires extensive 

changes to wing structure,

adds aircraft systems, and 

requires a new airplane flight

manual to address 

performance and flight

characteristics.

Change in type or number

of emergency exits or an

increase in the number of

passengers demonstrated.

No No Yes The new emergency egress

requirements exceed those 

previously substantiated. 

Comprehensive flight

deck upgrade. 

No No Yes Affects avionics and 

electrical systems integration

and architecture concepts and 

philosophies. This drives a 

reassessment of flight crew

workload and other human

factors issues, and requires a 

re-evaluation of the original 

design assumptions used for

the cockpit. 

Change in primary flight

controls to fly by wire 

(FBW) system. 

(Some airplanes have

some degree of FBW. 

Achieving full FBW may

be a not significant

change on some

airplanes.) 

Yes No Yes When the degree of change is

so extensive that it affects basic 

aircraft systems integration and

architecture concepts and 

philosophies. This drives a 

complete reassessment of flight

crew workload, handling 

qualities, and performance 

evaluation, which are different

from the original design

assumptions.

Page 22 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 2. EXAMPLES OF CHANGES FOR TRANSPORT AIRPLANES (Part 25) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Replace reciprocating

with turbo-propeller 

engines. 

Yes No No Requires extensive

changes to airframe

structure, addition of 

aircraft systems, and new 

airplane flight manual to 

address performance and 

flight characteristics. 

Typically a thrust 

increase of more than 

10 percent. 

No No Yes Requires resubstantiation

of powerplant installation, 

and has a marked affect on 

aircraft performance and 

flying qualities. 

Initial installation of an 

autoland system. 

No No Yes Baseline airplane not 

designed for autoland 

operation, potential crew 

workload and systems 

compatibility issues. 

Installation of a new 

fuel tank, (horizontal 

stabilizer tank or 

auxiliary fuel tank in 

the fuselage outside the 

wing in conjunction 

with increased 

maximum takeoff

weight and takeoff 

thrust) 

No No Yes Requires changes to

airframe, systems and 

AFM. Results in 

performance changes. 

Main deck cargo door 

installation. 

Yes No No Redistribution of internal 

loads, change in 

aeroelastic characteristics, 

system changes. 

Page 23 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 25) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR §

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Conversion from a 

passenger floor to a 

cargo floor and 

installation of a cargo

handling system. 

No No Yes Completely new floor

loading and design. 

Redistribution of internal 

loads, change in cabin 

safety requirements, 

system changes. 

Initial installation of an 

APU essential for 

aircraft flight 

operation. 

No No Yes Changes emergency

electrical power 

requirements, change in 

flight manual and 

operating characteristics. 

Page 24 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 2. EXAMPLES OF CHANGES FOR TRANSPORT AIRPLANES (Part 25) 

(CONTINUED)

The following are examples of NOT SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Alternate engine 

installation or hush kit at 

same position. 

No No No Typically it is not significant 

so long as there is not more 

than a 10 percent increase in 

thrust or a change in the 

principles of propulsion. 

Fuselage length changes 

– lengthen or shorten 

fuselage. 

No No No A small change in fuselage

length due to refairing the aft 

body or radome for cruise 

performance reasons, where 

such changes do not require 

extensive structural, systems, 

or AFM changes. 

Refairing of wing tip 

caps (for lights, fuel 

dump pipes) and 

addition of splitter plates 

to the trailing edge

thickness of the cruise 

airfoil. 

No No No Does not require extensive 

structural, AFM, or systems 

changes. 

Additional power used 

to enhance high altitude 

or hot day performance. 

No No No Usually no change in basic 

operating envelope. Existing

certification data can be 

extrapolated. Could be 

significant product change if 

the additional power is 

provided by installation of a 

rocket motor or additional, on 

demand engine due to changes 

in certification assumptions. 

Page 25 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 1. EXAMPLES OF CHANGES FOR SMALL AIRPLANES (Part 25) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a change 

to the general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a change 

to the principles 

of construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

General avionics 

changes. 

No No No These modifications are 

generally adaptive* in nature, 

and do not change the original 

certification assumptions, alter 

basic cockpit design

architecture concepts and 

philosophies, and do not have

a major impact on crew

workload or man/machine. 

*Adaptive means the change

adapts to the existing airplane 

buses, power, structure, etc. 

Initial installation of an 

autopilot system. 

No No No Modification is generally

adaptive in nature, with no 

change to original certification 

assumptions. 

Integrated modular 

avionics. 

No No No The basic functionality of the 

systems is unchanged. No 

change from analog to digital. 

Installation or 

rearrangement of an 

interior in an aircraft. 

No No No Special conditions could be 

required for new and novel 

features 

Change from assembled

primary structure to 

monolithic or integrally

machined structure. 

No No No Method of construction must 

be well understood. 

Modification to ice 

protection systems. 

No No No Recertification required, but 

certification basis is adequate. 

Brakes: design or 

material change, e.g., 

steel to carbon. 

No No No Recertification required, but 

certification basis is adequate. 

Page 26 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 2. EXAMPLES OF CHANGES FOR TRANSPORT AIRPLANES (Part 25) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a change 

to the general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a change 

to the principles 

of construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Redesign floor structure. No No No By itself, not a significant 

product change. It could be 

significant if part of a cargo

converted passenger airplane. 

Novel or unusual 

method of construction 

of a component. 

No No No The component change does 

not rise to the product level. 

Special conditions could be 

required if there are no 

existing regulations that 

adequately address these 

features. 

Initial installation of a 

non-essential APU. 

No No No A stand-alone initial APU

installation on an airplane 

originally designed to use 

ground/airport supplied 

electricity, and airconditioning. In this case, the 

APU would be an option to be 

independent of airport power. 

Page 27 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

The following are examples of SUBSTANTIAL changes: 

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR §

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Change from the 

number and or 

configuration of rotors 

(e.g., main & tail rotor 

system to two main 

rotors. 

NA NA NA Proposed change in design

is so extensive that a 

substantially complete 

investigation of 

compliance with the 

applicable regulations is 

required. 

