FAR Part 43 Appendix A: Major Alterations, Repairs, and Form 337

FAR Part 43 Appendix A is the FAA’s master list that sorts aircraft work into three buckets: major alterations, major repairs, and preventive maintenance. Which bucket a task falls into decides who is legally allowed to perform it, who can sign the aircraft off for return to service, and what paperwork has to reach the FAA. Get the classification wrong and the aircraft can lose its airworthy status even when the wrench work was done correctly.

The Major vs. Minor Threshold

Appendix A does not list every task a mechanic might encounter, so you need the underlying test first. Under 14 CFR 1.1, a major alteration is any change not already in the aircraft, engine, or propeller specifications that could 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.¹

A major repair uses a parallel test: a repair that, if done improperly, could appreciably affect those same airworthiness qualities, or that goes beyond accepted practices and elementary operations.¹ Anything below those thresholds is minor. Minor work still has to be done properly and logged, but it does not require FAA Form 337 or an Inspection Authorization signature. When a task is not explicitly listed in Appendix A but still meets the Part 1 test, treat it as major.

Major Alterations Listed in Paragraph (a)

Paragraph (a) breaks major alterations into airframe, powerplant, propeller, and appliance. The qualifier throughout is “when not listed in the aircraft specifications issued by the FAA.” If the type certificate data sheet already contemplates the change, it is not a major alteration under Appendix A.

Airframe

Changes to wings, tail surfaces, fuselage, engine mounts, control systems, landing gear, hulls or floats, and rotor blades qualify, along with modifications to spars, ribs, fittings, shock absorbers, bracing, cowlings, fairings, and balance weights.²

Two items on this list catch owners off guard. Any change to empty weight or balance that pushes the aircraft beyond its maximum certificated weight or CG limits is automatically a major alteration. And any change to the basic design of the fuel, oil, cooling, heating, cabin pressurization, electrical, hydraulic, de-icing, or exhaust systems counts. Swapping in an aftermarket exhaust or a different fuel system is not a task you can sign off yourself.²

Powerplant

The engine list captures converting an engine from one approved model to another when the conversion involves changes to compression ratio, propeller reduction gearing, or impeller gear ratios. Replacing engine structural parts with anything not supplied by the original manufacturer or specifically approved by the FAA is major, as is installing an unapproved accessory, removing one listed as required equipment, or converting the engine to a fuel grade not shown in its specifications.²

Propeller

Propeller major alterations cover changes in blade design, hub design, or governor and control design. Installing a propeller governor, feathering system, or de-icing system where none previously existed also qualifies, along with installing any parts not approved for the propeller.²

Appliance

Any change to the basic design of an appliance that does not follow the manufacturer’s recommendations or an FAA Airworthiness Directive is major. For radio and navigation equipment approved under a type certificate or Technical Standard Order, changes affecting frequency stability, noise level, sensitivity, selectivity, distortion, or the ability to pass environmental testing are also major alterations.²

Major Repairs Listed in Paragraph (b)

An alteration introduces something new; a repair restores what was damaged. Paragraph (b) uses the same four subcategories.

Airframe

Airframe major repairs center on primary structural members. Strengthening, reinforcing, splicing, or fabricating replacements for structural parts by riveting or welding is major work. The listed components include box beams, monocoque and semimonocoque wings and control surfaces, wing stringers, spars and spar flanges, fuselage longerons, engine mounts, landing gear brace struts, axles, wheels, and control columns, pedals, shafts, and brackets.³

Several task-based triggers apply regardless of the specific part. Repairing damaged metal or plywood stressed covering where the damage exceeds six inches in any direction is major. So is splicing skin sheets, making additional seams in skin sheets, repairing three or more adjacent wing ribs, and replacing fabric on fabric-covered parts like wings, fuselages, and control surfaces.³

Powerplant

The powerplant list is shorter than most owners expect. It covers separating or disassembling a crankcase or crankshaft on a reciprocating engine equipped with an integral supercharger, or one with other than spur-type propeller reduction gearing. Special structural repairs to engine parts by welding, plating, metalizing, or similar methods also qualify.²

Propeller

Propeller repair categories are wide because propeller failure is the most dangerous failure mode on a single-engine aircraft. The following are all major repairs:

