Subject: Airworthiness Compliance Checklists 

Used to Substantiate Major Alterations for 

Small Airplanes 

Date: 9/30/04

Initiated By: ACE-100 

AC No: 23-21

Change:

1. PURPOSE. 

a. This advisory circular (AC) provides guidance material for the creation and use of 

airworthiness compliance checklists that aid in substantiating requirements when performing 

major alterations to small airplanes. These checklists may be used by Airframe and Powerplant 

(A&P) mechanics and those holding Inspection Authorization (IA) privileges and by persons 

performing alterations (modifiers), under the privileges of an FAA Certificated Repair Station 

(CRS) operating under Title 14 Code of Federal Regulations (CFR) part 145 as well as by 

Federal Aviation Administration (FAA) Airworthiness Aviation Safety Inspectors (ASIs) to 

assist in determining that such alterations meet compliance with the Federal Aviation 

Regulations. These checklists will help identify all the data requirements and outline the data 

approval methods for a particular major alteration, as well as identify documentation required to 

further substantiate approval for returning the product to service. 

b. Use of these compliance checklists should be limited to alterations that have been 

determined to be “major” alterations, as defined in 14 CFR part 1, and are not major changes to 

the type design of the product as denoted in 14 CFR part 21, § 21.97, and which are not so 

complex that they require a Supplemental Type Certificate (STC), per FAA Order 8300.10. 

c. This AC is intended to aid persons performing an alteration using the guidance of 

AC 43-210, Standardized Procedures for Requesting Field Approval of Data, Major Alterations, 

and Repairs, as well as the ASI in determining requirements are met to satisfy airworthiness. 

d. This AC does not change any previously released FAA guidance material such as FAA 

Orders and advisory circulars listed in section 4 of this AC. The intent of this AC is to provide 

an aid to assist industry and the FAA with existing approval processes. 
AC 23-21 9/30/04 

e. Material in this AC is neither mandatory nor regulatory in nature and does not constitute 

a regulation. The use of these checklists during the return to service of a major alteration is not 

mandatory nor should it affect any previously acceptable method. 

2. BACKGROUND. 

a. The data and documentation requirements for major alterations vary considerably. This 

variation can be attributed to the following: 

• Differing complexity of an alteration, e.g. degrees of equipment and systems 

integration 

• Different sources and rigor of data submitted 

• Uncertainty of what data is actually required to show compliance with applicable 

regulations as prepared to substantiate approval by FAA. 

b. Standardization of data submission for alterations should be assured through the use of 

compliance checklists. The FAA will establish a library of compliance checklists that will be

periodically updated. Such a resource will enable modifiers and ASIs to better evaluate 

regulatory compliance of a modification on similar makes and models of aircraft. For example, 

in the first case, the installation of a multi-function display as interfaced with a weather detection 

sensor and Terrain Awareness and Warning System (TAWS). You can access the library to see 

if a compliance checklist exists for the same change. If so, you use that checklist as a template 

for the creation of your checklist. Each checklist identifies the pertinent regulation as the 

certification basis of the airplane for the alteration. It also lists the manner in which the data can 

be approved. 

c. This AC continues our commitment to improve the effectiveness and efficiency of the 

approval processes for major alterations by establishing an understanding of the needs and 

expectations of all parties involved. Reducing the time for applying for and receiving approval 

requires up front involvement between the FAA and the applicant to scope and plan the intended 

alteration project. Using a compliance checklist should result in a more effective use of FAA 

and industry resources by establishing more standard data and documentation criteria. 

3. APPLICABILITY. 

This AC applies to modifiers who use these airworthiness compliance checklists. These 

checklists are limited to use on small airplanes that have a maximum gross weight of 12,500 

pounds or less. Individual checklist “limitations” section may have restrictions and limitations 

that are applicable to a specific alteration. Some possible examples would include: airplanes that 

weigh 6,000 pounds or less; non-pressurized airplanes only; or single, non-turbocharged engine 

airplanes; airplanes that are to be operated under Visual Flight Rules (VFR) only; or airplanes 

manufactured prior to a certain date. Specific applicability depends on the determination of the 

major alteration and the resultant effect on the aircraft to remain in compliance with 14 CFR, 

part 23 or CAR 3. 