Change from an all 

metal rotorcraft to all 

composite rotorcraft. 

NA NA NA Proposed change in design

is so extensive that a 

substantially complete 

investigation of 

compliance with the 

applicable regulations is 

required. 

Page 28 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Comprehensive flight

deck upgrade. 

No No Yes The degree of change is so

extensive that it affects basic 

avionics and electrical systems

integration and architecture 

concepts and philosophies. This 

drives a complete reassessment 

of flight crew workload and 

other human factor issues, and 

requires a re-evaluation of the 

original design assumptions used

for the cockpit. 

Certification for flight

into known icing

conditions. 

No No Yes

(Fixed) flying controls

from mechanical to fly by

wire.

Yes Yes Yes

Addition of an engine; 

e.g., from single to twin

or reduction of the 

number of engines; e.g.,

from twin to single. 

Yes No Yes May be a substantial change

depending upon project 

details.

A fuselage modification

that changes the primary

structure, aerodynamics, 

and operating envelope 

sufficiently to invalidate 

the certification

assumptions.

Yes No Yes

Page 29 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Application of an

approved primary

structure to a different

approved model (e.g., 

installation on a former 

model of the main rotor 

approved on a new model

that results in increase 

performance). 

No Yes Yes

Extensive primary

structure changes from

metallic material to

composite material.

No Yes Yes Change in principles of

construction and assumptions

used for certification for the 

product level change. Changes 

of a few individual elements

from metal to composite are not 

typically considered a significant 

change. 

Emergency Medical 

Service (EMS) 

Configuration with

primary structural 

changes sufficient to 

invalidate the certification

assumptions.

No No Yes Many EMS configurations will 

not be classified as significant. 

Modifications made for EMS are 

typically internal, and the 

general external configuration is

normally not affected. These 

changes should not automatically

be classified as significant. 

Skid landing gear to

wheel landing gear or

wheel landing to skid. 

Yes No Yes If the rotorcraft is such that the 

skid or wheel configuration is 

inherent in the basic certification

design, the change may be not

significant.

Page 30 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

Examples of SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Change of the number 

of rotor blades. 

Yes No No Thee addition/deletion of 

rotorblades may not be 

significant, provided the 

remainder of the basic 

propulsion system remains 

essentially unchanged. 

Change tail anti-torque 

device (e.g., tail rotor, 

ducted fan or other 

technology). 

Yes Yes No

Page 31 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

The following are examples of NOT SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Emergency floats No No No Must comply with the specific 

applicable requirements for 

emergency floats. This

installation, in itself, does not 

change the rotorcraft

configuration, overall

performance, or operational

capability. Expanding an

operating envelope (such as

operating altitude and

temperature) and mission profile 

(such as passenger carrying 

operations to external load

operations, or flight over water, 

or operations in snow conditions) 

are not by themselves so different

that the original certification

assumptions are no longer valid at

the type certificated product level. 

FLIR or surveillance 

camera installation

No No No Additional flight or structural 

evaluation may be necessary but 

the change does not alter the basic 

rotorcraft certification.

Helicopter Terrain

Awareness Warning

System (HTAWS) for 

operational credit 

No No No Certificated per rotorcraft

HTAWS AC guidance material. 

Health Usage Monitoring

System (HUMS) for

Maintenance Credit 

No No No Certificated per rotorcraft HUMS

AC guidance material. 

Page 32 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Expanded limitations 

with minimal or no 

design changes, 

following further 

tests/justifications or 

different mix of 

limitations (CG limits, 

oil temperatures, 

altitude, 

minimum/maximum 

weight, minimum/max

external temperatures, 

speed, ratings structure) 

No No No Expanding an operating

envelope (such as operating

altitude and temperature) and 

mission profile (such as 

passenger carrying operations to 

external load operations, or 

flight over water, or operations 

in snow conditions) are not by

themselves so different that the 

original certification 

assumptions are no longer valid 

at the type certificated product 

level. 

Installation of a new 

engine type, equivalent 

to the former one; 

leaving aircraft 

installation and 

limitations 

substantially

unchanged 

No No No Refer to AC 27-1 or AC 29-2 

for guidance 

Windscreen installation No No No Does not change the rotorcraft 

overall product configuration. 

Page 33 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Snow skis, “Bear Paws” No No No Must comply with specific 

requirements associated with the 

change. Expanding an operating 

envelope (such as operating 

altitude and temperature) and

mission profile (such as passenger 

carrying operations to external 

load operations, or flight over 

water, or operations in snow

conditions) are not by themselves 

so different that the original 

certification assumptions are no

longer valid at the type

certificated product level. 

External cargo hoist No No No Must comply with the specific 

applicable requirements for external 

loads. This installation, in itself,

does not change the rotorcraft

configuration, overall performance, 

or operational capability. 

Expanding an operating envelope

(such as operating altitude and

temperature) and mission profile 

(such as passenger carrying 

operations to external load

operations, or flight over water, or 

operations in snow conditions) are 

not by themselves so different that 

the original certification

assumptions are no longer valid at

the type certificated product level. 

Page 34 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 3. EXAMPLES OF CHANGES FOR ROTORCRAFT (Parts 27 and 29) 

(CONTINUED)

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

IFR upgrades involving

installation of 

components (where the 

original certification 

does not indicate that 

the rotorcraft is not 

suitable as an IFR

platform, e.g., special 

handling concerns). 

No No No Not a rotorcraft level change. 

An upgrade to CAT A 

certification approval 

No No No Typically these are engine and 

drive systems rating changes 

appropriate for CAT A and 

rotorcraft performance 

requirements. These changes 

do not typically invalidate the 

certification assumptions, or 

change the general 

configuration of principles of 

construction. 

Reducing the number 

of pilots for IFR from 

2 to 1 

No No No May be significant if there are 

extensive equipment and design

changes such that the 

certification assumptions are 

invalidated or the general 

configuration of the rotorcraft is

changed. 

Page 35 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

(Parts 33 and 35) 

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Turbine Engines

Traditional turbofan to 

geared-fan engine 

Yes No Yes This change would affect the 

engine in terms of foreign 

object ingestion, etc. 

