Subject: MAINTENANCE AND ALTERATION Date: 10/7/02 AC No: 120-77 

DATA Initiated by: AFS-300 Change: 

1. PURPOSE. This advisory circular (AC) provides one means, but not the only means, of 

ensuring that the contemplated maintenance, alteration, or continue-in-service condition is in 

compliance with applicable regulations and existing policy. This AC is not mandatory and is not 

a regulation. This AC recommends acceptable methods by which: 

a. Air carriers and commercial operators, operating under Title 14 of the Code of Federal 

Regulations (14 CFR) Part 121, Part 129, section 129.14, and Part 135, section 135.411(a)(2), 

may apply the special provisions of Part 43, section 43.13(c). 

b. Repair stations certificated under 14 CFR Part 145 may perform work for the operators 

identified in paragraph a above in accordance with section 145.2 and the special provisions of 

section 43.13(c). 

c. These air carriers and commercial operators may continue articles in service or perform

maintenance and alterations in a different manner from the following: 

Methods, techniques, and practices contained in manufacturer’s Instructions for 

Continued Airworthiness (ICA), 

Maintenance and restoration/overhaul manuals (such as a Structural Repair Manual 

(SRM)), or 

Other methods, techniques, and practices acceptable to the Administrator. 

NOTE: The special provisions of section 43.13(c) do not exempt these air 

carriers and commercial operators from the “approved data” requirement of

sections 121.379(b) and 135.437(b), nor does it exempt a repair station from the 

requirement of 145.51(d)(3). 

2. SCOPE. This AC applies to 14 CFR Parts 121, 135, and section 129.14 operators that are 

required to provide a continuous airworthiness maintenance and inspection program and to the 

aircraft they operate. The aircraft include:
AC 120-77 10/7/02 

a. All that are type certificated in accordance with 14 CFR Part 25 and Civil Air Regulation 

(CAR) 4b, and 

b. Aircraft that have been type certificated in accordance with 14 CFR Parts 23, 27, 29, and 

Special Federal Aviation Regulation (SFAR) 41. 

3. REGULATIONS AND GUIDANCE MATERIAL.

a. Regulations. These acceptable means of compliance with operating regulations refer to

the applicable sections of 14 CFR Parts 91, 121, and 135.

Part 1, Definitions and abbreviations 

































Part 21, Certification procedures for products and parts 

Part 23, Airworthiness standards: Normal, utility, acrobatic, and commuter category

airplanes 

Part 25, Airworthiness standards: Transport category airplanes 

Part 27, Airworthiness standards: Normal category rotorcraft 

Part 29, Airworthiness standards: Transport category rotorcraft 

Part 33, Airworthiness standards: Aircraft engines 

Part 35, Airworthiness standards: Propellers 

Part 39, Airworthiness directives 

Part 43, Maintenance, preventive maintenance, rebuilding, and alteration 

Part 65, Certification: Airmen other than flight crewmembers 

Part 91, General operating and flight rules 

Part 119, Certification: Air carriers and commercial operators 

Part 121, Operating requirements: Domestic, flag, and supplemental operations 

Part 129, Operations: Foreign air carriers and foreign operators of U.S.-registered 

aircraft engaged in common carriage 

Part 135, Operating requirements: Commuter and on demand operations and rules 

governing persons onboard such aircraft 

Part 145, Repair stations 

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10/7/02 AC 120-77 

Part 183, Representatives of the Administrator 

SFAR 36 



































SFAR 41 

b. Related Guidance Materials (Latest Revisions):

(1) FAA Orders. Copies of the following Orders may be purchased from the 

Superintendent of Documents, P.O. Box 371954, Pittsburgh, PA, 15250-7954. These Orders 

may also be accessed online at http://www.faa.gov/RegulatoryAdvisory/index.htm. 

Order 8100.9, DAS, DOA, and SFAR 36 Authorization Procedures 

Order 8110.4, Type Certification Handbook 

Order 8110.37, Designated Engineering Representative (DER) Guidance Handbook 

Order 8130.2, Airworthiness Certification of Aircraft and Related Products 

Order 8300.10, Airworthiness Inspector’s Handbook 

Order 8310.6, Airworthiness Compliance Check Sheets Handbook 

(2) Advisory Circulars. You may obtain copies of these documents from the U.S. 

Department of Transportation, Subsequent Distribution Center, SVC-121.23, Ardmore East 

Business Center, 3341 Q 75th Avenue, Landover, Maryland 20785. 

AC 20-107 , Composite Aircraft Structure 

AC 20-114 , Manufacturers’ Service Documents 

AC 23.1309-1, Equipment, Systems, and Installations in Part 23 Airplanes 

AC 25.571-1, Damage Tolerance and Fatigue Evaluation of Structure 

AC 25.1309-1, System Design and Analysis 

AC 25.1529-1, Instructions for Continued Airworthiness of Structural Repairs on 

Transport Airplanes 

AC 27-1, Certification of Normal Category Rotorcraft

AC 29-2, Certification of Transport Category Rotorcraft

AC 33-2, Aircraft Engine Type Certification Handbook 

AC 33.4-1, Instructions for Continued Airworthiness 

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AC 120-77 10/7/02 

AC 43-9, Maintenance Records 

AC 43.13-1, Acceptable Methods, Techniques and Practices—Aircraft Inspection and 

Repair 

















AC 43.13-2, Acceptable Methods, Techniques and Practices—Aircraft Alterations 

AC 91-56, Continuing Structural Integrity Program for Large Transport Category

Airplanes

AC 91-60, The Continued Airworthiness of Older Airplanes 

AC 140-6, The Development and Use of Major Repair Data Under Provisions of 

Special Federal Aviation Regulation No. 36 

AC 145-3, Guide for Developing and Evaluating Repair Station Inspection 

Procedures Manuals 

(3) Other documents. 