Any repairs or straightening on steel blades
Repairing or machining steel hubs
Shortening blades on any propeller
On wood propellers: retipping, replacing outer laminations on fixed-pitch props, repairing elongated bolt holes, and inlay work
Repairing deep dents, cuts, or nicks on aluminum blades, and straightening them
Repair or replacement of internal elements of blades
Complete overhaul of controllable-pitch propellers
Repair of propeller governors

Every item above requires major-repair paperwork and a properly authorized signoff.²

Appliance

Appliance major repairs include calibrating instruments, calibrating radio equipment, rewinding field coils on electrical accessories, completely disassembling complex hydraulic power valves, and overhauling pressure-type carburetors and pressure-type fuel, oil, or hydraulic pumps.²

Preventive Maintenance a Pilot-Owner Can Do

Paragraph (c) is the section most owners care about because it defines what a pilot can legally do without a mechanic. Any holder of a pilot certificate issued under Part 61, other than a sport pilot, can perform preventive maintenance on an aircraft they own or operate, provided the aircraft is not used under Part 121, Part 129, or Part 135. Sport pilots can perform preventive maintenance only on aircraft they own or operate that hold a special airworthiness certificate in the light-sport category.⁴

Preventive maintenance is limited to specific tasks that do not involve complex assembly. The full list contains more than 30 items. The ones owners perform most often:

Removing, installing, and repairing landing gear tires
Cleaning and greasing landing gear wheel bearings
Adding oil, air, or both to landing gear shock struts
Replenishing hydraulic fluid, changing engine oil, and replacing or cleaning oil and fuel strainers and filter elements
Replacing or cleaning spark plugs and setting gap clearance
Replacing defective safety wiring or cotter keys
Replacing bulbs, reflectors, and lenses on position and landing lights, and troubleshooting broken landing-light wiring circuits
Replacing and servicing batteries
Replacing safety belts and replacing seats or seat parts with approved replacements, without disassembling primary structure
Replacing wheels and skis where no weight-and-balance recomputation is needed
Replacing any hose connection except hydraulic connections, and replacing prefabricated fuel lines
Replacing any cowling that does not require removing the propeller or disconnecting flight controls
Replacing side windows where the work does not interfere with structure or operating systems
Refinishing decorative coatings on non-structural surfaces, repairing cabin upholstery, and making small fabric patches that do not require rib stitching

The full list also covers replacing elastic shock absorber cords, lubrication that does not require disassembling structural components, applying preservative coatings, and small repairs to fairings and nonstructural cover plates.²

The thread running through every item is that none of them touch primary structure, flight controls, or operating systems. The moment your work requires disassembling a structural member or disconnecting a flight control, you have crossed out of preventive maintenance no matter how simple the task looks.

Who Can Perform the Work and Who Can Sign It Off

Appendix A tells you what category the work is in. 14 CFR 43.3 tells you who is authorized. This is where most confusion happens.

Holders of an FAA mechanic certificate, commonly the A&P for airframe and powerplant, can perform maintenance, preventive maintenance, and alterations within the scope of their ratings. Someone working under a mechanic’s direct supervision can perform the same work, but only if the supervisor personally observes the work to the extent necessary and is available for consultation. That supervised-work exception does not extend to inspections, or to the inspections required after major repairs and major alterations.⁴

The signoff distinction is the one that trips people up. A standard A&P mechanic can perform major repairs and major alterations, but cannot approve them for return to service. That authority belongs to a mechanic who holds an Inspection Authorization issued under 14 CFR 65.95. An IA holder can inspect and approve an aircraft for return to service after a major repair or major alteration, provided the work was done using FAA-approved technical data. IA holders also perform annual inspections and supervise progressive inspections.⁵ Certificated repair stations operating under Part 145 can both perform and approve major work within the scope of their ratings.⁴

Approved Data Required for Major Alterations

A major alteration has to be carried out using FAA-approved data, meaning drawings, specifications, and procedures that define exactly how the modification is to be made. Improvising a major alteration is not permitted no matter how experienced the mechanic.

The most common source of approved data is a Supplemental Type Certificate. An STC is issued when the FAA has approved a specific modification to an existing aircraft design, and it incorporates the original type certificate by reference.⁶ A new avionics suite or an engine conversion kit almost always comes with an STC.

