A major alteration under FAA rules is any change to an aircraft, engine, or propeller that could appreciably affect weight, balance, structural strength, performance, powerplant operation, or flight characteristics, or that cannot be accomplished through accepted practices and elementary operations.1eCFR. 14 CFR 1.1 – General Definitions Once a modification meets that definition, it must be supported by FAA-approved engineering data, performed by a certificated mechanic or repair station, approved for return to service by an FAA inspector, designee, or a mechanic holding an Inspection Authorization (IA), and documented on FAA Form 337 with a copy filed to the FAA within 48 hours. Miss any of those pieces and the aircraft is not legally airworthy.
The Two-Part Definition
The regulatory definition in 14 CFR 1.1 has two independent tests. A modification is major if it might appreciably affect weight, balance, structural strength, performance, powerplant operation, or flight characteristics. It is also major if it cannot be done using accepted practices or elementary operations.1eCFR. 14 CFR 1.1 – General Definitions Meeting either test is enough.
The word “appreciably” carries the weight. A swapped panel screw does not appreciably affect anything. Replacing a landing gear assembly with a different type almost certainly does. The standard is not whether the change definitely affects airworthiness, but whether it might. When the call is close, treating the work as major is the safer path: the paperwork burden of overdocumenting is minor compared to the consequences of an undocumented change surfacing later.
A minor alteration stays within accepted practices, uses straightforward methods, and does not meaningfully shift weight, balance, or performance. Minor alterations still require a maintenance record entry, but they do not need Form 337 or the formal approval chain.
Modifications That Are Automatically Major
Appendix A to 14 CFR Part 43 lists specific changes that count as major whenever they are not already accounted for in the aircraft’s FAA-issued specifications.2eCFR. 14 CFR Part 43 Appendix A – Major Alterations, Major Repairs, and Preventive Maintenance They fall into four groups.
Airframe
Modifications to wings, tail surfaces, the fuselage, engine mounts, control systems, and landing gear qualify as major. So do changes to the basic design of the fuel, oil, cooling, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust systems. Any change to empty weight or balance that pushes the aircraft past its maximum certified weight or center-of-gravity limits also lands here.
Powerplant
Converting an engine from one approved model to another is a classic powerplant major alteration, particularly when the conversion involves changes to compression ratio, propeller reduction gear ratios, or the swapping of major engine parts that require extensive rework and testing. Changes to the basic design of the engine’s fuel, oil, or cooling systems also trigger the classification.
Propeller
Propeller modifications not authorized in the propeller’s FAA-issued specifications count as major. The regulation names changes in blade design and changes in governor or control design as examples.
Appliance
Radio communication and navigation equipment approved under type certification or a Technical Standard Order is treated as a major alteration when changes affect frequency stability, sensitivity, noise level, distortion, or the equipment’s ability to meet environmental test conditions. Installing a modern avionics suite in an older aircraft frequently trips this rule because new equipment changes electrical loads and can interfere with existing systems.
Experimental Aircraft Are a Different World
If your aircraft holds an experimental airworthiness certificate and has never held any other type of airworthiness certificate, Part 43’s maintenance and alteration rules do not apply.3eCFR. 14 CFR 43.1 – Applicability The same exemption covers aircraft that received an experimental certificate after previously holding a light-sport special airworthiness certificate under specific provisions. Amateur-built experimental aircraft can be modified without Form 337 or the formal approval process, though the owner remains responsible for keeping the aircraft in a safe condition. If your aircraft was originally type-certificated and later moved to experimental status, Part 43 still applies to it.
What Counts as Approved Data
Every major alteration must be supported by FAA-approved data. This is the engineering justification proving the change meets airworthiness standards, and it is where owners most often go wrong. “Approved data” is not the same as “acceptable data” found in general maintenance manuals or manufacturer service bulletins. It must carry specific FAA authorization.
- Supplemental Type Certificates (STCs) authorize a specific modification for a particular aircraft model and include installation instructions and any flight manual supplements needed.
- Type Certificate Data Sheets (TCDSs) define the aircraft’s original design limits and can support alterations that stay within those parameters.