Note that this change is most 

likely substantial under 21.19

Low bypass ratio 

engine to high bypass 

ratio engine with an 

increased inlet area 

Yes No Yes Change in general

configuration 

Likely change in model 

designation 

Not interchangeable 

Assumptions for certification 

may no longer be valid in 

terms of ingestion, icing, etc. 

Note that this change is most 

likely substantial under 21.19

Turbojet to Turbofan Yes No Yes Change in general 

configuration 

Likely change in model 

designation 

Not interchangeable 

Assumptions for certification 

may no longer be valid lifting,

ingestion, icing, blade out 

criteria, etc. 

Note that this change is most 

likely substantial under 21.19

Page 36 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Turbo-shaft to turbopropeller Yes No Yes Change in configuration such 

as an additional gearbox 

Change in model designation 

Change in mission profile 

Assumptions for certification 

may no longer be valid in 

terms of flight envelope, 

ratings, etc 

Note that this change is most 

likely substantial under 21.19 

Conventional ducted 

fan to unducted fan 

Yes Yes Yes Change in configuration 

Change in type 

Not interchangeable 

Assumptions for certification 

may no longer be valid 

Note that this change is most 

likely substantial under 21.19

Conventional engine 

for subsonic operation 

to afterburning engine 

for supersonic 

operation 

Yes Yes Yes Change in configuration

Change in type 

Not interchangeable 

Assumptions for certification 

may no longer be valid 

Change in operating envelope 

Note that this change is most 

likely substantial under 21.19 

Page 37 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

(Parts 33 and 35) (CONTINUED)

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Increase/decrease in 

the number of 

compressor/turbine 

stages with resultant 

change in approved 

limitations* 

(* exclude life limits) 

No No Yes Change is associated with 

other changes that would 

affect the rating of the engine 

and have affected the dynamic 

behavior, in terms of backbone 

bending, torque spike effects 

on casing, surge and stall 

characteristics, etc.

New design fan blade 

and fan hub, or a 

bladed fan disk to a 

blisk, or a fan diameter 

change, that could not 

be retrofitted. 

Yes No Yes Likely change in model

designation 

Change is associated with 

other changes that would 

affect engine thrust/power 

limitations and have affected 

the dynamic behavior of the 

engine in terms of backbone 

bending, torque spike effects 

on casing, foreign object 

ingestion behavior, burst 

model protection for the 

aircraft. If there is a diameter 

change, installation will be 

also affected.

Page 38 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Hydro-Mechanical 

control to FADEC/EEC 

without hydro mechanical 

backup

Yes No Yes Change in engine control

configuration 

Likely change in model 

designation 

Not interchangeable 

Likely fundamental change to

engine operation 

Assumptions used for

certification are no longer valid or

were not addressed in the original 

certification, i.e., HIRF and

Lightning Protection, Fault

Tolerance, Software Certification

and other aspects associated with

FADEC/EEC’s systems.

A change in the 

containment case from

hard-wall to composite 

construction or vice versa,

that could not be 

retrofitted without 

additional major changes 

to the engine or restricting 

the initial limitations or

restrictions in the initial 

installation manual 

No Yes Yes Change in methods of

construction that have affected

inherent strength, backbone

bending, blade to case clearance 

retention, containment wave

effect on installation, effect on

burst model, torque spike effects. 

Replace gas generator 

(core, turbine/ 

compressor/combustor) 

with a different one that 

associated with changes

in approved limitations* 

*exclude life limits 

No No Yes Change is associated with other 

changes that would affect engine

thrust/power and have affected 

the dynamic behavior of the 

engine. Assumptions used for

certification may no longer be

valid

Page 39 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

(Parts 33 and 35) (CONTINUED)

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Piston Engines

Convert from Mechanical 

to Electronic Control 

System

Yes Yes No Change in engine configuration: 

Installation interface of engine

changed 

Changes to principles of

construction: Digital controllers 

and sensors require new

construction techniques and

environmental testing

Add Turbocharger that

increases performance 

and changes in overall 

product

Yes No Yes Change in general configuration: 

Installation interface of engine

changed (exhaust system)

Certification assumptions

invalidated: Change in operating

envelope and performance.

Convert from air-cooled 

cylinders to liquid cooled

cylinders 

Yes No Yes Change to general configuration: 

Installation interface of engine

changed (cooling lines from

radiator, change to cooling

baffles)

Certification assumptions

invalidated: Change in operating

envelope and engine temperature

requirements

Convert from sparkignition to compressionignition

Yes No Yes Change in general configuration: 

Installation interface of engine

changed (no mixture lever)

Certification assumptions

invalidated: Change in operating

envelope and performance. 

Page 40 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

The following are examples of SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i)

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i)

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii)

Notes

Propellers

Introduction of a 

different principle of 

blade retention 

Yes Yes No Change in propeller

configuration 

Likely change in model 

designation 

Propeller’s operating

characteristics and inherent 

strength require re-evaluation.

Page 41 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

(Parts 33 and 35) (CONTINUED) 

The following are examples of NOT SIGNIFICANT changes:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Turbine Engines

Change in the material 

from one type of metal 

to another type of 

metal of a compressor 

drum

No No No No change in performance

No likely change in model 

designation 

Assumptions are still valid

Increase/decrease in 

the number of 

compressor/turbine 

stages without resultant 

change in performance 

envelope 

No No No No change in performance

Model designation may or 

may not change

Assumptions are still valid

New components 

internal to the 

FADEC/EEC the

introduction of which 

does not change the 

function of the system

No No No No change in configuration 

Retrofitable 

Assumptions used for 

certification are still valid 

Possible changes in principles 

of construction are 

insignificant

Software changes No No No

Rub-strip design

changes 

No No No Component Level Change

A new combustor that 

does not change the 

approved limitations, 

or dynamic behavior* 

*exclude life limits 

No No No Component Level Change

Bearing changes No No No Component Level Change

Page 42 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

New blade designs

with similar material 

that can be retrofitted 

No No No Component Level Change

Fan blade redesign that 

can be retrofitted 

No No No Component Level Change

Oil tank redesign No No No Component Level Change

Change from one 

hydro-mechanical 

control to another 

hydro-mechanical 

control 

No No No Component Level Change

Change to limits on life 

limited components 

No No No Component Level Change

Changes to limits on 

exhaust gas 

temperature 

No No No

Changes in

certification 

maintenance 

requirements (CMR) 

with no configuration 

changes 

No No No

Bump ratings within 

the product’s physical 

capabilities that may be 

enhanced with gas path 

changes such as blade 

restaggered, cooling

hole patterns, blade 

coating changes, etc. 