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9/30/04 AC 23-21 

4. RELATED PUBLICATIONS. 

a. Federal Regulations

CAR 3, Part 3 of the Civil Air Regulations as amended through March 25, 1964 

CAR 4, Part 4 of the Civil Air Regulations as amended through March 25, 1964 

14 CFR part 21 – Certification Procedures for Products and Parts 

14 CFR part 23 – Airworthiness Standards: Normal, Utility, Acrobatic, and Commuter 

Category Airplanes 

b. Orders

FAA Order 8110.37C, Designated Engineering Representative (DER) Guidance Handbook, 

dated 9/30/98 

FAA Order 8110.45, Use of Data Approved by Designated Engineering Representatives to 

Support Major Alterations, dated 8/30/02 

FAA Order 8300.10, Volume 2, Chapter 1 Performing Field Approvals, contained within the 

Airworthiness Inspector’s Handbook, dated 6/30/04 

c. ACs 

AC 23-8B, Flight Test Guide for Certification of Part 23 Airplanes, dated 8/24/03 

AC 23.1309-1C, Equipment, Systems, and Installations in Part 23 Airplanes, dated 3/12/99 

AC 23.1311-1A, Installation of Electronic Displays in Part 23 Airplanes, dated 3/12/99 

AC 43.9-1E, Instructions for Completion of FAA Form 337 (OMB No. 2120-0020), Major 

Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), dated 5/21/87 

AC 43.13-1B, CHG 1, Acceptable Methods, Techniques, and Practices – Aircraft Inspection 

and Repair, dated 9/27/01 

AC 43.13-2A, Acceptable Methods, Techniques, and Practices – Aircraft Alterations, dated 

1/1/77 

AC 43-210, Standardized Procedures for Requesting Field Approval of Data, Major 

Alterations, and Repairs, dated 2/17/04 

Copies of the current publications of the Orders and ACs listed above can be obtained from the 

following address: U.S. Department of Transportation, Subsequent Distribution Office, Ardmore 

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AC 23-21 9/30/04 

East Business Center, 3341 Q 75th Avenue, Landover, MD 20785 or may be downloaded from the 

internet from the FAAs Regulatory and Guidance Library (RGL) at http://www.airweb.faa.gov/ac.

5. DEFINITIONS. 

Aircraft Certification Office (ACO) – In the context of this AC, the ACO is the cognizant 

office responsible for the issuance of the TC, amended TC, or STC. Upon request an ACO 

provides engineering assistance to an ASI making field approvals, and reviews (and in some

cases, approves) a final Aircraft Flight Manual Supplement (AFMS). An ACO may also be 

referred to as the Type Certificate Holding Office. 

Approval for Return to Service – The approval given by an appropriately rated person that 

enables an aircraft to be returned to service following alterations. 

Designated Engineering Representative (DER) – An FAA designated engineer who has been 

delegated a specific engineering discipline in which he can approve engineering data on behalf of 

the FAA. 

Existing Certification Basis – The Type Certification Basis is the applicable rules and any 

additional requirements that the applicant must show and use, and what the FAA must find 

compliant in order to grant a type certificate. 

FAA Form 337 (OMB Form No. 2120-0020) – An FAA form used to record a major repair 

or major alteration that becomes part of the maintenance record for the aircraft. Instructions for 

completing the Form 337 are contained within AC 43.9-1E. 

Field Approval – Approval by an ASI of a major alteration or repair, documented on a 

completed FAA Form 337, Major Repair and Alteration (Airframe, Powerplant, Propeller, or 

Appliance), or equivalent form and in a manner acceptable to the Administrator. In the absence 

of approved data, such as for an installation of a hand control to substitute for rudder and brake 

application to satisfy the needs of paraplegic person, a field approval may be granted for an 

alteration or repair by physical inspection or testing. 

Instructions for Continued Airworthiness (ICA) – The documentation that provides 

instruction on the maintenance of the airplane, engine, or propeller. 

Major Repair – A repair that, if done improperly, might appreciably affect weight, balance, 

structural strength, performance, powerplant operation, flight characteristics, or other qualities 

affecting airworthiness. A repair that is not completed according to accepted practices or cannot 

be made by elementary operations. 

Major Alteration – An alteration not listed in the aircraft, aircraft engine, or propeller 

specification. An alteration that might appreciably affect weight, balance, structural strength, 

performance, powerplant operation, flight characteristics, or other qualities affecting 

airworthiness, or an alteration that is not completed according to accepted practices or cannot be

made by elementary operations, is considered a major alteration. 