AN 8110.7, Approved Data 

FAA-G-8082-11, Inspection Authorization Knowledge Test Guide 

4. BACKGROUND. Air carriers and other certificate holders have cited the ambiguity of 

existing policies and procedures for developing, substantiating, and documenting maintenance 

and/or alteration data for civil aviation products. Usually, this confusion arises when a certificate 

holder performs a maintenance or alteration task (or allows an existing condition to remain in 

service) that differs from that called out in a manufacturer’s service document, such as a 

Structural Repair Manual, service bulletin, or maintenance manual. In response, the Federal 

Aviation Administration (FAA) Aircraft Certification and Flight Standards Services have jointly

developed this AC. 

5. DEFINITIONS. For the purposes of this AC, the following definitions apply:

a. Airworthiness Limitations Section. The FAA-approved section of the ICA that sets 

forth mandatory replacement times, structural inspection intervals, and related structural 

inspection procedures, such as those established in accordance with section 23.573 and 

section 25.571. Compliance with the Airworthiness Limitations section is required under 

section 43.16 and Part 91, section 91.403 unless an alternative is approved by the Administrator. 

(Reference sections 21.50(b), 43.16, and 91.403.) 

b. Airworthy. To be airworthy:

(1) The aircraft must conform to its Type Certificate (TC). Conformity to type design is 

considered attained when the aircraft configuration and the components installed are consistent 

with the drawings, specifications, and other data that are part of the TC and would include any

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10/7/02 AC 120-77 

Supplemental Type Certificate (STC) and field approved alterations incorporated into the 

aircraft. 

(2) The aircraft must be in a condition for safe operation. The condition of the aircraft 

relative to wear and deterioration (e.g., skin corrosion, window delamination/crazing, fluid leaks, 

tire wear, etc.) must be acceptable. 

c. Approved Data. Technical and/or substantiating data that has been approved by the 

FAA. 

d. Article. An aircraft, airframe, aircraft engine, propeller, appliance, or component part. 

e. Certificate Holder. Within the context of this AC, a person holding an Operating

Certificate (air carrier or commercial operator) issued under Part 119 and conducting operations 

under Part 121 or section 135.411(a)(2), or a Part 145 repair station performing work under 

section 145.2 (new rule cross-references section 145.205). This also includes section 129.14 

operators.

f. Certification Maintenance Requirements (CMR). Required periodic tasks established 

during the design certification of the airplane as an operating limitation of the TC. 

g. Change. A variation from the method, technique, or practice specified in the 

manufacturer’s service document for the article being maintained, altered, or otherwise 

evaluated. Such changes include, but are not limited to, developing new repairs, revising

existing work instructions, using a substituted part other than the one “called out” in a 

maintenance manual, or revising a damage limit or wear tolerance.

h. Continue-in-Service Condition. An article that is authorized to operate with wear or 

damage outside a specified limit, tolerance, or other parameter specified in a manufacturer’s

service document. 

i. FAA Designees. Designated Engineering Representatives (DER), Delegation Option 

Authorization (DOA) holders, and Designated Alteration Stations (DAS). 

j. Instructions for Continued Airworthiness (ICA). The methods, techniques, and 

practices for performing maintenance, preventive maintenance, and alterations, which are

provided by the design approval holder or its component manufacturers, and are considered 

acceptable to the Administrator under section 43.13(a). For example, under Part 25, appendix H, 

the ICA includes an airplane maintenance manual or section, maintenance instructions, and an 

Airworthiness Limitations section. (Reference sections 21.50(b), 25.1529, Part 25, appendix H, 

and 43.13(a).) 

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

specifications. 

(1) An alteration that might appreciably affect weight, balance, structural strength, 

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

airworthiness, or 

Par 5 Page 5
AC 120-77 10/7/02 

(2) An alteration that is not done according to accepted practices or cannot be done by

elementary operations. (Reference Part 1, section 1.1.) 

l. Major Repair. 

(1) A repair that, if improperly done, might appreciably affect weight, balance, structural 

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

airworthiness, or 

(2) A repair that is not done according to accepted practices or cannot be done by

elementary operations. (Reference Part 1, section 1.1.) 

m. Manufacturer’s Service Documents. Publications by a design approval holder that 

provide acceptable methods, techniques, and practices for performing maintenance, preventive 

maintenance, and alterations. They include, but are not limited to, maintenance manuals, 

restoration/overhaul manuals, ICAs, Component Maintenance Manuals, Structural Repair 

Manuals, service bulletins, letters, or other similar information. 

n. Methods, Techniques, and Practices. The step-by-step, “how-to” instructions for 

accomplishing maintenance, preventive maintenance, and alterations. These instructions are 

considered “acceptable to the Administrator” if the certificate holder shows that the instructions 

will return the aircraft, engine, or other article to its original or properly altered condition. 