No No No

Page 43 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

A change in principal 

physical properties and 

mechanics of load transfer 

of a material of primary

structure or highly loaded 

components. For example, 

change from traditional 

metal to either an exotic 

alloy or a composite 

material on a highly loaded 

component 

No No No Component Level Change

Piston Engine

A change in principal 

physical properties and 

mechanics of load transfer 

of a material of primary

structure or highly loaded 

components. For example, 

change from traditional 

metal to either an exotic 

alloy or a composite 

material on a highly loaded 

component 

No No No Component Level Change

New or redesigned 

cylinder head, or valves, 

or pistons

No No No

Changes in crankshaft No No No Component Level Change

Changes in crankcase No No No Component Level Change

Changes in carburetor No No No Component Level Change

Page 44 
4/28/03 AC 21.101-1 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Changes in mechanical 

fuel injection system

No No No

Changes in mechanical 

fuel injection pump

No No No Component Level Change

Engine model change to

accommodate new

airplane installation. No

change in principles of

operation of major 

subsystems; no significant 

expansion in power or 

operating envelopes or in

limitations

No No No

No change in basic 

principles of operation, or 

a simple mechanical 

change. For example, 

change from dual 

magneto to two single 

magnetos on a model

No No No

Subsystem change

produces no changes in

base engine input

parameters, and previous

analysis can be reliably

extended. For example, 

a change in turbocharger 

where induction system

inlet conditions remain

unchanged, or if changed,

the effects can be reliably

extrapolated 

No No No

Page 45 
AC 21.101-1 4/28/03 

Appendix 1 

APPENDIX 1. CLASSIFICATION OF CHANGES

FIGURE 4. EXAMPLES OF CHANGES FOR ENGINES AND PROPELLERS 

 (Parts 33 and 35) (CONTINUED) 

Examples of NOT SIGNIFICANT changes, continued:

Description of change Is there a 

change to the 

general 

configuration? 

14 CFR § 

21.101(b)(1)(i) 

Is there a 

change to the 

principles of

construction? 

14 CFR § 

21.101(b)(1)(i) 

Have the 

assumptions used 

for certification 

been invalidated? 

14 CFR § 

21.101(b)(1)(ii) 

Notes

Change in material of 

secondary structure or 

not highly loaded 

component. For 

example, a change

from metal to 

composite material in a 

non-highly loaded 

component, such as an 

oil pan that is not used 

as a mount pad 

No No No Component Level Change

Change in material that 

retains the physical 

properties and 

mechanics of load 

transfer. For example, 

a change in trace 

elements in a metal 

casting for ease of 

pouring or to update to 

a newer or more readily

available alloy with 

similar mechanical 

properties 

No No No Component Level Change

Propellers 

Change in the material 

of a blade bearing

No No No Component Level Change

Change to a component 

in the control system

No No No Component Level Change

Change to a propeller 

de-icer boot 

No No No Component Level Change

Page 46 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

1. INTRODUCTION.

a. The basic principal of enhancing the level of safety of changed aeronautical products is 

to apply the latest regulations for significant design changes, to the greatest extent practical. In

certain cases, the cost of complying fully with a later regulation may not be commensurate with 

the small safety benefit achieved. It is recognized that the existing fleet and newly produced 

airplanes, engines and propellers are safe, and any unsafe condition is immediately addressed 

through the airworthiness directive process. These factors form the basis by which compliance 

with the latest standard may be considered impractical, thereby allowing compliance with an

earlier regulation. This appendix gives one method of determining if compliance with a later 

regulation is impractical, however, this does not preclude the use of other methods for improving

the safety of aeronautical products. 

b. This AC recognizes that other procedures have been used for some products and have 

historically been accepted on a case-by-case basis. These procedures have not been fully

harmonized and may not be acceptable for all products. It is envisaged that other methods will 

be developed and become part of future versions of this AC. Regardless of which method is 

used, the method must show that a proposed certification basis is resource-effective when it is 

able to achieve a positive safety benefit for the overall product. 

c. In this regard, any method used must also encourage incorporating the safety

enhancements that will have the most dramatic impact on the accident rate and recognize the 

effective use of limited resources. This important point is illustrated graphically in the

accompanying figure. This figure notionally shows the interrelation between the total resources 

required for incorporating each potential safety enhancement with the corresponding net increase 

in safety benefit. 

Resources to implement 

the regulatory

requirement

Safety benefit of 

the regulatory

requirement

Potential safety enhancements 

Page 1 
AC 21.101-1 4/28/03 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

d. Typically one will find that there are proposals that can achieve a positive safety benefit 

that are resource effective. Conversely, there are proposals that may achieve a small safety

benefit at the expense of a large amount of resources to implement. Clearly, there will be a point 

where a large percentage of the potential safety benefit can be achieved with a reasonable 

expenditure of resources. The focus of the methods used should be to determine the most 

appropriate set of safety-significant regulatory standards relative to the respective cost to reach 

this point. 

e. This appendix provides procedural guidance for determining the practicality of applying

a requirement at a particular amendment level to a changed product. This guidance can be used 

to evaluate the safety benefit and resource impact of implementing the latest airworthiness 

requirements in the certification basis of a changed product. The procedure is generic in nature 

and describes the steps and necessary inputs that any applicant can use on any project to develop 

a position. 

f. The procedure is intended to be used, along with good engineering judgment, to evaluate 

the relative merits of a changed product complying with the latest regulations. It provides a 

means, but not the only means, for an applicant to present its position in regard to impracticality. 

g. The certification basis for a change to a product will not be at an amendment level 

earlier than the existing certification basis. Therefore, when determining the impracticality of

applying a requirement at the latest amendment level, only the increase in safety benefits and 

costs beyond compliance with the existing certification basis should be considered. 

h. The following are steps to determine the impracticality of applying a requirement at a

particular amendment level. The first step will be to identify the regulatory change being

evaluated. 