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9/30/04 AC 23-21 

6. DOCUMENTS REQUIRED FOR OBTAINING FIELD APPROVAL.

The applicant should submit the following to their local Flight Standards District Office 

(FSDO): 

• Completed FAA Form 337 with sufficient descriptive and detailed substantiating data to 

describe the alteration or repair 

• Completed airworthiness compliance checklist containing applicable data 

• Any other documentation required to describe limitations, emergency/abnormal 

procedures, normal operating procedures, performance, weight and balance, etc. such as would 

be contained within an Airplane Flight Manual Supplement (AFMS) 

• Applicable ICAs. (New ICAs or revisions to ICAs, which contain Airworthiness 

Limitations Section (ALS) of the ICA, require FAA approval by and must be coordinated with 

the Aircraft Certification Office (ACO). See paragraph 7f for restrictions on establishing, 

altering, or canceling airworthiness limitations. Your submitted ICAs without ALS are only 

accepted by the FAA). 

Individual airworthiness compliance checklists will have different data elements that are 

necessary to show compliance with the regulations. Also, some may require FAA ACO 

coordination for approval such as for approval of an AFMS, if otherwise not approvable by an 

ASI. This requirement depends on the alteration and will be conveyed on the checklist. Simple 

major alterations may not benefit from the use of a checklist when the applicant can show that 

the installation of equipment and system can be accomplished as a major alteration using 

approved data e.g. approved service bulletin, Airworthiness Directive (AD), STC, etc. This AC 

is not meant to imply that the use of these checklists are mandatory or impose the use of 

checklists in all cases, but it is meant to encourage their use. 

7. GENERAL OVERVIEW. 

a. The use of these compliance checklists should be limited to alterations that have been 

determined to be “major” alterations, as defined in 14 CFR, part 1, but are not so complex that 

they require an amended Type Certificate (TC) or STC per FAA Order 8300.10. Alterations that 

are classified as “minor” under 14 CFR, part 1, do not require the alteration be declared on an 

FAA Form 337, however the use of a compliance checklist would assist in making the 

determination that, in fact, the installation of all equipment, systems, and their components can 

be installed as a minor alteration. The first step in considering using these compliance checklists 

is making the determination that the major alteration is one that does not require an STC. 

b. The next step is to determine the aircraft’s certification basis, which will provide the 

amendment level of 14 CFR, part 23 (or predecessor Civil Aviation Regulation (CAR) 3, 

Bulletin 7, etc.). The alteration must show compliance to all applicable regulations at that 

amendment level. The certification basis of the airplane is found on the airplane’s Type 

Certificate Data Sheet (TCDS). The TCDS is the controlling document for an airplane, its model 

number and serial number. The TCDS can be found online at http://www.airweb.faa.gov/rgl. 

5 
AC 23-21 9/30/04 

You need to know the certification basis in order to complete the checklist form in the correct 

manner. The major alteration may be completed to a certification basis by applying later 

amendments of the regulations than the existing ones specified in the TCDS for the aircraft at the

time of its initial certification. 

c. The next step is to determine what data is required by the FAA to approve the major 

alteration as declared in Item (Block) 8 of the submitted FAA Form 337. Both descriptive and 

substantiating data must be provided in sufficient detail to describe the installation. A DER, 

FSDO ASI, or an ACO engineer can approve data. The “approval” methods of this data 

determine how the checklist is to be completed. The analysis and the flexibility of data approval 

methods depicted on the checklist illustrate the value in using these checklists during the job 

planning stages. For example, you may determine several compliance requirements for the 

installation of electrical equipment components, structural, wiring, antenna, or the associated 

electrical load analysis which is performed. You may possess approved data for the installation 

of the electrical equipment components and the antenna from the manufacturer, for example, you 

may use AC 43.13-1B as acceptable data for the wire type and gauge qualification, and you may 

use a DER to complete the approved data for electrical loads analysis. The checklist defines the 

contents of the data package and illustrates all items to be included within it. 

d. One of the methods of obtaining approved data for a particular alteration is to utilize a 

DER to provide necessary approved data. This methodology is described in FAA Order 

8110.45, “Use of Data Approved by Designated Engineering Representatives to Support Major 

Alteration,” dated 8/30/02; and Order 8110.37C, “Designated Engineering Representative (DER) 

Guidance Handbook,” dated 9/30/98. Both Orders can be found at the FAA website 

http://www.faa.gov. When you identify a requirement for data during an alteration, you may 

contact a DER with the authority, as listed in Order 8110.37C, to generate the report and submit 

approved data. The use of a DER to develop FAA approved data can save time because it is an 

efficient means to ensure completeness of the required data. When contacting a DER, only 

DERs specifically authorized by their managing ACO are permitted to approve data for major 

repairs and alterations. 

e. If you use a DER(s) to develop and approve data, you may receive feedback from the 

DER(s) regarding additional data requirements. For example, if you have contracted for an 

electrical DER to accomplish an electrical loads analysis, and he or she notices the need for, and 

recommends a structural review of the equipment installation, you should take that 

recommendation into consideration and include it as an additional checklist requirement. 

f. Neither the DER nor an ASI has the authority to establish, alter, or cancel the FAA 

approved airworthiness limitations contained in the ALS of an approved ICA. Only the product 

manufacturer has the authority to establish or alter the ALS and only the ACO can approve the 

ALS. 

