Understanding the boundary between major vs minor repairs and alterations—and knowing when to use FAA Form 337—is essential for aircraft mechanics, repair stations, and owners. Form 337 is the official document used to record major repairs and major alterations to airframes, powerplants, propellers, or appliances under FAA regulations.
Mistaking a major structural repair for a minor maintenance task violates airworthiness rules and invalidates aircraft logbook entries. Knowing how to classify maintenance work and execute Form 337 correctly ensures your aircraft records stand up to any regulatory audit or pre-purchase inspection.
Key Takeaways
- Minor repairs and alterations can be signed off directly in aircraft logbooks using standard maintenance data by certified mechanics.
- Major repairs and alterations require approved data and must be documented using FAA Form 337, signed by an IA mechanic or repair station.
- Form 337 has two sides: Block 3 records the description of work, while Block 6 records the return-to-service approval.
- Copies of Form 337 must be provided to the aircraft owner and submitted to the FAA registry for permanent airframe tracking.
Major vs Minor Maintenance: How Regulations Define the Boundary
Under 14 CFR Part 43 Appendix A, civil aviation regulations establish clear definitions separating minor maintenance from major work. A minor repair or alteration involves standard maintenance tasks or simple modifications whose execution does not appreciably affect weight, balance, structural strength, powerplant operation, or flight characteristics. These tasks are routinely signed off in logbooks by a standard A&P mechanic.
A major repair or alteration involves complex work where improper execution could degrade structural integrity or flight safety—such as splicing primary control cables, welding engine mount tubes, repairing stressed skin panels, or installing unapproved avionics suites. Major work cannot rely on standard shop practices alone; it requires specifically approved engineering data and formal administrative documentation.
| Maintenance Classification | Work Complexity | Required Approval Data | Sign-Off Document |
|---|---|---|---|
| Minor Repair | Standard component replacement | Standard maintenance manual / AC 43.13 | Aircraft Logbook Entry |
| Minor Alteration | Simple accessory addition | Standard acceptable practices | Aircraft Logbook Entry |
| Major Repair | Structural skin or spar fix | Approved engineering data | FAA Form 337 (Block 3 & 6) |
| Major Alteration | Engine swap or complex STC | STC package or Field Approval | FAA Form 337 + Logbook |
When and How to Execute FAA Form 337
Whenever a certificated mechanic or Part 145 repair station completes a major repair or major alteration, they must complete an FAA Form 337 (Major Repair and Alteration). The form is structured into two main operational sections:
- Block 3 (FAA Use Only): A detailed technical narrative explaining what was done, referencing approved engineering data (such as an STC number, manufacturer service bulletin, or Advisory Circular), and listing parts used.
- Block 6 (Approval for Return to Service): Signed exclusively by an A&P mechanic holding an active Inspection Authorization (IA), a designated engineering representative, or an authorized manager of a Part 145 repair station.
Two copies of the completed Form 337 must be generated: one is given to the aircraft owner to keep in the permanent maintenance records, and the second is mailed or submitted electronically to the FAA Aircraft Registration Branch in Oklahoma City.
Form 337s During Pre-Purchase Audits and Logbook Reviews
When evaluating a used aircraft acquisition following a structured pre-purchase inspection guide, scrutinizing past Form 337 filings is a top priority. Auditors cross-reference every Form 337 found in the logbook binder against the FAA registry file to ensure unapproved alterations were never performed.
Mismatched structural repairs, missing Form 337s for major engine overhauls, or undocumented STCs significantly depress aircraft valuation and create major hurdles during export or lease return inspections.
Frequently Asked Questions
Who is authorized to sign Block 6 of FAA Form 337?
Block 6 of FAA Form 337 can only be signed by an A&P mechanic holding an active Inspection Authorization (IA), an authorized representative of a certificated Part 145 repair station, or designated engineering representatives acting within their authorized specialty.
Do minor repairs require filing an FAA Form 337?
No. Minor repairs and minor alterations do not require Form 337. They are recorded directly in the aircraft maintenance logbooks by a certificated mechanic using standard acceptable maintenance practices.
What is an FAA Field Approval?
An FAA Field Approval is a process where an FAA Aviation Safety Inspector reviews and approves non-standard major repair or alteration data on a case-by-case basis, authorizing its installation via Form 337 when no pre-existing STC or manufacturer data exists.