(Reference sections 21.50(b), and 43.13(a).) 

o. Minor Alteration. An alteration other than a major alteration. (Reference section 1.1.) 

p. Minor Repair. A repair other than a major repair. (Reference section 1.1.) 

q. Structural Repair Manuals (SRM). These typically provide repair instructions, 

material substitutions, and allowable damage limits for the aircraft’s structure.

r. Substantiating Data. Technical data used to show that an article complies with the

applicable airworthiness standards (e.g., Parts 25 or 33). Compliance may be shown by tests, 

analysis, experience, and/or computations appropriate to the maintenance, alteration, or continuein-service condition of the article being evaluated. Substantiating data shown to comply with the

applicable airworthiness standards is acceptable to the Administrator. This is because it

establishes that the article meets the regulatory requirements and would be returned to its original

or properly altered condition by use of this data. (Reference sections 21.31, 25.603, and 

43.13(b).) 

s. Supplemental Type Certificate (STC). A certificate issued by the Administrator

approving a change in the type design of the product. (Reference section 21.117.) 

t. Technical Data. Drawings and specifications, including a list of drawings and 

specifications, needed to define the configuration and design features of a particular article, 

repair, or alteration. Typically, this includes information on materials, dimensions, and 

processes necessary to define structural strength, any required airworthiness limitations, and any

data necessary to determine the airworthiness, noise characteristics, fuel venting, and exhaust 

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10/7/02 AC 120-77 

emissions (as applicable) of the altered or repaired aircraft. Technical data also includes test data 

and engineering analyses and other engineering information, such as engineering handbooks or 

approved military or industry specifications. It may also include operational and service 

experience, maintenance and alteration experience, reliability data, and other documented factual 

information that can be shown to be directly applicable to the airworthiness of the article. 

(Reference Part 21, section 21.31.) 

u. Type Certificate. The type design, the operating limitations, the type certificate data

sheet (TCDS), the applicable regulations with which the Administrator records compliance, and 

any other conditions or limitations prescribed for the product in the pertinent airworthiness 

regulations. (Reference section 21.41.) 

v. Type Design. In pertinent part, this includes: 

(1) The drawings and specifications, and a listing of those drawings and specifications, 

necessary to define the configuration and the design features of the product shown to comply

with the requirements applicable to the product; 

(2) Information on dimensions, materials, and processes necessary to define the 

structural strength of the product; and 

(3) The Airworthiness Limitations section of the ICA as required by Parts 23, 25, 27, 29, 

33, and 35, or as otherwise required by the Administrator. 

6. AIRWORTHINESS DIRECTIVES, THE AIRWORTHINESS LIMITATIONS 

SECTION OF THE ICA, AND CERTIFICATION MAINTENANCE REQUIREMENTS. 

a. In several situations, prior approval must be obtained from the Administrator before a

certificate holder or a repair station may implement changes to documents that are required by

the regulations. For example, changes to Airworthiness Directives (AD) may not be made 

without the prior approval of the Administrator. Typically, this means that an alternative method 

of compliance (AMOC) must be obtained from the Aircraft Certification Office (ACO) or other 

designated FAA office. This is because Part 39, section 39.3 requires the operator of any

product to which an AD applies to comply with the provisions of an AD. The AMOC procedure 

is incorporated into an AD; therefore, complying with an AMOC constitutes compliance with the

AD. The aircraft manufacturer may have made arrangements for some of its DERs to approve 

repairs as an AMOC for an AD. 

b. The Airworthiness Limitations section includes each mandatory replacement time, 

structural inspection interval, and related structural inspection procedures approved under 

sections 23.573, 25.571, etc. (damage tolerance and fatigue evaluation of structure). Changes to 

the Airworthiness Limitations section of an ICA require the approval of the Administrator. (See

sections 43.16 and 91.403.) 

c. CMRs incorporated by reference into the appropriate TCDS also require prior approval of 

the Administrator. For example, the TCDS for the Boeing 777-200/300 series airplanes states 

that the CMRs are listed in either the FAA-approved Section 9 of the Boeing Maintenance 

Planning Document (Airworthiness Limitations and CMRs) or the applicable engine TCDS. The

Par 5 Page 7
AC 120-77 10/7/02 

TCDS for the aircraft specifically states that “the more restrictive requirement from these two 

documents shall be in force.” 

7. MANUFACTURERS’ SERVICE DOCUMENTS. 

a. Manufacturers’ service documents contain methods, techniques, and practices acceptable 

to the Administrator for performing maintenance, preventive maintenance, and alterations. 