8. HOW TO CREATE AND USE AIRWORTHINESS COMPLIANCE CHECKLISTS. 

This section describes how to use, and provides instructions for completing each section of 

the airworthiness compliance checklist. Each checklist lists the pertinent regulation at the 

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9/30/04 AC 23-21 

existing certification basis of the airplane at the time the alteration is performed. It also shows 

the manner or method by which the data can be approved. 

Each checklist is formatted with the same paragraphs with information specific to the 

particular alteration and with data specifically pertinent to that individual checklist. A discussion 

regarding each paragraph of a typical checklist found in the appendices is as follows: 

a. Paragraph ‘a’ of Checklist. The first page of each checklist is the table, which provides 

a means to show compliance to the applicable regulations associated with the major alteration to 

be accomplished. The compliance table contains the following: 

(1) Title Block. The title has a brief description of the major alteration to be 

accomplished. The title block contains information about the aircraft: Make, Model, and Serial 

Number blocks will be found on the aircraft manufacturer’s identification plate. The 

Registration Number is the same as shown on FAA Form 8050-3, Certificate of Aircraft 

Registration (N - number). The title block also contains the Statement of Applicability to be 

signed by the IA or a representative of the CRS, and states “I have determined the planned 

alteration to be in compliance with paragraph ‘c’, “checklist applicability”.” The intent is to 

have the IA or the CRS evaluate the aircraft and proposed alteration to determine if the use of the 

checklist is appropriate. The IA or CRS accomplishes this by evaluating the applicability 

requirements defined in paragraph c of the checklist. 

(2) The Body of the Compliance Table. The table outlines the applicable regulations, 

which need to be complied with, and the methods for showing compliance. The table contains 

eleven columns. The first four columns are used to show methods of compliance and 

completeness of compliance for the required line item as follows:

(a) Column 1 is titled “Item Completed Initials”. This column is used primarily 

by the applicant for initialing the completeness of that particular line item. When column 1 is 

completed and all the line items are initialed, the checklist is complete. 

(b) Columns 2, 3, and 4, “Planned FAA Approval Method” are used to indicate 

the data approval method. These columns are used during the planning stages of the alteration. 

The applicant will make selections for a planned data approval method for each line item in the 

table by checking the box in column 2, or 3, or by writing descriptions of data in column 4. 

Column 2 is labeled “DER, 8110-3.” The box in this column will be selected, if for that line 

item compliance will be documented on an FAA Form 8110-3 from a DER. Column 3 is labeled 

“FSDO/ASI, 337 Block 3.” The box in column 3 will be selected if it is intended that the ASI 

will approve the data in this line item. This is commonly accomplished by coordination between 

the modifier and the ASI during planning stages. If the column 3 box is selected, Block 3 of 

FAA Form 337 must be completed and signed by the ASI. Column 4 is titled “Other”. This 

column will be used when data approval is to be accomplished by means other than the use of an 

FAA Form 8110-3 with DERs signatures or by having the IA or CRS approve the data by 

signing Block 3 of the FAA Form 337. When using this column, enter the intended approval 

method. Some typical examples would include using a specified chapter and paragraph of 

AC 43.13-1B, STCs, or approved structural repair manuals. 

7 
AC 23-21 9/30/04 

(c) Column 5 is titled “Subject Evaluated.” This column contains a brief 

description of the engineering subject matter, which requires approved data to show compliance 

to the regulatory requirement of that particular line item.

(d) Columns 6 and 7 are titled “14 CFR or CAR 3”. Each of these columns lists 

the applicable 14 CFR or CAR 3 requirement that is associated with the certification basis of the 

airplane to find compliance for that particular line item. The determination of which column to 

select between “14 CFR” and “CAR 3” relates to the date of the initial or original the 

certification for the airplane. If your aircraft has a certification basis other than 14 CFR, part 23 

or CAR 3, such as CAR 4 or Bulletin 7, indicate as such in column 7 per tables 1-1, 2-1, 3-1, 4-1, 

and 5-1, note 5 in the tables. For example, if the aircraft had a certification date prior to 1966 

(the initial date for part 23), you would select the CAR 3 column. The amendment level of part 

23 at which each sub part was amended is listed next to the part. The requirement is to show 

compliance to the appropriate amendment for the existing certification basis of your airplane. 