Other approved-data sources include Airworthiness Directives, field approvals signed by an FAA inspector, and data approved by a Designated Engineering Representative hired by the owner. The Form 337 must identify which source was used. If the source is not an STC or Airworthiness Directive, the Form 337 needs the signature of the FAA inspector or DER who approved the data, in addition to the mechanic who performed the work and the IA who inspected it.

Minor alterations can be performed using “acceptable data” rather than “approved data.” Acceptable data includes manufacturer’s maintenance manuals, FAA advisory circulars, and other industry-accepted methods. The gap between approved and acceptable data is one of the most practically significant differences between major and minor work.

Maintenance Records and Form 337

Every person who performs maintenance, preventive maintenance, rebuilding, or an alteration has to create a maintenance record entry. The required content:

A description of the work, or a reference to acceptable data describing it
The date the work was completed
The name of the person who did the work, if different from the person approving it
The signature, certificate number, and type of certificate held by the person approving the work for return to service

That signature is the approval for return to service, but only for the specific work described.⁷ When a pilot-owner performs preventive maintenance, they log the entry themselves and sign it with their pilot certificate number, acting as both the person performing the work and the person approving it.

Major repairs and major alterations require an additional layer: FAA Form 337.⁸ Appendix B to Part 43 sets the filing process:

The person performing the work executes Form 337 in at least duplicate
One signed copy goes to the aircraft owner
A second copy must be forwarded to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours after the aircraft is approved for return to service

The 48-hour clock starts when the aircraft is approved for return to service, not when the work is completed.⁹

Weight, Balance, and Flight Manual Updates

If a major alteration changes the empty weight or center of gravity, the weight-and-balance data has to be updated before the next flight. Appendix A flags changes to empty weight or balance that push the aircraft beyond its certificated weight or CG limits as major alterations in their own right.²

Beyond weight and balance, no person may approve an aircraft for return to service after a repair or alteration that changes the operating limitations or flight data in the approved flight manual without revising those documents accordingly.¹⁰ A new engine model may change Vne, fuel burn, or performance charts. Flying with outdated limitations puts the aircraft out of compliance even when the mechanical work was faultless.

Consequences of Getting the Classification Wrong

Unauthorized maintenance, improper signoffs, and false record entries carry real federal exposure. The FAA can impose civil penalties on individuals and organizations for maintenance violations, and 49 U.S.C. § 46316 provides for criminal prosecution: a person who knowingly and willfully violates aviation safety regulations can face fines under Title 18 and imprisonment of up to five years.¹¹ Certificate action is the more immediate risk for most mechanics and pilots: the FAA can suspend or revoke mechanic certificates, pilot certificates, and repair station certificates for maintenance violations.

For an owner, the practical result of unauthorized or improperly documented work is that the aircraft loses its airworthiness status. Without valid records or with undocumented major alterations, it cannot legally fly and is nearly impossible to sell or insure at standard rates. The 48-hour Form 337 filing exists so the FAA’s records match the aircraft’s actual condition; missing filings surface later at annual inspections, pre-purchase inspections, and accident investigations, when the cost of fixing them is highest.

1
eCFR. 14 CFR 1.1 General Definitions
2
eCFR. Appendix A to Part 43 – Major Alterations, Major Repairs, and Preventive Maintenance
3
Legal Information Institute. 14 CFR Appendix A to Part 43 – Major Alterations, Major Repairs, and Preventive Maintenance
4
eCFR. 14 CFR 43.3 Persons Authorized to Perform Maintenance, Preventive Maintenance, Rebuilding, and Alterations
5
eCFR. 14 CFR 65.95 Inspection Authorization Privileges and Limitations
6
Federal Aviation Administration. Supplemental Type Certificates
7
eCFR. 14 CFR 43.9 Content, Form, and Disposition of Maintenance, Preventive Maintenance, Rebuilding, and Alteration Records
8
Federal Aviation Administration. Advisory Circular 43.9-1G – Instructions for Completion of FAA Form 337
9
eCFR. Appendix B to Part 43 – Recording of Major Repairs and Major Alterations
10
eCFR. 14 CFR Part 43 – Maintenance, Preventive Maintenance, Rebuilding, and Alteration
11
GovInfo. 49 USC 46316 – General Criminal Penalty