(1) Step 1: Identify the Regulatory Change Being Evaluated. In this step, 

document: 

(a) The specific requirement (for example, 14 CFR § 25.365); 

(b) The amendment level of the existing certification basis for the requirement; and 

(c) The latest amendment level of the requirement. 

(2) Step 2: Identify the Specific Hazard that the Requirement Addresses. 

(a) Each requirement and subsequent amendments are intended to address a hazard 

or hazards. In this step the specific hazard(s) is/are identified. This identification will allow for

a comparison of the effectiveness of amendment levels of the regulation at addressing the hazard. 

Page 2 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(b) In many cases the hazard and the cause of the hazard will be obvious. When 

the hazard and its related cause are not immediately obvious, it may be necessary to review the 

preamble of the regulation. It may also be helpful to discuss the hazard with the responsible 

FAA office. 

(3) Step 3: Review the Consequences of the Hazard(s). 

(a) Once the hazard has been identified, it is possible to identify the types of

consequences that may occur because of the presence of the hazard. More than one consequence 

can be attributed for the same hazard. Typical examples of consequences would include, but are 

not be limited to: 

1 Incidents where only injuries occurred;

2 Accidents where less than 10 percent of the passengers died; 

3 Accidents where 10 percent or more passengers died; and 

4 Accidents where a total hull loss occurred. 

(b) The preamble to the regulation may provide useful information regarding the 

consequences of the hazard the requirement is intended to address. 

(4) Step 4: Identify the Historical and Predicted Frequency of Each Consequence. 

(a) Another source for determining impracticality is the historical record of the

consequences of the hazard that led to a requirement or an amendment to a requirement. From 

these data, a frequency of occurrence for the hazard can be determined. It is important to 

recognize that the frequency of occurrence may be higher or lower in the future. Therefore, it 

also is necessary to predict the frequency of future occurrences. 

(b) More than one consequence can be attributed for the same hazard. Therefore, 

when applicable, the combination of consequences and frequencies of those consequences should 

be considered together. 

(c) The preamble of the regulation may provide useful information regarding the 

frequency of occurrence. 

(5) Step 5: Determine How Effective Full Compliance with the Latest Amendment 

of the Requirement Would Be at Addressing the Hazard. 

(a) When each amendment is promulgated, it is usually expected that compliance 

with the requirement would be completely effective at addressing the associated hazard. It is 

Page 3 
AC 21.101-1 4/28/03 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

expected that the hazard would be eliminated, avoided, or dealt with. However, in a limited 

number of situations, this may not be the case. It is also possible that earlier amendment levels 

may have addressed the hazard but were not completely effective. Therefore, in comparing the

benefits of compliance with the existing certification basis to the latest amendment level, it is 

useful to estimate the effectiveness of both amendment levels in dealing with the hazard. 

(b) It is recognized that the determination of levels of effectiveness is normally of a

subjective nature. Therefore, prudence should be exercised when making these determinations. 

In all cases, it is necessary to document the assumptions and data that support the determination. 

(c) The following five levels of effectiveness are provided as a guideline 

1 Fully effective in all cases. Compliance with the requirement eliminates

the hazard or provides a means to avoid the hazard completely. 

2 Considerable potential for eliminating or avoiding the hazard. 

Compliance with the requirement eliminates the hazard or provides a means to avoid completely

the hazard for all probable or likely cases, but it does not cover all situations or scenarios. 

3 Adequately deals with the hazard. Compliance with the requirement 

eliminates the hazard or provides a means to avoid the hazard completely in many cases. 

However, the hazard is not eliminated or avoided in all probable or likely cases. Usually this 

action only addresses a significant part of a larger or broader hazard. 

4 Hazard only partly addressed. In some cases compliance with the

requirement partly eliminates the hazard or does not completely avoid the hazard. The hazard is 

not eliminated or avoided in all probable or likely cases. Usually this action only addresses part

of a hazard. 

5 Hazard only partly addressed but action has negative side effect. 

Compliance with the requirement does not eliminate or avoid the hazard or may have negative

safety side effects. The action is of questionable benefit. 

(6) Step 6: Determine Resource Costs and Cost Avoidance. 

(a) There is always cost associated with complying with a requirement. This cost

may range from minimal administrative efforts to the resource expenditures that support full 

scale testing or the redesign of a large portion of an aircraft. However, there are also potential

cost savings from compliance with a requirement. For example, compliance with a requirement

may avoid aircraft damage or accidents and the associated costs to the manufacturer for 

investigating accidents. Compliance with the latest amendment of a requirement may also help a 

foreign authority certificate a product. 

Page 4 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(b) When determining the impracticality of applying a requirement at the latest 

amendment level, only the incremental costs and safety benefits from complying with the

existing certification basis should be considered. 

(c) When evaluating the incremental cost, it may be beneficial for the applicant to

compare the increase in cost to comply with the latest requirements to the cost to incorporate the

same design feature in a new airplane. In many cases an estimate for the cost of incorporation in 

a new airplane is provided in the regulatory evaluation by the Administrator, which was 

presented when the corresponding regulation was first promulgated. Incremental costs of 

retrofit/incorporation on existing designs may be higher than that for production. Examples of 

costs may include but are not limited to: 

(1) Costs: The accuracies of fleet size projections, utilization, etc. may be 

different than that experienced for derivative product designs and must be validated. 

(aa) Labor: Work carried out in the design, fabrication, inspection, 

operation or maintenance of a product for the purpose of incorporating or demonstrating

compliance with a proposed action. Non-recurring labor requirements, including training, should 

be considered. 

