FAA Form 337: Major vs. Minor, Approved Data, Field Approvals, and Who Can Actually Sign It
Updated: Sep 26
FAA Form 337, officially titled Major Repair and Alteration (Airframe, Powerplant, Propeller, or Appliance), is a critical document in aviation maintenance and regulatory compliance. For aircraft owners, pilots, and maintenance professionals, understanding when and how it's used is essential to maintaining an aircraft's legal airworthiness — but the details behind it matter: what actually makes a repair or alteration "major" (a specific regulatory definition, not a judgment call), the difference between "approved data" and merely "acceptable data," who can legally approve a major job versus merely perform it, and the several paths to getting the approved data an alteration requires. Getting these right is what separates a clean maintenance record from an airworthiness problem waiting to surface at the next annual or sale.
This post covers Form 337 in practical depth: what it is and its regulatory basis, the major-versus-minor definitions under Part 43, the critical approved-data versus acceptable-data distinction, the three paths to approved data (STC, field approval, DER), who can approve major work for return to service, the filing timeline and recordkeeping, and why it all matters for airworthiness and resale.
Study this full length lesson (video, podcast, flashcards, and quiz) here: Full Length Lesson >
What FAA Form 337 Is
FAA Form 337 is the standardized document used to record major repairs and major alterations made to an aircraft airframe, engine (powerplant), propeller, or appliance.
What it documents:
Major repairs (significant repairs affecting airworthiness)
Major alterations (significant changes to the type design)
The specific work performed and the approved data used
A permanent record of the modification/repair
The regulatory basis:
The requirement to document maintenance is in 14 CFR 43.9 (major repairs/alterations require Form 337)
14 CFR 43.5 addresses approval for return to service
Part 43 Appendix A DEFINES what is major vs. minor (the definitional backbone)
Part 43 Appendix B addresses the recording/reporting (including the 337)
Why the form exists:
Major work significantly affects the aircraft
It needs standardized, permanent documentation
The FAA and future owners need the record
Traceability of significant modifications
The permanent-record aspect:
Once completed, Form 337 becomes a permanent part of the aircraft's records
It stays with the aircraft (through resales)
Provides the modification history
Essential documentation
Major vs. Minor: The Regulatory Definitions
The major/minor distinction isn't a judgment call — it's defined in Part 43 Appendix A, and understanding it is key.
The general principle:
A "major" repair or alteration is one that, if done improperly, could significantly affect airworthiness — weight and balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities
A "minor" repair or alteration is anything that isn't major
Appendix A lists specific examples
Major Repairs (Appendix A examples):
Repairs to structural components (wing spars, ribs, fuselage structure)
Repairs to primary flight control surfaces and their systems
Engine mount repairs
Repairs requiring special jigs, fixtures, or equipment
Repairs affecting structural strength
The "if done wrong, it could be dangerous" category
Major Alterations (Appendix A examples):
Changes to the wings, control surfaces, or structure
Engine changes (installing a different engine model)
Propeller changes (different propeller)
Installing equipment that affects the type design (some avionics)
Changes to the fuel or oil systems
Anything altering the approved type design significantly
Minor Repairs and Alterations:
Everything not classified as major
Routine repairs and simple changes
Documented via logbook entry (not Form 337)
The default category
Why the distinction matters:
Major = Form 337 required, approved data required, higher-level approval
Minor = logbook entry, acceptable data sufficient, A&P can approve
The classification drives the documentation and approval requirements
Misclassifying major as minor is a common, serious error
The judgment reality:
Appendix A gives examples, but classification sometimes requires judgment
When in doubt, treat it as major (the conservative approach)
A mechanic or the FAA can help classify
Get it right — the consequences differ
Approved Data vs. Acceptable Data: The Critical Distinction
The heart of what makes major work different: it requires APPROVED data, not merely acceptable data. This distinction is often misunderstood.
Acceptable data:
Data that is acceptable to the FAA for MINOR repairs and alterations
Includes manufacturer's maintenance manuals, AC 43.13-1B (the acceptable methods handbook), manufacturer service information
Sufficient for minor work
Doesn't require specific FAA approval
Approved data:
Data that is specifically APPROVED by the FAA for MAJOR repairs and alterations
Required for major work
The higher standard
Must be FAA-approved (not just acceptable)
The key difference:
MINOR work: acceptable data is enough
MAJOR work: approved data is REQUIRED
Approved data has FAA approval; acceptable data doesn't need it
This is why major alterations need approved data (STC, field approval, etc.)