For example, if your airplane’s certification basis is 14 CFR, part 23, amendment 37, and the 

checklists identify a line item “14 CFR, part 23, § 23.301, N, 28, 42, 48” you are required to 

show compliance to 14 CFR part 23, § 23.301, as written in amendment 23-28. Amendments 42 

and 48, which may be more stringent are not applicable to your aircraft when using this 

checklist. In this example, you would circle the “28” and show compliance by using column 2-4, 

as required. Note: There may be times when compliance with the most recent amendment 

would be desirable to satisfy certain operational conditions, or in order to comply with special 

conditions or other requirement to mitigate safety risk. 

(e) Column 8 is titled “Items to Consider or Intent of the Regulation”. This 

column contains examples of the items under which regulatory compliance of the particular line 

item is identified. 

(f) Column 9 is titled “DER Authority”. This column identifies the specific DER 

authority delegated to him or her to execute the FAA Form 8110-3 “Statement of Compliance 

with the Federal Aviation Regulations” for the respective approval basis. These authorities are 

identified within Order 8110.37, as amended. A DER may also be required to have the authority 

delegated to him or her by the cognizant Aircraft Certification Office in order to perform 

different functions than those identified in Order 8110.37, as amended. If eligible, the respective 

DER must indicate additional authorized functions in paragraph ‘h’ of Table 1-1 of the checklist. 

(g) Column 10 is titled, “Other Guidance”. Listed here are other sources of 

information, ACs or Orders that may help in preparation. 

b. Checklist Table. The Checklist Table is a description of checklist intent.

c. Approval, Paragraph ‘c’ of Checklist. Alterations meeting the scope described in 

paragraph c may be approved by using this checklist and completing the FAA Form 337. The IA 

can approve this alteration and the airplane incorporating it for return to service using DER data 

by completing Block 7, “Approval for Return to Service,” of FAA Form 337. The FAA usage of 

Block 3, “For FAA Use Only,” will not be required to accomplish this checklist for alterations 

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9/30/04 AC 23-21 

that fall within the scope of paragraph ‘c’, providing that no data approval is required by 

AFS/ASI.

d. Airworthiness Compliance Checklist Applicability, Paragraph ‘d’ of Checklist. 

(1) This paragraph stipulates all the limitations and restrictions of the usage of the 

checklist. The following limitations, restrictions, and requirements are common to all checklists: 

(a) Checklists do not add to or detract from any existing FAA regulations. Some

installations may have additional regulatory requirements beyond those listed below. An 

alteration cannot override an AD. If an alteration affects an AD, contact the FSDO or ACO for 

assistance. 

(b) A foreign country may require more documentation of airworthiness than a 

copy of the FAA Form 337 before it will license an airplane that has been altered with DER data 

or via the field approval method. The nature and amount of additional documentation required 

depends upon the terms of the bilateral agreement between the United States and the importing 

country. Consult the applicable bilateral agreement and comply with its terms before exporting 

an altered airplane. Refer any questions regarding compliance with a bilateral agreement to your 

local Manufacturing Inspection District Office (MIDO). 

(c) Alterations must be compatible with previous alterations and the current 

configuration. 

(d) Checklists for alterations requiring revision of the FAA approved limitations 

section of the Aircraft Flight Manual (AFM) or Flight Manual Supplement (FMS) require ACO 

coordination (reference 14 CFR, part 23, §§ 23.1581 through 23.1589). 

(e) Checklists that require changes to the ALS of the Instructions for Continued 

Airworthiness, as described by 14 CFR, require ACO coordination for approval. 

(2) Other restrictions or limitations applicable to the individual alteration are stipulated. 

One example would be the following: 

This checklist is to be used only on the following:

(a) Airplanes of 6,000 pounds or less maximum gross weight, 

(b) Airplanes having a single, naturally aspirated reciprocating piston engine, 

(c) Unpressurized airplanes. 

e. Checklist Use, Paragraph ‘e’ of Checklist. In this paragraph specific installation 

instructions, installation requirements, continued airworthiness requirements, and inspections are 

outlined. Discussion is provided regarding specific engineering examination requirements, 

installation limitations, and any ICAs required. Specific engineering support data required for 

9 
AC 23-21 9/30/04 

the alteration is outlined in detail. Specific installation requirements, specific things to watch out 

for, are outlined, and any specific instructions for continued airworthiness are stipulated. For 

example: 

(1) If adding or relocating equipment, specific installation requirements should be 

stipulated in this section.