(bb) Capital: Construction of new, modified or temporary facilities for

design, production, tooling, training, or maintenance. 

(cc) Material: Cost associated with product materials, product 

components, inventory, kits, and spares. 

(dd) Operating Costs: Costs associated with fuel, oil, fees, and 

expendables. 

(ee) Revenue/Utility Loss: Costs resulting from earning/usage capability

reductions from departure delays, product downtime, capability reductions of performance loss 

due to seats, cargo, range, or airport restrictions. 

(2) Cost Avoidance: 

(a) Avoiding cost of accidents, including investigation of accidents, 

lawsuits, public relations activities, insurance, and lost revenue. 

(b) Foreign Certification: Achieve a singular effort that would 

demonstrate compliance to the requirements of most certifying agencies, thus minimizing

certification costs. 

Page 5 
AC 21.101-1 4/28/03 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(7) Step 7: Document Conclusion. 

(a) Once the information from previous steps has been documented and reviewed, the applicant’s 

position and rationale regarding practicality can be documented. Examples of possible positions 

would include but are not limited to: 

1 Compliance with the latest requirement is necessary. The applicant would 

pursue the change at the latest amendment level.

2 Compliance with an amendment level between the existing certification 

basis and the latest amendment would adequately address the hazard at an acceptable cost, while

meeting the latest amendment level would be impractical. The applicant would then propose the 

intermediate amendment level of the requirement. 

3 The increased level of safety is not commensurate with the increased costs 

associated with meeting the latest amendment instead of the existing certification basis. 

Therefore, the applicant would propose the existing certification basis. 

4 The results of this analysis were inconclusive. Further discussions with 

the FAA are warranted. 

NOTE: This process may result in a required certification basis that

renders the proposed modification economically not viable. 

2. EXAMPLES OF HOW TO CERTIFY CHANGED AIRCRAFT. The following

examples are for transport airplanes and illustrate the typical process an applicant follows. The

process will be the same for all product types. 

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

(1) This change is part of a significant transport airplane change that increases 

passenger payload and gross weight by extending the fuselage 20 feet. To accommodate the 

higher design weights and increased braking requirements, and to reduce runway loading, the 

applicant will change the landing gear from a two-wheel to four-wheel configuration; this 

changes the debris scatter on the wing from the landing gear. The new model airplane will be

required to comply with the latest applicable regulations based on the date of application. 

(2) The wing will be strengthened locally at the side of the body and at the attachment 

of engines and landing gear, but the applicant would not like to alter wing access panels and the 

fuel tank access covers. Although the applicant recognizes that the scatter pattern and impact 

loading on the wing from debris being thrown from the landing gear will change, he proposes 

that it would be impractical to redesign the fuel tank access covers.

Page 6 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(3) Step 1: Identify the Regulatory Change Being Evaluated.

(a) The existing certification basis of the airplane that is being changed is Part 25 

prior to Amendment 25-69.

(b) Amendment 25-69 added the requirement that fuel tank access covers on 

transport category airplanes be designed to minimize penetration by likely foreign objects, and be

fire resistant.

(4) Step 2: Identify the Specific Hazard that the Regulation Addresses. Fuel tank 

access covers have failed in service due to impact with high-energy objects such as failed tire 

tread material and engine debris following engine failures. In one accident, debris from the 

runway impacted a fuel tank access cover, causing its failure and subsequent fire, which resulted 

in fatalities and loss of the airplane. Amendment 25-69 will ensure that all access covers on all 

fuel tanks are designed or located to minimize penetration by likely foreign objects, and are fire

resistant.

(5) Step 3: Review the History of the Consequences of the Hazard(s). Occurrences 

with injuries and with more than 10percent deaths.

(6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence.

(a) In 200 million departures of large jets:

(1) One occurrence with more than 10 percent deaths; and

(2) One occurrence with injuries.

(b) There is no reason to believe that the future rate of accidents will be 

significantly different than the historical record. 

(7) Step 5: Determine How Effective Full Compliance with the Latest Amendment 

of the Regulation Would Be at Addressing the Hazard. 

(a) Considerable potential for eliminating or avoiding the hazard. 

(b) Compliance with Amendment 25-69 eliminates the hazard or provides a means 

to avoid the hazard completely for all probable or likely cases. However, it does not cover all 

situations or scenarios. 

Page 7 
AC 21.101-1 4/28/03 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(8) Step 6: Determine Resource Costs and Cost Avoidance. 

(a) Costs: 

1 For a newly developed airplane, there would be minor increases in labor 

resulting from design and fabrication. 

2 There would be a negligible increase in costs related to materials, 

operating costs, and revenue utility loss. 

(b) Cost Avoidance: 

1 There were two accidents in 200 million departures. The applicant 

believes that it will manufacture more than 2,000 of these airplanes or derivatives of these 

airplanes. These airplanes would average five flights a day. Therefore, statistically there will be

accidents in the future if the hazard is not alleviated. Compliance will provide cost benefits 

related to avoiding lawsuits, accident investigations, and public relation costs. 

2 There are cost savings associated with meeting a single certification basis 

for FAA and foreign regulations. 

(9) Conclusion. It is concluded that compliance with the latest regulation increases the

level of safety at a minimal cost to the applicant. Based on the arguments and information 

presented by the applicant through the issue paper process, the Administrator determined that

meeting the latest amendment would be practical. 

b. Example 2: 14 CFR § 25.365 Pressurized Compartment Loads.

(1) For the product change described in Example 1, the lengthened fuselage affects the 

size of the main deck passenger compartment and the lower center cargo compartment. The 

applicant plans to comply fully with the latest pressurized compartment loads, except for one

interior partition for which the applicant believes compliance would be impractical. 

(2) The applicant proposes to increase the length of the fuselage by installing fuselage

plugs. This change affected the size of the main deck passenger compartment and the lower 

center cargo compartment. 

(3) Step 1: Identify the Regulatory Change Being Evaluated. 