Manufacturers’ service documents include maintenance manuals, restoration/overhaul manuals, 

ICA, component maintenance manuals, SRM, service bulletins, service letters, and other similar 

information. Under section 21.50(b), design approval holders are required to prepare and make 

ICA available to operators, repair stations, and others required to comply with ICA. While the

FAA does not approve all service documents, it approves the Airworthiness Limitations section 

of the ICA. Typically, for Part 25 airplanes, both the SRM and service bulletins are approved by

the FAA. 

b. Under 14 CFR Parts 65, 121, 135, and 145, maintenance providers are responsible for 

obtaining FAA approval of technical data prior to approving a major repair or major alteration 

for return to service. Sometimes, manufacturers elect to obtain the FAA’s approval of technical

data supporting a maintenance or alteration action before issuing a service document. This is 

often the most efficient and cost-effective way to approve technical data because it eliminates the 

need for affected operators and maintenance providers to secure independent approval before 

doing the work. However, it remains the operator’s responsibility to ensure that the 

Administrator’s approval of technical data is obtained, if required, before approving for return to 

service a repair or alteration based on the advice or recommendation of a manufacturer. A No 

Technical Objection (NTO) letter or a statement that a particular maintenance or alteration action 

is “DER approvable” does not constitute FAA approval. However, such a statement may be

considered, when supported along with other technical information, when evaluating the 

airworthiness of a particular maintenance or alteration action or continue-in-service condition. 

c. When a manufacturer has obtained the FAA’s approval of technical data, a service 

document is often marked “FAA-Approved,” “FAA/DER-Approved,” or “DER-Approved.” 

Although the entire document is reviewed by the FAA, only the technical data (i.e., engineering

information) associated with it is approved. In many cases, the technical data is not included in 

the body of the service document. This is because that document typically contains only the 

instructions (methods, techniques, and practices) for performing a particular repair or alteration, 

rather than the engineering information that established compliance with the applicable

airworthiness standards. Refer to AC 20-114, Manufacturers’ Service Documents, for methods a 

manufacturer may use to indicate that the FAA has approved service information.

8. THE STRUCTURAL REPAIR MANUAL, MAINTENANCE AND 

RESTORATION/OVERHAUL MANUALS, AND THE INSTRUCTIONS FOR 

CONTINUED AIRWORTHINESS.

a. Under section 21.50(b), the holder of a design approval for which application was made 

after January 28, 1981 must prepare and give the ICA to the owner of the aircraft. The holder of 

the design approval must thereafter make it available to any other person (such as repair stations) 

required to comply with the ICA’s instructions. For transport category aircraft certificated under 

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10/7/02 AC 120-77 

Part 25, the ICA requirements are in section 25.1529 and Part 25, appendix H. Similar ICA 

requirements exist for small airplanes, rotorcraft, engines, and propellers. (Reference Parts 23, 

27, 33, and 35.)

b. The SRM provides repair instructions, material substitutions, and allowable damage limits 

data for continue-in-service condition of the aircraft’s structure. Thus, the SRM sets forth 

acceptable methods, techniques, and practices for performing structural repairs in the same 

manner as other service documents (e.g., maintenance, component, or restoration/overhaul 

manual in compliance with 43.13(a)). Manufacturers typically request FAA approval of the 

SRM so the operator or a repair station may have ready access to approved data to perform major 

repairs and return aircraft to service in accordance with the airworthiness standards and 

sections 121.379(b) and 145.201(c)(2) (formerly 145.51(d)). 

(1) It is the operator’s responsibility to classify repairs as major or minor. An operator 

may use an FAA-approved SRM to accomplish repairs it has classified as either major or minor. 

An operator must support changes to repairs. 

(2) The SRM also provides limits of damage that an operator may use to continue an

aircraft in service without repair. These damage limits are not necessarily the maximum limits 

that the aircraft is capable of withstanding while still complying with the airworthiness standards. 

An operator intending to operate an aircraft with damage beyond a manufacturer’s established 

damage limit must support the position that the aircraft continues to meet the airworthiness 

standards. 

c. Performing maintenance in accordance with the SRM is one way to maintain certain areas 

of an aircraft in an airworthy condition. Aircraft structural designs are certificated to meet the 

applicable airworthiness standards that were in effect when the TC was issued. These rules form

the type certification basis for the aircraft. Any subsequent repairs must continue to comply with 

applicable airworthiness standards and be accomplished in accordance with Part 43. 

d. In addition to providing acceptable methods, techniques, and practices for performing

maintenance, the SRM may also be used, when supported by appropriate substantiating data, to 

develop new or different repairs, or to authorize damage limits beyond those set forth in that 

manual. The SRM may also contain specific procedures authorizing continued aircraft operation 

for a period of time before a permanent repair is required to be completed. 

e. The SRM is not considered part of the Airworthiness Limitations section of the ICA or the

CMR. Moreover, the SRM is not part of the type design, even though it is often referenced on 

the TCDS under the heading of “Service Information,” as it is for the Boeing 777-200/300 series 

airplanes.