(2) Appropriate operations advisory information should be included in the AFM/FMS. 

(3) Installation must comply with installation instructions and limitations from the 

component manufacturer. 

(4) Any additional ICAs are properly documented per 14 CFR, part 23, § 23.1529, 

requirements. 

f. Necessary Approvals, Paragraph ‘f’ of Checklist. This paragraph specifies the 

necessary approvals to complete the checklist and where on the checklist to indicate the 

approvals. Specific sections of the tables will be filled out by DERs, CRSs, A&Ps, and/or IAs.

g. Applicable Guidance Material, Paragraph ‘g’ of Checklist. This paragraph allows for 

additional applicable guidance material to be added to the checklist. 

h. Evidence of DER Authority to Approve, Paragraph ‘h’ of Checklist. This paragraph 

provides an opportunity for a DER to present his authority to sign and approve a particular 

engineering discipline that has been presented to him in a manner other than in accordance with 

FAA Order 8110.37. 

i. Complete Checklist Process. Send a copy of the completed checklist and supporting 

data to the local FSDO office which will, in turn, forward it to the FAA Aircraft Registration 

Branch, Oklahoma City, Oklahoma, for inclusion in the aircraft record. Submit originals to the 

aircraft owner or operator to be kept with the aircraft records.

9. BENEFITS. The benefits of using the airworthiness compliance checklist during return to 

service of specific alterations on small airplanes are numerous: 

a. The data package requirements for submittal to the FAA are known. 

b. A specific list of all necessary data requirements and means of obtaining FAA approval is 

stipulated. 

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9/30/04 AC 23-21 

c. The overall benefit to the applicant and the FAA is the standardization of requirements 

for a particular small airplane alteration and a more predictable, streamlined FAA approval 

process. The use of the airworthiness compliance checklists will eliminate the need to recreate 

repetitively, from scratch, similar applicable airworthiness requirements. 

David R. Showers 

Acting Manager, Small Airplane Directorate 

Aircraft Certification Service 

11 and 12 

9/30/04 AC 23-21 

APPENDIX 1

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Appendix 1 

A1-2 
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Appendix 1 

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AC 23-21 9/30/04 

Appendix 1 

A1-4 
9/30/04 AC 23-21 

Appendix 1 

A1-5 
AC 23-21 9/30/04 

Appendix 1 

b. Checklist Table. This checklist is issued for guidance purposes to help show 

completeness of documentation for the FAA field approval and return to service process. The 

checklist is intended to provide guidance information during the approval process and then 

returning the airplane to service. An airplane alteration is eligible to use this checklist when it 

has met the requirements outlined in paragraph 8d of this AC. 

c. Approval. Alterations meeting the scope described in paragraph c may be approved by 

using this checklist and completing FAA Form 337. The IA can approve this alteration return to 

service using DER data entirely approved by completing Block 7, “Approval for Return to 

Service,” of FAA Form 337. The FAA usage of Block 3, “For FAA Use Only,” will not be 

required to accomplish this checklist for alterations that fall within the scope of paragraph c, 

providing that no data approval is required by AFS/ASI. Should an IA have any questions about 

the applicability or use of this checklist, they are encouraged to consult with their local FSDO. 

d. Checklist Applicability. This alteration must be in compliance with paragraph 8d of this 

AC, which outlines all the limitations, restrictions and requirements that are applicable to all 

checklists. This checklist is to be used only on alterations to the following: 

• Airplanes of 6,000 pounds or less maximum gross weight 

• Airplanes having a single, naturally aspirated reciprocating piston engine 

• Unpressurized airplanes

• Belt driven installations of alternator or generator only. 

e. Checklist Use. Inspect the alteration and examine its supporting data to verify that they 

meet the following criteria:

(1) The FAA Form 337 must include engineering analysis to support applicability of 

installation to aircraft application. Such engineering analysis should include weight, installation, 

operational Revolutions Per Minute (RPM), heat dissipation compatibility, amperage output 

compatibility and operational rotation direction compatibility. 

(2) If the new alternator or generator replaces an existing alternator or generator, utilize 

Table 1-2 for applicability issues.

(3) Appropriate operational advisory information should be included in the AFM or 

FMS. 

