Under 14 CFR Part 43, major repairs and alterations are maintenance jobs that either appear on the Appendix A list or could appreciably affect an aircraft’s weight, balance, structural strength, performance, powerplant operation, or flight characteristics if done improperly. That classification decides three things at once: whether you need FAA-approved data before starting, whether the work must be recorded on FAA Form 337, and who is allowed to approve the aircraft for return to service when the job is done. Everything else in Part 43 flows from getting this call right.
How the FAA Draws the Line
The definitions live in 14 CFR 1.1. A major alteration is any alteration not already listed in the aircraft, engine, or propeller specifications that could appreciably affect weight, balance, structural strength, performance, powerplant operation, or flight characteristics, or that goes beyond accepted practices and elementary operations. A major repair uses nearly identical language, framed as risk: if the repair were done improperly, could it appreciably affect those same qualities? If so, it is major.1eCFR. 14 CFR 1.1 – General Definitions
A minor repair or minor alteration is simply one that does not meet the major threshold. There is no separate definition with its own criteria. If the work does not appear in Appendix A to Part 43 and does not trigger the broad language of 14 CFR 1.1, it is minor by default.2Federal Aviation Administration. Advisory Circular 43-210A – Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations
For a borderline job, Advisory Circular 43-210A provides flowcharts to work through. Start with Appendix A. If the task matches something on that list, it is major. If it does not match but could still appreciably affect airworthiness under the 14 CFR 1.1 language, it is still major. Only when neither test is met does the work qualify as minor.
Concrete Examples From Appendix A
Appendix A to Part 43 is the closest thing to a definitive checklist, catalogued by airframe, powerplant, propeller, and appliance. Reviewing it before starting significant work is the fastest way to avoid a classification mistake.
Airframe
Airframe major alterations include changes to wings, tail surfaces, or control surfaces that affect flutter or vibration characteristics. Redesigning a fuel, oil, pressurization, electrical, hydraulic, or de-icing system also qualifies, as does installing a different engine mount designed for a new engine type.3eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance
On the repair side, any work that involves strengthening, reinforcing, splicing, or fabricating primary structural members counts as major. That covers spar repairs, fuselage longeron reinforcement, and stressed-skin patches larger than six inches in any direction. Splicing or adding seams to skin sheets falls here too.3eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance
Powerplant and Propeller
Converting an engine to run on a different fuel grade is a powerplant major alteration. So is converting from one approved engine model to another when compression ratios, reduction gear ratios, or major internal parts change and the engine needs extensive rework and testing. For propellers, changes in blade design and changes to the governor or control system are both major alterations, because both directly affect thrust characteristics and flight safety.3eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance
Appliances
An appliance alteration becomes major when it changes the basic design of the appliance outside the manufacturer’s recommendations or an Airworthiness Directive. Changes to radio or navigation equipment that affect frequency stability also qualify, even if the external installation looks identical.3eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance
Who Can Perform the Work vs. Who Can Sign It Off
Performing the work and approving the aircraft for return to service are two separate regulatory acts. This is the point where projects most often stall.
A certificated mechanic holding an airframe or powerplant rating can perform major repairs and alterations within the scope of that rating. A certificated repair station can do the same within its ratings. Someone working under the direct supervision of a certificated mechanic can also perform the hands-on work, but that supervised person cannot conduct the inspection that follows.4eCFR. 14 CFR 43.3 – Persons Authorized To Perform Maintenance, Preventive Maintenance, Rebuilding, and Alterations
Approval for return to service is narrower. Under 14 CFR 43.7, the people authorized to approve major work are a mechanic with an Inspection Authorization, a certificated repair station, the original manufacturer (for work it performed), and air carriers operating under Parts 121 or 135.5eCFR. 14 CFR 43.7 – Persons Authorized To Approve Aircraft, Airframes, Aircraft Engines, Propellers, Appliances, or Component Parts for Return to Service
For most general-aviation aircraft, that means an Inspection Authorization holder. The IA must personally perform the conformity inspection, and this responsibility cannot be delegated. The inspection includes verifying the data is FAA-approved, confirming the work matches the approved data, checking that operating limitations have been updated if affected, and ensuring the weight-and-balance records and equipment lists are revised when appropriate.6Federal Aviation Administration. Inspection Authorization Information Guide The IA then signs Block 7 of FAA Form 337 and returns both copies to the person who performed the work.
An A&P mechanic without an Inspection Authorization can perform major repairs and alterations all day, but that same mechanic cannot approve the airplane for return to service afterward. Line up an IA or a repair station before the job starts, not after.
Approved Data vs. Acceptable Data
The kind of technical data you need depends entirely on whether the work is major or minor. Major work requires data that has been formally approved by the FAA. Minor work only needs data that is acceptable to the FAA.2Federal Aviation Administration. Advisory Circular 43-210A – Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations
Approved data means someone at the FAA or an FAA-delegated authority has reviewed and formally signed off on the engineering behind the work. Common sources include Supplemental Type Certificates, Airworthiness Directives, FAA-approved manufacturer repair manuals, and data packages approved through the field approval process. The approval is documented with a letter, stamp, or signature in Block 3 of Form 337.
Acceptable data has not necessarily been reviewed by the FAA in advance. The person using it must be able to demonstrate that it meets regulatory standards, but the FAA bears the burden of showing it is unacceptable if a dispute arises later.2Federal Aviation Administration. Advisory Circular 43-210A – Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations Using merely acceptable data for a major repair is a regulatory violation, even if the repair itself is flawless.
