FAA Major and Minor Repairs Explained: Reading 14 CFR Part 43 Appendix A and Form 337 from the UK

In the FAA system, a repair is major when 14 CFR Part 43 Appendix A lists it: work that strengthens, reinforces, splices or manufactures primary structural members, such as spars, longerons, wing main ribs or engine mounts, plus named cases like stressed-skin damage exceeding six inches in any direction or three or more adjacent ribs. Major repairs need approved data and an FAA Form 337. Minor work can use acceptable data such as AC 43.13-1. This is American law: the base doesn't cover UK CAA or EASA rules.

Why would a British reader care? Part 43 states that it applies to aircraft having a U.S. airworthiness certificate, so if you own, maintain, buy or inspect such an aircraft, its records will be written in FAA vocabulary, and students comparing maintenance systems will find the FAA's major/minor split a useful reference point. This guide sets out the FAA rule precisely, from the eCFR text of Part 43 and the FAA AMT General Handbook, both indexed in the Kopik AC 43.13 base.

How the FAA defines "major"

The FAA AMT General Handbook (FAA-H-8083-30B, Chapter 2) describes a major action as one that "might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness" and that is not done according to accepted practices or cannot be done by elementary operations. In practice, it sends technicians to 14 CFR part 1 and to Part 43 Appendix A, which lists the cases.

The handbook also distinguishes repairs (returning the aircraft to its previous condition, even an engine change if it is the engine specified for the type) from alterations (changing the aircraft, such as winglets, new avionics or an engine not listed in the Type Certificate Data Sheet). Appendix A has separate lists for each.

The airframe list, with its thresholds

Appendix A (b)(1) names the airframe parts whose strengthening, reinforcing, splicing or manufacture, or replacement by fabrication such as riveting or welding, is a major repair. Among them: box beams, spars and spar flanges, wing stringers or chord members, wing main ribs and compression members, brace struts, engine mounts, fuselage longerons, bulkhead and truss members, main seat support braces, landing gear brace struts, axles, wheels, skis, and control system parts such as columns, pedals, shafts, brackets or horns.

Read the thresholds literally. A 4-inch skin repair is not major because of its size, but it could still be major if it involves splicing a skin sheet. Two adjacent ribs are below the threshold; three are not.

Where AC 43.13-1B draws its own lines

• Ribs: manufacturer repair schemes are acceptable, "but any other methods of reinforcement are major repairs and require approved data" (paragraph 4-58c).
• Fabric: recovering or repairing with a fabric or coating other than the type used by the original manufacturer is considered a major alteration (paragraph 2-5).
• Blind rivets in airframe sheet metal repairs must be authorised by the airframe manufacturer or approved by an FAA representative (paragraph 7-2j).
• Engine mounts: if the manufacturer stress-relieved the mount after welding, it should be re-stress-relieved after weld repairs (Chapter 4).

Approved data versus acceptable data

This is where AC 43.13 is most often misunderstood. Per the AMT General Handbook, minor repairs and alterations need only acceptable data, "such as manufacturers' maintenance manuals or AC 43.13-1", and are recorded with a logbook entry. Major repairs and alterations require approved data: Airworthiness Directives, STCs, TCDS, DER-specific delegations, FAA-approved manufacturer service bulletins. Where no approved data exists, a technician may ask the FAA for a field approval on Form 337.

So AC 43.13-1B does not, on its own, approve a major repair, and it never replaces the manufacturer's manual: 14 CFR 43.13(a) puts the current maintenance manual or Instructions for Continued Airworthiness first. One nuance for alterations: AC 43.13-2B says its data may be used as approved data for major alterations when the chapter, page and paragraph are entered in block 8 of Form 337 and the user has determined it is appropriate to the product, directly applicable, and not contrary to manufacturer's data.

How an FAA field approval works

1. The technician completes the front of Form 337 through block 6, leaving block 3 open for the FAA.
2. Block 8 on the reverse describes the intended work and the substantiating reference data.
3. The form goes to the local FAA Flight Standards District Office (FSDO) for review by an aviation safety inspector, who may consult other specialists.
4. If the data complies, the inspector enters an approval statement in block 3, either for the data alone or after physically inspecting the aircraft, in each case approved for use only on the aircraft described, subject to conformity inspection by a person authorised under § 43.7.

In other words, the approval attaches to one aircraft and one job. A field-approved Form 337 found in another aircraft's records is not a ready-made approval for yours.

Reading a Form 337 in the aircraft records

If you are going through the records of a US-certificated aircraft, each Form 337 marks a major repair or alteration. The AMT General Handbook explains its main items:

• Item 1: aircraft data from the dataplate, with the registration mark compared against the registration form.
• Item 3: completed only when the FAA granted a field approval; often blank because existing approved data was used.
• Item 5: marked either "Repair" or "Alteration".
• Item 7: approval for return to service, completed by the IA or an authorised individual from a repair station.
• Item 8 (reverse side): description of the work, which must reference the approved data used. This is the line to check.

Under Part 43 Appendix B(a), the form is executed at least in duplicate, a signed copy goes to the owner, and a copy goes to the FAA Aircraft Registration Branch in Oklahoma City within 48 hours of return to service. A certificated repair station may instead use a signed work order and maintenance release for major repairs (Appendix B(b)).

Who may do and sign what, in the FAA system

• Mechanic certificate holders work and approve return to service as provided in Part 65 (§§ 43.3(b), 43.7(b)).
• The handbook cites 14 CFR 65.95(a)(2): "an IA must perform an annual inspection".
• Work done under supervision does not extend to any inspection performed after a major repair or alteration (§ 43.3(d)).
• Pilot-owners may do preventive maintenance from the Appendix A(c) list (for example, simple fabric patches without rib stitching, or replacing and servicing batteries) on aircraft not used under parts 121, 129 or 135 (§ 43.3(g)).
• Part 43 does not apply to experimental-certificated aircraft unless they previously held another kind of airworthiness certificate (§ 43.1(b)).

Questions such as "I patched a 4-inch dent in my aluminium wing skin myself, does that need an FAA major repair sign-off?" get a cited answer in the base. The regulation is on eCFR, 14 CFR Part 43, and the handbook on the FAA website.