9. MAINTENANCE PERFORMANCE RULES. 

a. Section 43.13(a) states that maintenance, preventive maintenance, and alterations may be 

performed in accordance with either of the following:

(1) Methods, techniques, and practices in the maintenance manual or the ICA prepared 

by the manufacturer, or 

Par 8 Page 9
AC 120-77 10/7/02 

(2) Other methods, techniques, and practices acceptable to the Administrator. 

b. Under section 43.13(b), each person must perform work in such a manner and use 

materials of such a quality that the article worked on will be returned to at least its original or

properly altered condition. This does not mean that an article must be maintained in factory-new 

condition. Rather, it requires that the maintenance, preventive maintenance, and alterations 

result in an airworthy product (i.e., one that complies with Part 25, any other applicable 

airworthiness standards (such as Part 33), and the pertinent operating rules). Generally, that is 

accomplished by following the maintenance manuals prepared by the manufacturers, or using

other information acceptable to the Administrator, such as ACs. 

c. Section 43.13(c) states that (unless otherwise notified by the Administrator) the methods, 

techniques, and practices in the air carrier’s maintenance manual constitute an acceptable means 

of compliance with section 43.13. Therefore, an air carrier that follows its maintenance manual 

procedures not only complies with sections 121.367 and 121.369 (see paragraph 11), but also 

complies with section 43.13(a) and (b). The special conditions of section 43.13(c) 

notwithstanding, each operator and repair station is responsible for determining that major 

repairs and major alterations have been accomplished in accordance with technical data approved 

by the Administrator. (Reference sections 121.379(b), 135.437(b), and 145.201(c)(2) (formerly

145.51(d)). 

d. Pursuant to section 145.57 (new rule cross-references sections 145.109 and 145.201), 

repair stations are also required to comply with section 43.13(a) and (b). In addition, when they

perform work for Part 121 air carriers, repair stations must do so in accordance with the carrier’s 

maintenance manual, as required by section 145.2 (new rule cross-references section 145.205). 

When a repair station follows an air carrier’s manual, it will also be in compliance with 

section 43.13(a) and (b).

10. METHODS, TECHNIQUES, AND PRACTICES VERSUS TECHNICAL DATA. 

a. The terms “methods, techniques, and practices” and “technical data” have often been 

confused. While the concepts are related, each has a distinct meaning in 14 CFR. 

(1) The methods, techniques, and practices referenced in section 43.13(a) are the stepby-step instructions for performing maintenance (including inspections), preventive 

maintenance, and alterations. These “how-to” instructions are normally contained in 

manufacturers’ maintenance manuals and other service documents, and are usually based on 

approved technical data developed by the approval holder. 

(2) Technical data consists primarily of engineering information. It includes drawings

and specifications that define the configuration and design characteristics of an article, and 

information on materials, dimensions, and processes necessary to define structural strength. 

Substantiating data is technical data used to establish that the aircraft or other article, if repaired

or altered using the instructions that the substantiating data supports, would be returned to its 

original or properly altered condition in accordance with section 43.13(b). The operator is 

responsible for showing that any substantiating data is directly related to the change that is being

evaluated. This is especially important when it is necessary to substantiate a change using

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10/7/02 AC 120-77 

service experience, maintenance and alteration experience, and reliability data. Accordingly, the

technical data, when substantiated, is considered acceptable to the Administrator even in cases 

when it does not require the FAA’s explicit approval.

b. Maintenance manuals and/or restoration/overhaul manuals may contain technical data. 

For example, if the manual specifies the required materials, dimensions, or other design

information, this represents technical data. 

c. An allowable damage limit in an SRM represents an acceptable method for accomplishing

a repair or authorizing a continue-in-service condition. The TC holder’s damage “limit” is based 

on approved substantiating data—i.e., an engineering analysis showing that even with such 

damage, the airplane would then still meet the appropriate airworthiness standards. In many

cases a higher limit can also be shown to comply with those standards. (Refer to paragraph 12.) 

11. MAINTENANCE AND RESTORATION/OVERHAUL MANUALS. 

a. Parts 121, 135, and Section 129.14 Operators. 

(1) Under sections 121.133 and 121.135, an air carrier must prepare and keep current a 

manual containing these items, among others: 

Instructions and procedures for performing maintenance, preventive maintenance,

and servicing 

Time limitations, or standards for determining time limitations, for

restoration/overhaul, inspections, and checks of aircraft and related components 

Airworthiness inspections, including instructions covering procedures, standards, 

responsibilities, and authority of inspection personnel 

(2) Additional maintenance manual requirements are in sections 121.367 and 121.369. 

These rules also require the carrier to follow its manual when performing maintenance,

preventive maintenance, and alterations. Similar rules exist for Part 135 and section 129.14 

operators. 

(3) An operator’s maintenance manual generally includes two basic kinds of 

information. The first is product-specific and consists of the methods, techniques, and practices 

for performing maintenance and alterations on specific aircraft, engines, and related components. 

The second includes the carrier’s maintenance practices, policies, and procedures, usually found 

in a General Maintenance Manual, Maintenance Policies and Procedures Manual, or similar

document. Both kinds of information—the product-specific and the operator’s general and 

policies and procedures manuals—are part of the operator’s manual system required by Parts 121 

and 135. When preparing its maintenance manual, the operator may designate the 

manufacturer’s manuals as its own, develop its own maintenance information, or use some 

combination of the two. However, once its manual is established, the operator is required to 

comply with its terms. 

