Welding on aircraft structures, engine mounts, fuel systems, and exhaust components is not simply a matter of having a skilled welder and the right torch. Federal Aviation Administration (FAA) regulations establish a layered framework of rules that determine who is authorized to perform welded repairs, what data must be used, and when additional approvals are required before the aircraft can be returned to service. For an Aviation Maintenance Technician (AMT) holding an Airframe rating, mastering this framework is as important as mastering the metallurgy and technique of welding itself. A technically perfect weld performed without proper approval or documentation can still render an aircraft unairworthy in the eyes of the FAA.
This article walks through the regulatory foundation for welded repairs, the approval pathways available to AMTs, the role of Approved Maintenance Data, and the practical steps required to close out welded repair work correctly.
The Regulatory Foundation
The authority governing aircraft maintenance and repairs begins with 14 CFR Part 43, which defines who may perform maintenance, what standards apply, and what records must be kept. Part 43, Section 43.13 requires that any person performing maintenance use methods, techniques, and practices acceptable to the FAA Administrator — specifically those described in the aircraft manufacturer's current maintenance manual, Instructions for Continued Airworthiness (ICA), or other FAA-approved data. This rule applies directly to welding: you cannot simply decide to weld a cracked spar fitting using a technique you learned in trade school if the manufacturer specifies an entirely different procedure or material.
14 CFR Part 65 governs the certification of AMTs. An Airframe-rated mechanic is authorized to perform maintenance, preventive maintenance, and alterations on airframe structures — which includes welding on structural components, provided the work falls within the scope of their certificate and rating. However, holding an Airframe rating alone does not automatically grant authority to perform all welded repairs. Certain repairs are classified as major repairs, and these carry additional regulatory requirements.
Major vs. Minor Repairs: Why the Distinction Is Critical
Part 43, Appendix A divides repair work into major repairs and minor repairs. This distinction is the single most important classification decision an AMT makes before picking up a torch.
A minor repair is one that, if improperly performed, has only a slight effect on the weight, structural strength, flight characteristics, or other airworthiness qualities of the aircraft. Minor repairs can be approved by a certificated AMT with the appropriate rating using an FAA Form 337 is not required for minor repairs — the AMT simply signs off the maintenance record per Part 43.9.
A major repair is defined as one that might appreciably affect structural strength, flight characteristics, powerplant operation, or other airworthiness qualities if improperly done. Appendix A to Part 43 lists specific examples of major repairs to airframe structure. Welding falls squarely into this category in several situations, including: welding on primary structural members, welding on fittings or attachments, welding on seats, berths, safety belts, or fuel system components, and welding on exhaust manifolds. Any welded repair meeting the definition of a major repair requires approved data and must be documented on FAA Form 337 (Major Repair and Alteration form), which is then submitted to the local Flight Standards District Office (FSDO).
Approved Data: What It Is and Where It Comes From
Before performing a welded repair classified as major, the AMT must have approved data in hand. Approved data is documentation that the FAA has accepted as technically adequate to restore airworthiness. It is not simply any technical reference — it has a specific regulatory meaning.
- Manufacturer's Maintenance Manual or Structural Repair Manual (SRM): If the manufacturer's FAA-approved manual contains a specific repair procedure for the type of weld damage encountered, that data is considered approved. The AMT must follow the procedure exactly, including material specifications, filler materials, heat treatment requirements, and inspection criteria.
- Airworthiness Directives (ADs): ADs issued under 14 CFR Part 39 may mandate specific welded repairs or inspections. Compliance with an AD is mandatory and constitutes approved data when a repair is performed as described in the AD.
- Advisory Circulars (ACs): AC 43.13-1B, Acceptable Methods, Techniques, and Practices — Aircraft Inspection and Repair, is widely used as an approved data source for repairs on non-pressurized aircraft not covered by specific manufacturer instructions. Chapter 4 of AC 43.13-1B covers welding and provides material specifications, joint design guidance, and inspection criteria for gas and arc welding on steel tube structures. However, AMTs must confirm with their local FSDO that the use of AC 43.13-1B is acceptable for the specific repair, since it is not automatically approved data for all aircraft types.