(a) The existing certification basis of the airplane that is being changed includes 

14 CFR § 25.365 at Amendment 25-54. The initial release of 14 CFR § 25.365 required that the 

interior structure of passenger compartments be designed to withstand the effects of a sudden 

Page 8 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

release of pressure through an opening resulting from the failure or penetration of an external

door, window, or windshield panel, or from structural fatigue or penetration of the fuselage, 

unless shown to be extremely remote. 

(b) Amendment 25-54 revised 14 CFR § 25.365 to require that the interior 

structure be designed for an opening resulting from penetration by a portion of an engine, an

opening in any compartment of a size defined by 14 CFR § 25.365(e)(2), or the maximum 

opening caused by a failure not shown to be extremely improbable. 

(c) Amendment 25-71 extended the regulation to all pressurized compartments, 

not just passenger compartments, and to the pressurization of unpressurized areas. The later 

regulation had previously been identified as an unsafe feature under 14 CFR § 21.21(b)(2). 

(4) Step 2: Identify the Specific Hazard that the Regulation Addresses. The 

hazard is a catastrophic structure and/or system failure produced by a sudden release of pressure 

through an opening in any compartment in flight. This opening could be caused by an 

uncontained engine failure, an opening of a prescribed size due to the inadvertent opening of an 

external door in flight, or an opening caused by a failure not shown to be extremely improbable. 

The opening could be produced by an event that has yet to be identified. 

(5) Step 3: Review the History of the Consequences of the Hazard(s). 

(a) Occurrences with injuries, less than 10 percent deaths, and more than 10 

percent deaths. 

(6) Step 4: Identify the Historical and Predicted Frequency of Each Consequence. 

(a) In 200 million departures of large jets: 

1 Two occurrences with more than 10 percent deaths; 

2 One occurrence with less than 10 percent deaths; and 

3 One occurrence with injuries. 

(b) There is no reason to believe that the future rate of accidents will be 

significantly different than the historical record. 

(7) Step 5: Determine How Effective Full Compliance with the Latest Amendment 

of the Regulation Would Be at Addressing the Hazard. 

(a) Fully effective in all cases. Compliance with Amendment 25-71 eliminates the

hazard or provides a means to avoid the hazard completely. 

Page 9 
AC 21.101-1 4/28/03 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(b) Considerable potential for eliminating or avoiding the hazard. Compliance

with Amendment 25-54 eliminates the hazard or provides a means to avoid the hazard

completely for all probable or likely cases, but it does not cover all situations or scenarios. 

(c) Adequately deals with the hazard. Compliance with the original certification 

basis eliminates the hazard or provides a means to avoid the hazard completely in many cases. 

However, the hazard is not eliminated or avoided in all probable or likely cases. Usually this 

action only addresses a significant part of a larger or broader hazard. 

(d) Design changes made to the proposed derivative airplane bring it nearly into 

compliance with 14 CFR § 25.365 at Amendment 25-71. Analyses show that one interior 

partition would fail when subjected to the pressure differential defined by the latest regulation. 

However, its failure would not have an impact on continued safe flight and landing. This is 

because none of the critical or essential systems are affected by failure of this partition, and its 

failure would not present a hazard to a crewmember. Design solutions were considered for this 

partition, including structural reinforcement and additional venting area, but all were found to 

require extensive redesign. With this design the applicant believes that most of the safety

benefits have been achieved fully with Amendment 25-71. 

(8) Step 6: Determine Resource Costs and Cost Avoidance. 

(a) Costs: 

1 For a newly developed airplane, there would be a considerable increase in

costs related to labor and capital to comply with Amendment 25-71 instead of the original

certification basis.

2 There would be a negligible increase in costs related to materials, 

operating costs, and revenue utility loss.

3 There would be savings in both labor and capital costs if compliance were

shown to Amendment 25-54 instead of Amendment 25-71.

(b) Cost Avoidance: 

(1) There were four accidents in 200 million departures. The applicant 

believes that it will manufacture more than 2,000 of these airplanes or derivatives of these 

airplanes. These airplanes would average five flights a day. Therefore, statistically there will be

accidents in the future if the hazard is not alleviated. Compliance will provide cost benefits 

related to avoiding lawsuits, accident investigations, and public relation costs. 

Page 10 
4/28/03 AC 21.101-1 

Appendix 2 

APPENDIX 2. PROCEDURE FOR EVALUATING IMPRACTICALITY OF APPLYING

LATEST REQUIREMENTS TO A CHANGED PRODUCT

(CONTINUED) 

(2) There are cost savings associated with meeting a single certification basis 

for FAA and foreign regulations. 

(9) Step 7: Document Conclusion Regarding Practicality. The design is in 

compliance with 14 CFR § 25.365 at Amendment 25-54 and nearly in full compliance to 

Amendment 25-71. The design would adequately address the hazard at an acceptable cost. 

Therefore, based on arguments of impracticality discussed in an issue paper, the Administrator

accepts the applicant’s proposal to comply with 14 CFR § 25.365 at Amendment 25-54.

Page 11 

4/28/03 AC 21.101-1 

Appendix 3 

APPENDIX 3. THE USE OF SERVICE EXPERIENCE IN THE CERTIFICATION 

PROCESS 

1. INTRODUCTION. Service experience may support the application of an earlier regulatory

standard if, in conjunction with the applicable service experience and other compliance 

measures, the earlier standard provides a level of safety comparable to that provided by the latest 

requirements. The applicant must provide sufficient substantiation to allow the Administrator to 

make this determination. A statistical approach may be used, subject to the availability and 

relevance of data, but sound engineering judgment must be used. For service history to be 

acceptable, the data must be both sufficient and pertinent. The essentials of the process involve: 

a. A clear understanding of the requirement change and the purpose for the change; 

b. A determination based on detailed knowledge of the proposed design feature; 

c. The availability of pertinent and sufficient service experience data; and 

d. A comprehensive review of that service experience data. 

2. GUIDELINES. The Issue Paper process would be used, and the applicant should provide 

documentation to support the following: 

a. The identification of the differences between the requirement in the existing basis and 

the requirement as amended, and the effect of the change in the requirement. 

b. A description as to what aspect(s) of the latest requirements the proposed changed 

product would not meet. 

c. Evidence showing that the proposed certification basis for the changed product, together 

with applicable service experience, provides a level of safety consistent with complying with the

latest requirements. 

d. A description of the design feature and its intended function. 

e. Data for the product pertinent to the requirement. 