Par 10 Page 11
AC 120-77 10/7/02 

(4) Operators may generally change their maintenance manuals without obtaining prior 

FAA acceptance. These changes, however, must be acceptable to the FAA. If an operator is

following its maintenance manual, including its procedures for processing proposed changes to 

service documents, section 43.13(c) states that the operator will be in compliance with 

section 43.13(a) and (b). However, if the Administrator notifies an operator that a procedure in 

its manual is unacceptable, the carrier would no longer be authorized to use that procedure until 

the issue is resolved. Many operators have adopted procedures in their maintenance manuals to 

evaluate proposed changes to existing manufacturers’ service documents. An operator’s 

procedures must ensure that changes (and continue-in-service conditions) result in an aircraft

configuration that meets the airworthiness standards. The procedures must also ensure that when 

interim changes are made, an appropriate maintenance action is accomplished at the appropriate

time, as determined during the analysis phase of the operator’s procedures. These changes (or

variations) may involve the use of a different maintenance procedure, such as an interim repair, 

an equivalent part substitution, inspection technique, or revising repair or damage “limits,” 

including those in an SRM. In some cases, revising a limit results in an article being approved 

for return to service without a repair having been performed. 

(5) Operators typically process proposed changes through their engineering

organizations. The engineers document the damage, condition, or contemplated action; conduct 

a technical analysis to determine whether the change would comply with the pertinent

airworthiness standards (i.e., Parts 25, 33, etc.); and classify the maintenance action, alteration 

action, or continue-in-service condition as major or minor. If major, the technical and 

substantiating data supporting the change must be approved by the Administrator. If minor, an 

engineering analysis should be performed and documented in accordance with the procedures in 

the operator’s maintenance manual, appropriate to the complexity of the situation. The change

may be implemented if the analysis reveals that the proposed change would comply with the 

applicable airworthiness standards. 

(6) Some operators may desire repair stations to develop technical and substantiating

data on their behalf. These operators should have procedures in their maintenance manuals 

governing the manner in which this data may be evaluated under their procedures, including

appropriate documentation and approval. 

b. Repair Stations. 

(1) Pursuant to section 145.2 (new rule cross-references section 145.205), repair stations 

performing work for air carriers must follow the procedures in the air carrier’s manual for 

developing, documenting, and using technical data. An air carrier may use the technical data, 

substantiating data, or approved data developed by the repair station under this AC in accordance 

with the air carrier’s procedures. 

(2) A repair station manual submitted in accordance with section 145.45 (new rule crossreferences section 145.207) governs the operations of certificated repair stations. Like the air 

carrier maintenance manual, the repair station manual is an accepted manual. This manual must 

be followed by the repair station when it performs maintenance, preventive maintenance, and 

alterations under Part 145. Unless otherwise prohibited by the repair station manual, repair 

stations may also implement changes to their manuals without obtaining prior FAA acceptance. 

Page 12 Par 11 
10/7/02 AC 120-77 

If the FAA notifies a repair station that a particular change to its manual would not comply with 

14 CFR, the repair station would be required to rescind the change. 

(3) Like air carriers, repair stations are governed by section 43.13 when they perform 

work. They must use methods, techniques, and practices acceptable to the Administrator, which 

are normally those in the manufacturer’s or air carrier’s service documents. However, when they

are working on behalf of non-air carrier customers, the regulations allow repair stations to use 

other methods, techniques, and practices that are acceptable to the Administrator for performing

the work. Paragraph 12(a) describes the general requirements for demonstrating that a particular

variation from an existing service document would be acceptable to the Administrator. 

(4) Like air carriers, a repair station must also obtain FAA approval of the technical data

supporting a major repair or major alteration.

12. EVALUATING PROPOSED CHANGES TO DATA IN SERVICE DOCUMENTS. 

a. General. Before an air carrier or repair station may change a repair, limit, or other

procedure in an SRM or other service document, the following three requirements must be met. 

First, the change must be processed in accordance with procedures in the air carrier’s 

maintenance manual. Second, the change must be shown to comply with the relevant 

airworthiness standards. Compliance with these standards will establish that the article will be

returned to its original or properly altered condition in accordance with section 43.13(b). Third, 

if the change is classified as “major” in accordance with the operator’s procedures, the technical

data supporting the change must be approved before the article may be approved for return to 

service. 

b. Manual Procedures. The certificate holder’s manual procedures should be sufficiently

detailed, but flexible enough to allow it to classify a change. If required, it should be able to 

obtain FAA-approved data, apply the approved data to the aircraft, and approve the aircraft for 

return to service. To help accomplish this, the certificate holder may take advantage of the latest

communication media (fax, electronic mail, video, telex, etc.). These media can also be used to 

collect and transmit substantiating data in support of a proposed change. A certificate holder 

should not rely only on verbal communications. This could interfere with the proper 

classification of the data required to determine airworthiness. If a certificate holder intends to 

use these media, it should have procedures in its manual that explain how the media will be used. 

Procedures should also provide for a continuous loop of timely information and communications 

among the certificate holder, FAA principal inspector, ACO, designees, and the manufacturer, as 

appropriate. The procedures to evaluate proposed changes to service documents should be 

included in the air carrier manual. The procedures should contain the following elements: 

(1) A method of ensuring only appropriately qualified and experienced personnel will 

make the engineering determinations required. 