- FAA-Approved Supplemental Type Certificates (STCs): If a repair is part of an alteration covered by an STC, the STC holder's approved data governs the welding procedures.
- DER-Approved Data: A Designated Engineering Representative (DER) authorized in the structural discipline can develop and approve repair data for a specific aircraft and specific repair. This pathway is used when no manufacturer data exists and the repair is complex enough to require engineering analysis. The DER's approval is noted on the FAA Form 337.
The FAA Form 337 Process
When a major welded repair is completed, the AMT or Repair Station must complete FAA Form 337. This two-page form captures the aircraft identification information, a description of the work performed, the approved data used, and the signature of the approving authority. For a certificated AMT performing a major repair, the mechanic signs Block 6 of the Form 337, certifying that the repair was performed according to approved data and that the aircraft is airworthy for return to service. The original is given to the aircraft owner for the aircraft records, and a copy is sent to the FAA within 48 hours, ultimately becoming part of the permanent aircraft record maintained by the FAA Aircraft Registration Branch in Oklahoma City.
Certificated Repair Stations operating under 14 CFR Part 145 may have different internal approval authority depending on their ratings and Operations Specifications, but they are still bound by Part 43 requirements for approved data and Form 337 documentation on major repairs.
Material and Process Specifications
Part of the approval data framework involves material traceability. When welding on a primary structure, the replacement material must meet the same or equivalent specification as the original. For steel tube fuselage structures, this typically means 4130 chromoly steel meeting the appropriate military or ASTM specification. Using uncertified or substituted material, even if the weld appears excellent, violates the approved data requirement and may constitute an unapproved alteration rather than a proper repair.
Welding processes must also match approved data. Gas welding (oxyacetylene), TIG (GTAW), and MIG (GMAW) each have specific applications and restrictions on aircraft. AC 43.13-1B provides guidance on when each process is appropriate. For example, gas welding has historically been specified for thin-wall 4130 steel tube structures because of the broad heat zone and slower cooling rate, which reduces the risk of cracking in the heat-affected zone. TIG welding is generally preferred for stainless exhaust components. Substituting a welding process not authorized by the approved data requires its own engineering justification.
Key Numbers and Rules
- 14 CFR Part 43, Appendix A — defines major and minor repairs; welding on structural members, fittings, and fuel systems is listed as a major repair.
- FAA Form 337 — required for every major repair or major alteration; original goes with aircraft records, copy to FAA.
- AC 43.13-1B, Chapter 4 — the primary reference for welding methods and practices on non-pressurized general aviation aircraft when no specific manufacturer data exists.
- 14 CFR Part 43.9 — mandates a maintenance record entry for all work; minor repairs require a record entry signed by the approving AMT; major repairs also require Form 337.
- 14 CFR Part 43.13 — requires use of manufacturer's current instructions or other FAA-approved data; establishes the standard of care for all maintenance.
Common Test Traps
- Assuming all welding is a major repair. Welding on non-structural components such as minor brackets or fairings may qualify as a minor repair. Carefully apply Appendix A criteria before making that determination.
- Confusing AC 43.13-1B as universally approved. This AC is acceptable methods and practices, but it is not automatically approved data for all aircraft types. Its applicability must be confirmed with the FSDO for the specific repair.
- Forgetting the Form 337 copy routing. The FAA copy goes to the FAA Aircraft Registration Branch in Oklahoma City — not to the FSDO. Many test questions and practical situations expose this confusion.
- Believing an Airframe certificate alone covers all welded repairs. Major repairs require approved data regardless of the technician's skill or rating. An Airframe-rated AMT without approved data cannot legally approve a major welded repair for return to service.
- Overlooking material traceability. Using 4130 steel from an unverified source, even if chemically equivalent, may not satisfy the approved data requirement. Material certifications must accompany structural stock used in approved repairs.