Major Repairs, Major Alterations, Field Approvals, and Form 337
Classification first
Part 1 defines major repair and major alteration as those not listed in the aircraft, engine, or propeller specifications that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other airworthiness qualities, or that cannot be done by elementary operations or accepted practices. Part 43 Appendix A gives category examples (airframe, powerplant, propeller, appliance) and Appendix B describes how the work is recorded. Appendix A also lists preventive maintenance - a common distractor set.
Approved data
A major alteration may only be approved for return to service when accomplished with approved data: an STC, data approved with a type certificate, a manufacturer's approved structural repair manual drawing, or an FAA field approval. Generic guidance such as AC 43.13-1B or 43.13-2B is acceptable data, not approved data, for a major alteration. Designees (DERs) and organization designees can approve data; an IA cannot.
Documenting it
FAA Form 337 records the major repair or alteration: aircraft and owner identification, a description of the work, the approval-for-return-to-service signature by the IA or repair station, and, where the FAA grants a field approval, the FAA approval block citing the approved data. One copy is filed with the FAA and one becomes part of the aircraft records; logbook entries still accompany the form.
Traps
Field approval is FAA action, not IA action - an IA who lacks approved data must stop and seek it. Minor alterations need only a logbook entry, but minor versus major is decided by effect on the aircraft, not by how much time the job took. Structural repairs outside published repair manual limits require FAA-approved data.
Rule of thumb
No approved data, no approval signature.