A&P mechanics cannot approve the technical data themselves. If the data needed for a major job has not already been approved through an STC, AD, or other channel, the mechanic must obtain approval before beginning. Advisory Circular 43-210A is blunt about this: do not start work until all data is approved, because the FAA may not approve the package as submitted, and any work performed before approval may not conform to the final approved design.2Federal Aviation Administration. Advisory Circular 43-210A – Standardized Procedures for Obtaining Approval of Data Used in the Performance of Major Repairs and Major Alterations
Field Approval When No Approved Data Exists
When no Supplemental Type Certificate or previously approved data covers a planned modification, you can seek a field approval from the local Flight Standards District Office. A field approval covers one specific aircraft by serial number and is documented on FAA Form 337.7Federal Aviation Administration. Field Approval Process
You determine that the proposed work qualifies as a major repair or alteration under 14 CFR 1.1 and Appendix A, prepare a data package, and submit it with a completed Form 337 to the FSDO. The FSDO may find the data adequate and have an Aviation Safety Inspector sign Block 3 approving it; may send the package back for supplementation; may forward it to an Aircraft Certification Office for deeper engineering review; or may determine that the proposed alteration exceeds what a field approval can cover and must go through the Supplemental Type Certificate process instead.
A Designated Engineering Representative can also approve data for major repairs and alterations using FAA Form 8110-3, but the DER’s approval covers only the engineering data. The installation still has to be approved for return to service by an IA holder or repair station after the conformity inspection.8Federal Aviation Administration. Documenting Compliance Findings – Using FAA Form 8110-3
Form 337, Filing, and Logbook Entries
FAA Form 337 is the official record for every major repair and major alteration performed on a civil aircraft, and the regulation requires it to be executed in at least duplicate.9eCFR. 14 CFR Part 43 Appendix B – Recording of Major Repairs and Major Alterations
The form captures identifying information for the aircraft — nationality and registration marks, make, model, and serial number — along with the owner’s name and address.10Legal Information Institute. 14 CFR Appendix B to Part 43 – Recording of Major Repairs and Major Alterations Block 8, the description of work performed, is the most scrutinized section and must describe the actual work in enough technical detail for a future inspector to understand exactly what was done. Block 3 is where data approval lives; an FAA Aviation Safety Inspector signs there once the data package is found compliant. Block 7 is the return-to-service approval, signed by the IA holder or repair station representative after the conformity inspection.11Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337
Certificated repair stations performing major repairs under a manual or specifications acceptable to the FAA may use the customer’s work order instead of Form 337. The repair station gives the owner a signed maintenance release identifying the aircraft, describing where the repair was made, and containing a return-to-service statement, and must keep a duplicate of the work order for at least two years.9eCFR. 14 CFR Part 43 Appendix B – Recording of Major Repairs and Major Alterations
The 48-Hour Filing Deadline
Once the work is complete and approved, one signed copy of Form 337 goes to the aircraft owner and one copy goes to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the aircraft is approved for return to service. The mailing address is P.O. Box 25504, Oklahoma City, OK 73125.11Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337 The 48-hour clock starts when the authorized person signs Block 7, not when the mechanic finishes turning wrenches. The aircraft is eligible to fly as soon as that signature is obtained; you do not need to wait for the Registration Branch to process or acknowledge the form.
The FAA also offers an electronic Form 337 system at eforms.faa.gov. Forms submitted electronically are forwarded automatically to the Registration Branch. The mechanic, designee, or FAA inspector must be authorized by their Responsible Flight Standards Office to use it, and you cannot mix methods on a single form: it must be submitted either entirely electronically or entirely on paper.12Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337
Logbook Entries
Beyond Form 337, the person performing the work must make an entry in the aircraft’s maintenance records. The entry must include a description of the work performed (or a reference to acceptable data), the date of completion, and the signature and certificate number of the person approving the work.13eCFR. 14 CFR 43.9 – Content, Form, and Disposition of Maintenance Records If the work affects weight and balance, the changes must also be entered in the aircraft’s weight-and-balance records with a reference to the Form 337.
What Happens If You Get It Wrong
Misclassifying major work as minor, skipping Form 337, or using unapproved data are the kinds of violations that attract FAA attention during ramp inspections and accident investigations.
Certificate actions are the more immediate threat for mechanics. The FAA can suspend a mechanic certificate for a fixed number of days as a disciplinary measure, suspend it indefinitely until the mechanic demonstrates competency, or revoke it entirely when the FAA determines the holder is no longer qualified.14Federal Aviation Administration. Legal Enforcement Actions Revocation means starting the certification process over from scratch.
Civil penalties are the other tool. The FAA can assess penalties of up to $100,000 against an individual per violation. In practice, penalties for each violation generally range from $1,100 to $75,000 depending on the provision violated and whether the violator is an individual airman, an individual who is not an airman, a small business, or a larger entity.14Federal Aviation Administration. Legal Enforcement Actions Falsifying or omitting material information from maintenance records can result in both civil penalties and certificate action under 14 CFR Part 3.
Aircraft owners feel it too. An airplane with incomplete or missing 337 records loses its airworthiness documentation trail. That can ground the aircraft until the records are reconstructed, and it almost always reduces resale value, because buyers and pre-purchase inspectors check 337 history carefully and treat gaps as red flags.