Examples of approved data:
Supplemental Type Certificates (STCs)
Type Certificate Data Sheets (TCDS)
FAA-approved portions of maintenance manuals
Airworthiness Directives (the AD itself is approved data for its compliance)
Manufacturer data specifically FAA-approved
DER-approved engineering data
Field approvals (approval of specific data by an FAA inspector)
Examples of acceptable data (for minor work):
AC 43.13-1B (Acceptable Methods, Techniques, and Practices)
Manufacturer maintenance manuals (the non-approved portions)
Manufacturer service bulletins/instructions (generally)
Sufficient for minor repairs/alterations
Why AC 43.13-1B is "acceptable" not "approved":
AC 43.13-1B contains acceptable methods
It's acceptable data for MINOR work
For MAJOR work, it's acceptable ONLY if the FAA accepts it as approved data for that specific case (or it's used as a basis)
Generally, major work needs specifically approved data
A common point of confusion
The practical impact:
Before a major alteration, you need approved data
Where does it come from? (STC, field approval, DER — below)
Without approved data, the major alteration can't be legally done
Approved data is the prerequisite
The Three Paths to Approved Data
For a major alteration, you need approved data. There are three main ways to get it, which the basic treatment doesn't fully lay out.
Path 1 — Supplemental Type Certificate (STC):
An STC is FAA-approved data for a specific modification
A company develops a modification, gets it approved, and sells the STC
Installing per the STC uses approved data
The most common path for popular modifications
Examples: an avionics upgrade STC, an engine conversion STC, an autopilot STC
How STCs work:
The STC holder has already gotten FAA approval for the modification
You buy the STC (the right to use the approved data)
Install per the STC instructions
The Form 337 references the STC as the approved data
Clean and straightforward
Path 2 — Field Approval:
When no STC or existing approved data exists for a specific alteration
An FAA inspector (from the FSDO) can grant a "field approval"
The inspector approves the specific data for that specific aircraft
Documented on the Form 337 (block 3, the FAA approval block)
A one-off approval
How field approvals work:
You (or your mechanic) submit the proposed alteration and data to the FSDO
An FAA inspector reviews it
If approved, the inspector signs the Form 337 approving the data
The alteration is now legal (for that aircraft)
More involved than an STC
The field-approval tightening (post-2005):
Field approvals became HARDER to obtain after policy changes around 2005-2006
The FAA restricted the use of "sample" 337s and limited what inspectors could field-approve
More alterations now require an STC or DER-approved data
Field approvals are less freely granted than they once were
A practical reality for owners
Path 3 — DER-Approved Data (Engineering):
A Designated Engineering Representative (DER) can approve engineering data on the FAA's behalf
For complex alterations without an STC
The DER develops/approves the engineering data
More expensive but available for custom work
Used when an STC doesn't exist and a field approval isn't available
How DER data works:
Hire a DER to develop and approve the engineering data
The DER-approved data is approved data
Use it for the alteration
Reference it on the Form 337
The path for custom/complex modifications
Choosing the path:
STC available: use it (easiest)
No STC, simple alteration: field approval (if the FSDO grants it)
No STC, complex: DER-approved data
The path depends on the alteration and what's available
When Form 337 Is Required (and Not)
The clear triggers for the form.
Form 337 IS required when:
A MAJOR repair is performed
A MAJOR alteration is completed
Approved data (such as an STC) is used to modify the aircraft in a major way
The work falls in the "major" category (Part 43 Appendix A)
Form 337 is NOT required for:
Minor repairs or alterations (logbook entry instead)
Preventive maintenance performed by pilots (logbook entry)
Routine maintenance (logbook entry)
Anything in the "minor" category
The documentation by category:
MAJOR repair/alteration: Form 337 (plus logbook entry)
MINOR repair/alteration: logbook entry only
Preventive maintenance (pilot): logbook entry only
The category determines the documentation
Why the trigger matters:
Major work needs the 337 (and approved data and proper approval)
Doing major work without the 337 is a documentation failure (unairworthy)
Know when the form is required
The classification drives it
Who Performs vs. Who Approves for Return to Service
A critical distinction the basic treatment underdevelops: performing the work and approving it for return to service are different, and major work has specific approval requirements.
