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FAA Form 337: Major Repairs and Alterations Explained

Updated: Dec 19, 2025

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 Form 337 is used is essential to maintaining an aircraft’s legal airworthiness.


This article explains what Form 337 is, when it’s required, who completes it, and how it affects aircraft records and resale value.



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What Is FAA Form 337?

FAA Form 337 is used to document major repairs and major alterations made to:

  • Aircraft airframes

  • Engines (powerplants)

  • Propellers

  • Appliances


The form provides a standardized way to record significant maintenance actions that affect an aircraft’s configuration, performance, or structural integrity.


Once completed, Form 337 becomes a permanent part of the aircraft’s maintenance records.


Major vs. Minor Repairs and Alterations

Major Repairs

A major repair is a repair that:

  • If improperly done, could affect weight and balance

  • Could affect structural strength

  • Could impact performance or flight characteristics


Examples:

  • Structural repairs to wings or fuselage

  • Repairs to primary flight control structures

  • Engine mount repairs


Major Alterations

A major alteration is a change that:

  • Alters the aircraft’s approved type design

  • Affects weight, balance, structural strength, or performance

  • Is not done according to approved data in the aircraft’s maintenance manual


Examples:

  • Installation of new avionics not previously approved

  • Engine or propeller changes

  • Modifications requiring a Supplemental Type Certificate (STC)


When Is Form 337 Required?

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


Form 337 is not required for:

  • Minor repairs or alterations

  • Preventive maintenance performed by pilots

  • Routine maintenance documented via logbook entries


Who Completes and Signs Form 337?

  • The form is typically completed by a certificated mechanic or repair station

  • An Inspection Authorization (IA) is required for:

    • Certain alterations

    • Field approvals

  • The person approving the work signs the form, certifying compliance with FAA regulations and approved data


Approved Data and Field Approvals

Approved Data

Approved data may include:

  • Supplemental Type Certificates (STCs)

  • FAA-approved maintenance manuals

  • Manufacturer service instructions

  • Engineering data approved by the FAA


Field Approvals

When approved data does not already exist, an FAA inspector may grant a field approval, documented on Form 337, allowing the alteration to be legally performed.


Filing and Recordkeeping Requirements

Once completed:

  • One copy of Form 337 is placed in the aircraft’s permanent records

  • One copy is submitted to the FAA Aircraft Registry


Modern submissions are often electronic, but older aircraft may have decades of paper 337 forms on file.


Why Form 337 Matters to Aircraft Owners

Airworthiness

Failure to properly document major repairs or alterations:

  • Can render an aircraft unairworthy

  • May result in enforcement action

  • Can cause issues during annual inspections


Resale Value

Missing or incomplete Form 337 documentation:

  • Raises red flags during pre-purchase inspections

  • Can reduce aircraft value

  • May delay or prevent aircraft sales


Insurance and Liability

Accurate documentation protects owners by:

  • Demonstrating regulatory compliance

  • Providing traceability for modifications


Common Form 337 Mistakes

  • Misclassifying major alterations as minor

  • Missing FAA-approved data references

  • Incomplete descriptions of work performed

  • Failure to submit the form to the FAA

  • Inconsistent logbook entries


These errors often surface during inspections or aircraft sales.


Final Thoughts

FAA Form 337 plays a vital role in documenting the maintenance history of an aircraft. Properly completed and filed forms ensure transparency, maintain legal airworthiness, and preserve aircraft value.


For aircraft owners, understanding Form 337 helps avoid costly compliance issues and ensures confidence in the aircraft’s maintenance pedigree.



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