(1) Service experience from such data sources as the following:

(a) Accident reports; 

(b) Incident reports; 

(c) Service bulletins; 

(d) Airworthiness directives; 

(e) Repairs; 

Page 1 
AC 21.101-1 4/28/03 

Appendix 3 

APPENDIX 3. THE USE OF SERVICE EXPERIENCE IN THE CERTIFICATION 

PROCESS (CONTINUED) 

(f) Modifications; 

(g) Flight hours/cycles for fleet leader and total fleet; 

(h) World airline accident summary data;

(i) Service difficulty reports; 

(j) National Transportation Safety Board reports; and 

(k) Warranty, repair and parts usage data. 

(2) Show that the data presented represent all relevant service experience for the 

product, including the results of any operator surveys, and is comprehensive enough to be 

representative. 

(3) Show that the service experience is relevant to the issue. 

(4) Identification and evaluation of each of the main areas of concern with regard to:

(a) Recurring and/or common failure modes; 

(b) Cause; 

(c) Probability, by qualitative reasoning; and 

(d) Measures already taken and their effects. 

(5) Relevant data pertaining to aircraft of similar design and construction may be

included. 

(6) Evaluation of failure modes and consequences through analytical processes. The 

analytical processes should be supported by: 

(a) A review of previous test results; and 

(b) Additional detailed testing.

f. A conclusion that draws together the data and the rationale. 

g. These guidelines are not intended to be limiting, either in setting required minimum 

elements or in precluding alternative forms of submission. Each case may be different, based on 

the particulars of the system being examined and the requirement to be addressed. 

Page 2 
4/28/03 AC 21.101-1 

Appendix 3 

APPENDIX 3. THE USE OF SERVICE EXPERIENCE IN THE CERTIFICATION 

PROCESS (CONTINUED) 

3. EXAMPLE: 14 CFR § 25.1141(f) TRANSPORT AIRPLANES. 

a. The following example, for transport airplanes (14 CFR § 25.1141(f) Auxiliary Power 

Unit (APU) Fuel Valve Position Indication System), illustrates the typical process an applicant 

follows. The process will be the same for all product types. 

b. This example comes from a new generation model transport airplane where significant 

changes were made to the main airframe components, engines and systems, and APU. The 

baseline airplane has an extensive service history. The example shows how the use of service 

experience supports a finding that compliance with the latest regulation would not contribute 

materially to the level of safety, and that application of the existing certification basis (or earlier

amendment) would be appropriate. The example is for significant derivatives of transport 

airplanes with extensive service history, and illustrates the process, following the guidelines in 

this appendix, but does not include the level of detail normally required. 

(1) The differences between the regulation in the existing certification basis and the

regulation as amended, and the effect of the change in the regulation. The existing certification 

basis of the airplane that is being changed is the initial release of Part 25. Amendment 25-40 

added requirement 14 CFR § 25.1141(f), which mandates that power-assisted valves must have a 

means to indicate to the flight crew when the valve is in the fully open or closed position, or is 

moving between these positions. 

(2) What aspect of the proposed changed product would not meet the latest regulations? 

The proposed APU fuel valve position indication system does not provide the flight crew with 

fuel valve position or transition indication and, therefore, does not comply with the requirements 

of 14 CFR § 25.1141(f). 

(3) Evidence that the proposed certification basis for the changed product, together with 

applicable service experience of the existing design, provide a level of safety comparable to that

intended by the latest regulation. The APU fuel shut-off valve and actuator are unchanged from 

those used on the current family of airplanes, and have been found to comply with the earlier 

Amendment 25-11 of 14 CFR § 25.1141(f). The existing fleet has achieved approximately (#) 

flights during which service experience of the existing design has been found to be acceptable. If

one assumes a complete APU cycle, i.e., start-up and shutdown for each flight, the number of 

APU fuel shut-off valve operations would be over 108

 cycles, which demonstrates that the valve 

successfully meets its intended function and complies with the intent of the regulation. In

addition, the system design for the changed product incorporates features that increase the level 

of functionality and safety. 

(4) A description of the design feature and its intended function. The fuel shut-off 

valve, actuator design, and operation is essentially unchanged; with the system design ensuring

that the valve is monitored for proper cycling from closed to open at start. If the valve is not in 

the appropriate position (i.e., closed), then the APU start is terminated, an indication is displayed 

Page 3 
AC 21.101-1 4/28/03 

Appendix 3 

APPENDIX 3. THE USE OF SERVICE EXPERIENCE IN THE CERTIFICATION 

PROCESS (CONTINUED) 

Page 4 

on the flight deck, and any further APU starts are prevented. Design improvements using the 

capability of the APU Electronic Control Unit (ECU) have been incorporated in this proposed 

product change. These design changes ensure that the fuel valve indication system will indicate 

failure of proper valve operation to the flight crew, but the system does not indicate valve 

position as required by 14 CFR § 25.1141(f). 

(5) Data for the product pertinent to the regulation. An issue paper was coordinated, 

included data, or referenced reports, documenting relevant service experience that has been 

compiled from incident reports, fleet flight hour/cycle data, and maintenance records. The issue 

paper also discussed existing and proposed design details, failure modes and analyses showing to 

what extent the proposed airplane complies with the latest amendment of 14 CFR § 25.1141. 

Information is presented to support the applicant’s argument that compliance with the latest 

amendment would not materially increase the level of safety. Comparative data pertaining to 

aircraft of similar design and construction are also presented. 

(6) Conclusion, drawing together the data and rationale. The additional features 

incorporated in the APU fuel shut-off valve will provide a significant increase in safety to an 

existing design with satisfactory service experience. The applicant proposes that compliance 

with the latest amendment would not materially increase the level of safety, and that compliance 

with 14 CFR § 25.1141 at Amendment 25-11 would provide an acceptable level of safety for the 

proposed product change.