- Airworthiness Directives (ADs) that mandate a modification serve as approved data for that alteration.
- FAA field approvals cover one-time alterations to a single aircraft when no existing STC fits.
The person performing the work identifies the approved data source on Form 337. The approving inspector or IA holder then verifies the completed work actually conforms to that data.
When No STC Exists: The Field Approval
A field approval is the path when you want to modify one aircraft and nothing off the shelf covers the change. It is common for one-off projects like a non-standard instrument panel or a custom interior configuration.
You prepare a data package describing the proposed alteration and submit it with FAA Form 337 to your local Flight Standards District Office (FSDO).4Federal Aviation Administration. Field Approval Process The FSDO reviews the submission and may meet with you to assess scope and complexity. From there, one of four things happens:
- The data is adequate, the inspector signs Block 3 of Form 337 approving the data, and the project moves forward.
- More engineering data is requested.
- The project is referred to the Aircraft Certification Office for deeper review.
- The alteration is deemed complex enough that a full Supplemental Type Certification is required instead.
Field approvals work well for straightforward modifications. Structural changes and anything that affects flight characteristics tend to get elevated, so plan for extra time on those.
Who Does the Work and Who Signs It Off
Two roles exist in the major alteration process, and they are often held by different people. Confusing them is a common source of grief.
A certificated mechanic holding airframe and powerplant ratings can perform the alteration and sign the conformity statement on Form 337, confirming the work was completed as described.5Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337 Certificated repair stations can also perform and sign for the work.
Approval for return to service is a separate authority. A Flight Standards inspector, an FAA designee, or a mechanic holding an Inspection Authorization can sign Block 7 of Form 337 to clear the aircraft for flight. The IA holder must personally inspect the completed work to verify it conforms to the approved data, and that responsibility cannot be delegated.6Federal Aviation Administration. Inspection Authorization Information Guide An IA can approve the work but not the underlying engineering data; the data must already carry FAA approval before the IA signs.
Form 337, the Logbook, and the 48-Hour Deadline
FAA Form 337, “Major Repair and Alteration,” is the backbone of the paperwork. It must be executed in at least two copies.7Legal Information Institute. 14 CFR Appendix B to Part 43 – Recording of Major Repairs and Major Alterations The form captures the aircraft’s nationality and registration marks, make, model, and serial number, along with a description of the alteration and the approved data source that authorized it. Vague descriptions are the most common reason forms come back. Be specific about the parts installed, part numbers, and the engineering basis.
Once the approving person has inspected the work and signed the form, one signed copy stays with the aircraft’s permanent records. A second copy must reach the FAA Aircraft Registration Branch in Oklahoma City within 48 hours of the aircraft being approved for return to service.8Federal Aviation Administration. AC 43.9-1G – Instructions for Completion of FAA Form 337 Missing that 48-hour window is one of the more common compliance failures and leaves a gap in the FAA’s records for your aircraft.
Form 337 does not replace the logbook entry. The person who performed the work must also record the alteration in the aircraft’s maintenance records, including a description of the work, the completion date, and the signature and certificate number of the person approving it.9eCFR. 14 CFR 43.9 – Content, Form, and Disposition of Maintenance, Preventive Maintenance, Rebuilding, and Alteration Records Referencing the Form 337 in the logbook ties the two records together for future inspectors.
What Happens If You Skip the Process
An undocumented or improperly approved major alteration is not a paperwork problem. The aircraft loses its airworthiness status, and flying it violates federal regulations.
Insurance follows the same logic. Aircraft policies require the insured aircraft to be airworthy and in compliance with airworthiness requirements. An unapproved modification gives the insurer grounds to reduce or deny a claim if the modification caused or contributed to a loss. Some policies also require notification of any material change in configuration; skipping that notification can leave a later claim uncovered.
On the enforcement side, the FAA can suspend or revoke the certificate of a mechanic or IA holder who performs or approves major alterations without following the process. Pilots who knowingly fly an aircraft with an unapproved major alteration face potential certificate action and civil penalties. Falsifying maintenance records, including Form 337, carries separate penalties under 14 CFR 43.12. Shortcuts on major alterations are not worth the exposure.