(4) Many aircraft installations have noise filters installed on the aircraft firewall or 

firewall area. These filters have amperage ratings that should not be exceeded by the amperage 

increases of newly installed generators or alternators. 

(5) When installing generators or alternators with increased amperage output from

original, use the drive belt of the smaller original unit. This will maintain the original failure 

mode characteristics of the original installation. 

A1-6 
9/30/04 AC 23-21 

Appendix 1 

(6) Installation must comply with installation instructions and limitations of component 

manufacture and aircraft standards. 

(7) Any additional ICAs are properly documented per 14 CFR, part 23, § 23.1529, 

requirements. 

f. Necessary Approvals. 

(1) All initials must be complete in column one, with “Planned FAA Approval 

Method” selected in column 2-4. 

(2) If column 3 is selected on any item, Block 3 of the FAA Form 337 must be signed 

by an ASI.

(3) An ACO engineer or FSDO inspector may sign in place of any DER signature on 

any data approval. 

g. Applicable Guidance Material. 

h. Evidence of DER Authority to Approve. 

i. Complete Checklist Process. Send a copy of the completed checklist and reporting data 

to the local FSDO office, which will, in turn, forward it to the FAA Aircraft Registration Branch, 

Oklahoma City, Oklahoma, for inclusion in the aircraft record. Submit originals to the aircraft 

owner or operator to be kept with the aircraft records. 

A1-7 
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Appendix 1 

A1-8 
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Appendix 1 

A1-9 
AC 23-21 9/30/04 

Appendix 1 

b. Checklist Table. This checklist is issued for guidance purposes to help show 

completeness of documentation for the FAA field approval and return to service process. The 

checklist is intended to provide guidance information during the approval process and then 

returning the airplane to service. An airplane alteration is eligible to use this checklist when it 

has met the requirements outlined in paragraph 8 of this AC. 

c. Approval. Alterations meeting the scope described in paragraph d may be approved by 

using this checklist and completing FAA Form 337. The IA can approve this alteration return to 

service using DER data entirely approved by completing Block 7, “Approval for Return to 

Service,” of FAA Form 337. The FAA usage of Block 3, “For FAA Use Only,” will not be 

required to accomplish this checklist for alterations that fall within the scope of paragraph c, 

providing that no data approval is required by AFS/ASI. Should an IA have any questions about 

the applicability or use of this checklist, they are encouraged to consult with their local FSDO. 

d. Checklist Applicability. This alteration must be in compliance with paragraph 8 of this 

AC, which outlines all the limitations, restrictions and requirements that are applicable to all 

checklists. This checklist is to be used only on or alterations to the following: 

• Airplanes of 12,500 pounds or less maximum gross weight. 

e. Checklist Use. Inspect the alteration and examine its supporting data to verify that they 

meet the following criteria:

(1) Installation must comply with installation instructions and limitations of component 

manufacture and aircraft standards. 

(2) Any additional ICAs are properly documented per 14 CFR, part 23, § 23.1529, 

requirements. 

f. Necessary Approvals. 

(1) All initials must be complete in column one with “Planned FAA Approval Method” 

selected in column 2-4. 

(2) If column 3 is selected on any item, Block 3 of the FAA Form 337 must be signed 

by the ASI.

(3) An ACO engineer or FSDO inspector may sign in place of any DER signature on 

any data approval. 

g. Applicable Guidance Material. 

h. Evidence of DER Authority to Approve. 

i. Complete Checklist Process. Send a copy of the completed checklist and reporting data 

to the local FSDO office, which will, in turn, forward it to the FAA Aircraft Registration Branch, 

Oklahoma City, Oklahoma, for inclusion in the aircraft record. Submit originals to the aircraft 

owner or operator to be kept with the aircraft records. 

A1-10 
9/30/04 AC 23-21 

Appendix 1 

A1-11 
AC 23-21 9/30/04 

Appendix 1 

A1-12 
9/30/04 AC 23-21 

Appendix 1 

A1-13 
AC 23-21 9/30/04 

Appendix 1 

b. Checklist Table. This checklist is issued for guidance purposes to help show 

completeness of documentation for the FAA field approval and return to service process. The 

checklist is intended to provide guidance information during the approval process and then 

returning the airplane to service. An airplane alteration is eligible to use this checklist when it 

has met the requirements outlined in paragraph 8 of this AC. 

c. Approval. Alterations meeting the scope described in paragraph d may be approved by 

using this checklist and completing FAA Form 337. The IA can approve this alteration return to 

service using DER data entirely approved by completing Block 7, “Approval for Return to 