(2) A method of determining whether the proposed change requires FAA approval 

because it affects an AD, CMR, Airworthiness Limitation, Minimum Equipment List (MEL), 

maintenance program tasks, intervals, etc., and if the change requires FAA approval, procedures 

for obtaining that approval. 

Par 11 Page 13
AC 120-77 10/7/02 

(3) A method of determining whether the proposed change is major or minor (see 

Appendix 1) and, if major, procedures to obtain FAA approval (or its equivalent, such as SFAR 

36 in the case of repair data) of the technical data supporting the change. 

(4) A method of developing technical/engineering justification, which includes the 

following: (Also see paragraphs 12d through f, below.) 

A review to determine if the existing technical data is sufficient to proceed with 

the proposed change.

A method of consulting with the manufacturer, as appropriate. Such consultation 

brings together the best available knowledge and experience in the areas of 

design, production changes, and operations to ensure a well-designed repair. 

A procedure for obtaining additional technical data, as necessary. 

A procedure for substantiating compliance with pertinent airworthiness 

regulations, including citing the specific 14 CFR Part(s) for which compliance has 

been determined. This should be accomplished through tests, analyses, and/or 

computations appropriate to the complexity of the change being proposed. 

NOTE: In each case, the procedures must establish the airworthiness 

standards that are applicable to the change and provide for documenting 

that the aircraft configuration after the change meets those standards. 

Document the substantiation process. 

(5) The method of notification/reporting to principal inspector(s) of changes 

accomplished, as appropriate. 

(6) For repair stations, a method of coordinating with the air carrier as required by

section 145.2 (new rule cross-references section 145.205). 

(7) A method of retaining the records of each action consistent with the recordkeeping

requirements of 14 CFR. 

c. Evaluating the Proposed Change. When determining whether a contemplated 

change from an existing service document is major or minor, the focus should be the change’s 

effect on the capability of the aircraft configuration after the change to meet the applicable

airworthiness standards. For example, a structural repair performed in accordance with an SRM 

may call for a specific size fastener, fastener edge margin, finish, corrosion treatment, or similar

method, technique, or practice. In evaluating whether a proposed change (such as using an 

oversized fastener or a different finish than specified in the SRM) is major or minor, the analysis 

should address whether the change (including its effect on the repair and the aircraft 

configuration) will meet the definition of a major repair in Part 1. This is because the FAA only

considered the configuration of the aircraft with the repair described in the repair instructions 

when it previously approved the technical data supporting that repair in the SRM. This 

evaluation should be made within the meaning of Part 1. 

Page 14 Par 12 
10/7/02 AC 120-77 

NOTE: Only major changes to FAA-approved technical data require

FAA approval.

d. Showing Compliance with the Relevant Airworthiness Standards. Regardless of 

whether a proposed change is classified as major or minor, the certificate holder must establish 

that after the change the aircraft configuration complies with the pertinent airworthiness 

standards in Parts 23, 25, 27, 29, 33, 35, etc. This would therefore return the article to its 

original or properly altered condition in accordance with section 43.13(b). The certificate holder 

must document its determination so that the FAA can evaluate whether it is following its manual 

procedures. The certificate holder’s procedures must ensure that the evaluation considers the 

effect of the change on the entire aircraft or article. This includes any other changes (approved 

or acceptable) that are affected by the change under consideration. The following procedure 

(paragraph 12f) will help certificate holders evaluate whether appropriate technical and 

substantiating data exists to accomplish the proposed change. The nature and extent of the data 

required will vary according to the complexity of the change and the other factors described 

below. 

e. Initial Assessment.

(1) Description of the Affected Article. This description should include, but is not 

limited to: 

Nomenclature

Part number 

Assembly number 

Serial number(s), if applicable 

Make and model of the article

If known or as appropriate: hours or cycles, time-in-service, time since new, time 

since last maintenance action 

(2) Description of the Discrepancy or Contemplated Change. A complete description 

of the damage, discrepancy, or change should include, but is not limited to: 

Specific location on the article

Type (i.e., corrosion, limit exceeded, cracks, dents) 

Extent and/or dimensions 

Remaining dimensions of the affected area(s) or part(s) 

Associated, hidden and/or adjacent discrepancies, damage, or alteration(s) 

Par 12 Page 15 
AC 120-77 10/7/02 

Previous maintenance action(s)or disposition(s) thereof, or alteration(s) at or 

adjacent to the affected area(s) or part(s) 

Cause, if known (i.e., environmental, accidental, design) 

(3) Review Existing Data. As appropriate, review and document existing information 

(e.g., applicable maintenance documents, procedures, or process specifications) to determine

whether it applies to the proposed change. 

(4) Circumstances in Which Additional Data May Be Required. Situations in which 

further data collection and analysis may be required include, but are not limited to: 

Noticing that no methods, techniques, and practices are specified in the applicable 

service document, or the existing condition is beyond allowable limits or repair 

tolerances

Performing an action in a different manner than prescribed by manufacturers’ 

service documents 

Using maintenance or alteration data intended for a specific product on a like 

product 

Observing a substitution of materials or processes 

Noticing that an AD applies to the product in the area of the damage, discrepancy,

or contemplated change

Discovering that the area of the damage, discrepancy, or contemplated change has 

been defined as a primary structure or will create a Principle Structural Element 

(PSE) (see section 25.571) 

Noticing that CMRs apply in the area of the damage, discrepancy, or 

contemplated change

Discovering that the article has an Airworthiness Limitation 

f. Data Development. 