The two roles:
Performing the repair/alteration (the physical work)
Approving for return to service (certifying it's done correctly per approved data — the legal signoff)
These can be different people
The approval is what makes the aircraft airworthy again
Who can APPROVE a MAJOR repair/alteration for return to service:
An A&P mechanic with Inspection Authorization (IA)
A certificated repair station (appropriately rated)
The manufacturer (in some cases)
NOT a regular A&P alone (this is the key point)
The A&P limitation:
A regular A&P mechanic (without IA) CAN PERFORM major repairs/alterations
But CANNOT approve them for return to service
An IA (or repair station) must approve the major work
This trips up many people
Why the IA is required for major work:
Major work significantly affects airworthiness
It requires the higher-level inspection/approval (IA)
The IA verifies the work was done per approved data
The IA signs the Form 337 (block 7, the return-to-service approval)
The comparison:
MINOR repair/alteration: an A&P can perform AND approve for return to service
MAJOR repair/alteration: an A&P can perform, but an IA (or repair station) must approve
The difference is the approval authority
Major work needs IA-level approval
The field-approval nuance:
For a field approval, the FAA inspector approves the DATA (block 3)
The IA (or repair station) still approves the WORK for return to service (block 7)
Two different approvals on the form
Data approval vs. return-to-service approval
The Form 337 Structure and Filing
How the form is completed and filed, expanded.
The form's key blocks:
Aircraft identification (make, model, serial, registration)
Owner information
The type of work (repair or alteration, and the component)
Description of the work and the approved data used (block 8 — the detailed description)
The approval for return to service (block 7 — signed by the IA/repair station)
The FAA approval (block 3 — for field approvals)
The description requirement:
Block 8 describes the work in detail
References the approved data (STC number, field approval, DER data)
Enough detail to understand what was done
Incomplete descriptions are a common error
The filing timeline:
The person approving the work completes the Form 337
A copy is given to the aircraft owner
A copy is submitted to the FAA
The FAA submission is generally required within a specified time (the mechanic/repair station submits it to the FAA Aircraft Registry — historically within 48 hours of approval for return to service)
The dual-copy requirement:
One copy to the aircraft's permanent records (stays with the aircraft)
One copy to the FAA Aircraft Registry (Oklahoma City)
Both required
The FAA copy goes in the aircraft's file at the Registry
The FAA Registry file:
The FAA maintains a file for each aircraft
Form 337s are filed there
You can order the aircraft's records from the FAA (the CD/file of all 337s and documents)
Useful for pre-purchase research
Electronic vs. paper:
Modern submissions are often electronic
Older aircraft may have decades of paper 337s
Both are part of the record
The history accumulates
Weight and Balance After a Major Alteration
A practical consequence the basic treatment omits.
The requirement:
A major alteration that changes weight or CG requires a weight-and-balance revision
The aircraft's empty weight and CG must be recalculated
The equipment list updated
New weight-and-balance data created
Why it's necessary:
Adding/removing equipment (avionics, etc.) changes the weight and CG
The weight-and-balance data must reflect the current configuration
An outdated W&B (not reflecting the alteration) is a problem
The current W&B must be aboard
The connection to the 337:
The major alteration is documented on the 337
The weight-and-balance revision accompanies it
The updated W&B goes in the aircraft records
Part of completing the alteration properly
The practical point:
After a major alteration affecting weight/CG, update the W&B
Ensure the current W&B is aboard
The alteration isn't complete without it
Often overlooked
Why Form 337 Matters to Owners
The stakes, expanded.
Airworthiness:
Failure to properly document major repairs/alterations can render the aircraft unairworthy
The 337 (with approved data and proper approval) is required for major work
Missing/improper 337s = airworthiness problem
Can result in enforcement action
Issues surface at the annual
The unapproved-major-alteration problem:
A major alteration done without approved data and a proper 337 is a serious issue
The aircraft may not conform to an approved configuration
Unairworthy until resolved
Can be expensive to fix (getting retroactive approval)
Resale value:
Missing or incomplete 337 documentation raises red flags in pre-purchase inspections
Buyers (and their mechanics) scrutinize the 337s
Undocumented or improperly documented alterations reduce value
Can delay or kill a sale
The pre-purchase scrutiny:
A pre-purchase inspection reviews all 337s
Verifies alterations were done with approved data and proper approval
Identifies undocumented or improper modifications
A major factor in the aircraft's value and salability
Insurance and liability:
Accurate documentation demonstrates regulatory compliance
Provides traceability for modifications
Protects the owner
Important for claims and liability
Common Form 337 Mistakes
The errors, expanded.