Service,” of FAA Form 337. The FAA usage of Block 3, “For FAA Use Only,” will not be 

required to accomplish this checklist for alterations that fall within the scope of paragraph c, 

providing that no data approval is required by AFS/ASI. Should an IA have any questions about 

the applicability or use of this checklist, they are encouraged to consult with their local FSDO. 

d. Checklist Applicability. This alteration must be in compliance with paragraph 8 of this 

AC, which outlines all the limitations, restrictions and requirements that are applicable to all 

checklists. This checklist is to be used only on repairs or alterations to the following: 

• Airplanes of 6,000 pounds or less maximum gross weight 

• Airplanes having a single, naturally aspirated reciprocating piston engine 

• Unpressurized airplanes.

e. Checklist Use. Inspect the alteration and examine its supporting data to verify that they 

meet the following criteria:

(1) Appropriate operational advisory information should be included in the AFM or 

FMS. 

(2) Installation must comply with installation instructions and limitations of component 

manufacture and aircraft standards. 

(3) Any additional ICAs are properly documented per 14 CFR, part 23, § 23.1529, 

requirements. 

f. Necessary Approvals. 

(1) All initials must be complete in column one, with “Planned FAA Approval 

Method” selected in column 2-4. 

(2) If column 3 is selected on any item, Block 3 of the FAA Form 337 must be signed 

by the ASI.

(3) An ACO engineer or FSDO inspector may sign in place of any DER signature on 

any data approval. 

A1-14 
9/30/04 AC 23-21 

Appendix 1 

g. Applicable Guidance Material. 

h. Evidence of DER Authority to Approve. 

i. Complete Checklist Process. Send a copy of the completed checklist and reporting data 

to the local FSDO office, which will, in turn, forward it to the FAA Aircraft Registration Branch, 

Oklahoma City, Oklahoma, for inclusion in the aircraft record. Submit originals to the aircraft 

owner or operator to be kept with the aircraft records. 

A1-15 
AC 23-21 9/30/04 

Appendix 1 

A1-16 
9/30/04 AC 23-21 

Appendix 1 

A1-17 
AC 23-21 9/30/04 

Appendix 1 

b. Checklist Table. This checklist is issued for guidance purposes to help show 

completeness of documentation for the FAA field approval and return to service process. The 

checklist is intended to provide guidance information during the approval process and then 

returning the airplane to service. An airplane alteration is eligible to use this checklist when it 

has met the requirements outlined in paragraph 8 of this AC. 

c. Approval. Alterations meeting the scope described in paragraph d may be approved by 

using this checklist and completing FAA Form 337. The IA can approve this alteration return to 

service using DER data entirely approved by completing Block 7, “Approval for Return to 

Service,” of FAA Form 337. The FAA usage of Block 3, “For FAA Use Only,” will not be 

required to accomplish this checklist for alterations that fall within the scope of paragraph c, 

providing that no data approval is required by AFS/ASI. Should an IA have any questions about 

the applicability or use of this checklist, they are encouraged to consult with their local FSDO. 

d. Checklist Applicability. This alteration must be in compliance with paragraph 8 of this 

AC, which outlines all the limitations, restrictions and requirements that are applicable to all 

checklists. This checklist is to be used only on alterations to the following: 

• Airplanes of 12,500 pounds or less maximum gross weight. 

e. Checklist Use. Inspect the alteration and examine its supporting data to verify that they 

meet the following criteria:

(1) Appropriate operational advisory information should be included in the AFM or 

FMS. 

(2) Installation must comply with installation instructions and limitations of component 

manufacture and aircraft standards. 

(3) Any additional ICAs are properly documented per 14 CFR, part 23, § 23.1529, 

requirements. 

f. Necessary Approvals. 

(1) All initials must be complete in column one, with “Planned FAA Approval 

Method” selected in column 2-4. 

(2) If column 3 is selected on any item, Block 3 of the FAA Form 337 must be signed 

by the ASI.

(3) An ACO engineer or FSDO inspector may sign in place of any DER signature on 

any data approval. 

g. Applicable Guidance Material. 

h. Evidence of DER Authority to Approve. 

A1-18 
9/30/04 AC 23-21 

Appendix 1 

i. Complete Checklist Process. Send a copy of the completed checklist and reporting data 

to the local FSDO office, which will, in turn, forward it to the FAA Aircraft Registration Branch, 

Oklahoma City, Oklahoma, for inclusion in the aircraft record. Submit originals to the aircraft 

owner or operator to be kept with the aircraft records. 

A1-19