(1) Technical Qualifications. The operator’s procedures should prescribe the steps for 

reviewing, approving, and overseeing engineering authorizations, and specifying the minimum 

qualifications (particularly knowledge of applicable airworthiness standards) for the individuals 

engaged in these activities. These procedures should identify an individual who is responsible 

for the process, can delegate final engineering approval authority, and has the following

qualifications: 

A thorough working knowledge of the applicable airworthiness standards of 

14 CFR 

Page 16 Par 12 
10/7/02 AC 120-77 

At least 1 year of satisfactory experience in processing engineering work, in direct 

contact with the FAA for type certification or major repair/alteration approval 

At least 8 years of aeronautical experience, which may include the 1 year of 

processing engineering work described above 

(2) Technical Data. The data needed to substantiate changes may range from 

maintenance information referencing existing technical data to (in the case of critical systems)

critical engine parts and primary structure, fully documented engineering analyses, and/or test(s). 

Additional data (such as endurance runs or destructive testing) should be collected if both of the 

following conditions apply: 

The proposed change uses processes that affect the part’s performance 

characteristics, structural strength, or material properties 

Available data, analysis, or computation cannot satisfactorily establish 

compliance with the applicable airworthiness standards 

(a) Assessment. As appropriate to the action being considered, the functions, 

interactions, environment, and consequence of the article’s failure, as applicable, must be 

assessed. The assessment must determine the extent of analysis and the expertise needed to

develop technical and substantiating data to comply with the airworthiness regulations. This 

assessment may include, but is not limited to: 

Functional information describing how the article functions within the

assembly, how the assembly functions within the system, and/or how the 

system functions within the product 

Operating environment of the product 

Interaction with other systems and consequences of failure that could be most 

influenced by the maintenance, disposition, or alteration. Consider the most 

probable effect on its ability to perform its intended function 

Effect of the maintenance, alteration, or continued in service condition on the 

article’s ability to meet the airworthiness standards specified in its type

certification basis, including its operational characteristics or performance 

(b) Substantiating Data. The technical data should be evaluated against each 

applicable regulatory section. The evaluation should be documented with sufficient detail to 

determine whether the technical data meets the applicable airworthiness standards. 

(c) Methods, Techniques, and Practices. The methods, techniques, and practices 

should establish, as applicable, detailed work instructions, inspection criteria (incoming (initial), 

in-process, and/or final inspection), and description of or reference to processes and the 

instructions. For example, processes and instructions could include: 

i. Heat-treating, surface treatment, blending, and welding;

Par 12 Page 17
AC 120-77 10/7/02 

ii. Consideration of specialized processes on engine hardware, such as, but not 

limited to: 

HPT turbine blade repair (especially single crystal alloys)

LPT turbine blade repair

Subjecting engines and parts thereof to specialized processes (acid 

stripping, ultrasonic cleaning, etc.) outside of the manufacturer’s ICA 

iii. Continuous inspection requirements and/or future maintenance action 

requirements; and 

iv. Acceptance of condition without further specific maintenance action. 

13. RETENTION OF RECORDS. The above documentation must be retained for the period 

specified in sections 91.417(b), 121.380(c), 135.439(b), 145.61 (new rule cross-references 

section 145.219), and/or SFAR 36, as appropriate. Because the operator is responsible for the 

continuing airworthiness of these changes, it should retain these records for a sufficient period. 

These records may need to be used as reference material in addressing possible airworthiness 

concerns related to a repair or alteration. 

/s/ 

James J. Ballough 

Director, Flight Standards Service 

Page 18 Par 12 
10/7/02 AC 120-77 

 Appendix 1 

APPENDIX 1. MAJOR AND MINOR DETERMINATIONS—REPAIRS, 

ALTERATIONS, AND CONTINUE-IN-SERVICE CONDITIONS

DOES CHANGE

AFFECT WEIGHT?

ALTERATION

LISTED IN MFR'S

SPECS?

START

No

No

No

No

No

No

No

No

No

Yes

No

Yes

No

Yes

Yes

Yes

Yes

Yes

Yes

Yes

Yes

BALANCE?

STRUCTURAL

STRENGTH?

PERFORMANCE

POWER PLANT

OPERATION?

FLIGHT

CHARACTERISTICS

?

OTHER

AIRWORTHINESS

QUALITIES?

PRINCIPAL

STRUCTURAL

ELEMENT?

CONTINUE IN SERVICE

CONDITION?

Yes

Yes

Yes

MAJOR

IF IMPROPERLY DONE,

WOULD APPRECIABLY

AFFECT AIRWORTHINESS?

MINOR

No

MAJOR AND MINOR DETERMINATIONS --

REPAIRS, ALTERATIONS,

AND CONTINUE IN SERVICE CONDITIONS

No

ACCEPTED PRACTICES OR

ELEMENTARY OPERATIONS?

REPAIR?

ALTERATION

Page 1 (and 2)