Misclassifying major alterations as minor:
Treating a major alteration as minor (logbook entry, no 337)
Avoids the approved-data and IA-approval requirements (improperly)
A serious error
Surfaces at inspection or sale
Missing FAA-approved data references:
The 337 must reference the approved data (STC, field approval, DER)
Without the reference, the approval basis is unclear
Incomplete documentation
A common deficiency
Incomplete descriptions of work performed:
Block 8 must describe the work adequately
Vague descriptions are insufficient
Enough detail to understand the modification
Frequently too sparse
Failure to submit the form to the FAA:
The FAA copy must be submitted to the Registry
Failing to submit means the FAA file is incomplete
Required step
Sometimes missed
Using acceptable data where approved data is required:
Major work needs APPROVED data (not just acceptable)
Using AC 43.13 alone for a major alteration (without it being approved for that case) is improper
The approved-data requirement
A subtle but important error
A regular A&P approving major work:
Major work needs IA (or repair station) approval for return to service
An A&P alone can't approve it
A common misunderstanding
The approval authority matters
The consequences:
These errors surface during annuals and pre-purchase inspections
Can render the aircraft unairworthy
Reduce value, delay sales
Avoidable with proper procedures
Final Thoughts
FAA Form 337 plays a vital role in documenting the significant maintenance history of an aircraft — and doing it right requires understanding the framework behind it. Major repairs and alterations (defined in Part 43 Appendix A) require approved data (not merely acceptable data), which comes via an STC, a field approval, or DER-approved engineering data. The work can be performed by an A&P, but major repairs and alterations must be approved for return to service by an IA or an appropriately rated repair station, and the completed form is filed both in the aircraft's permanent records and with the FAA Registry.
Properly completed and filed 337s ensure transparency, maintain legal airworthiness, and preserve aircraft value. For owners, understanding the major-versus-minor distinction, the approved-data requirement, and the approval authority helps avoid costly compliance issues — and ensures confidence in the aircraft's maintenance pedigree, whether at the next annual or the next sale.
On the Written Test and Checkride
Form 337 and major/minor classification appear on tests and checkride orals (especially for CFI and maintenance-focused questions). The most commonly tested topics:
What Form 337 documents (major repairs and alterations)
Major vs. minor (Part 43 Appendix A)
Approved data vs. acceptable data
STCs and field approvals
Who approves major work for return to service (IA/repair station)
The dual-copy filing (aircraft records + FAA Registry)
Quick Reference
FAA Form 337:
Documents MAJOR repairs and alterations
Airframe, powerplant, propeller, appliance
Regulatory basis: 14 CFR 43.9, Part 43 Appendices A and B
Permanent part of the aircraft records
Major vs. Minor (Part 43 Appendix A):
Major: could significantly affect airworthiness (weight/balance, structure, performance, flight characteristics)
Minor: everything not major
Major → Form 337; Minor → logbook entry
When in doubt, treat as major
Approved Data vs. Acceptable Data:
Acceptable data: for MINOR work (AC 43.13-1B, manuals)
Approved data: for MAJOR work (FAA-approved)
Major work REQUIRES approved data
Approved Data Sources:
STC (Supplemental Type Certificate)
TCDS, FAA-approved manual portions
Field approval (FAA inspector approves specific data)
DER-approved engineering data
The AD itself (for AD compliance)
Three Paths to Approved Data:
Path | When |
STC | Modification already approved (buy it) |
Field approval | No STC, FSDO approves (harder post-2005) |
DER data | Complex/custom, engineer-approved |
When 337 Is Required:
Major repair or alteration
NOT for minor work or pilot preventive maintenance (logbook instead)
Perform vs. Approve for Return to Service:
A&P: can PERFORM major work, but CANNOT approve it
IA or repair station: must APPROVE major work for return to service
Minor work: A&P can perform AND approve
Field Approval Note:
FAA inspector approves the DATA (block 3)
IA/repair station approves the WORK (block 7)
Field approvals harder to get after ~2005
Filing:
One copy: aircraft permanent records
One copy: FAA Aircraft Registry (within ~48 hours)
Order aircraft records from the FAA for history
Weight and Balance:
Major alteration affecting weight/CG requires a W&B revision
Update the equipment list and W&B data
Current W&B must be aboard
Common Mistakes:
Misclassifying major as minor
Missing approved-data references
Incomplete work descriptions
Not submitting to the FAA
A&P approving major work (needs IA)
Key Principle:
Form 337 documents major repairs and alterations (defined in Part 43 Appendix A), which require approved data — via an STC, a field approval, or DER-approved data — not merely acceptable data. An A&P can perform the work, but an IA or repair station must approve major work for return to service. The completed form is filed in both the aircraft records and with the FAA Registry, and a major alteration affecting weight or CG requires a weight-and-balance revision.
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Author: Nathan Hodell
CFI, CFII, MEI, ATP, Creator and CEO
Nathan is an aviation enthusiast with thousands of hours of flying and dual instruction over the past 15+ years. Through his aviation career he has been able to earn his ATP, fly as an airline pilot, own/operate flight schools, and create and host wifiCFI.