2.4 FAA Form 337 Major Repairs and Major Alterations

2.4 FAA Form 337 Major Repairs and Major Alterations

When maintenance activities exceed the threshold of routine maintenance, aviation technicians enter the realm of major repairs and major alterations. These critical operations require strict adherence to approved technical data, formal documentation on FAA Form 337, and specialized return-to-service authorizations.

1. Classification under 14 CFR Part 43 Appendix A

Appendix A of Part 43 provides the regulatory benchmark for classifying maintenance tasks:

Major Alteration (Airframe, Powerplant, Propeller, Appliance)

A major alteration is an alteration not listed in the product's type certificate data sheet (TCDS) that:

1. Might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness; OR
2. Is not done according to accepted practices or cannot be done by elementary operations.

Examples of Major Alterations:

• Installing an unapproved avionics package or autopilot system.
• Converting an engine to use an alternate fuel or turbocharger system.
• Installing floats, skis, or extended-range fuel tanks.
• Changes to the basic wing or control surface design.

Major Repair (Airframe, Powerplant, Propeller, Appliance)

A major repair is a repair that:

1. If improperly done, might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other airworthiness qualities; OR
2. Is not done according to accepted practices or cannot be done by elementary operations.

Examples of Major Repairs:

• Splicing of skin sheets, stringers, spar flanges, or wing ribs.
• Straightening or welding of structural steel tubing fuselage members.
• Overhaul of a supercharger, turbocharger, or pressure carburetor.
• Straightening or shortening of propeller blades.
• Rewinding of an internal appliance armature.

Comparative Matrix: Major vs. Minor vs. Preventive Maintenance

2. Anatomy and Execution of FAA Form 337

FAA Form 337 is an official federal document consisting of two pages and eight specific data blocks:

                             FAA FORM 337 DATA BLOCKS
  ┌────────────────────────────────────────┬────────────────────────────────────────┐
  │ Block 1: Aircraft Identity             │ Block 2: Owner Information             │
  │ (Make, Model, Serial No., Tail N#)     │ (Name and mailing address)             │
  ├────────────────────────────────────────┴────────────────────────────────────────┤
  │ Block 3: For FAA Use Only (Field Approvals / Special Authorizations)            │
  ├────────────────────────────────────────┬────────────────────────────────────────┤
  │ Block 4: Type of Action                │ Block 5: Unit Identification           │
  │ [X] Major Repair  [X] Major Alteration │ (Airframe, Engine, Prop, or Appliance) │
  ├────────────────────────────────────────┴────────────────────────────────────────┤
  │ Block 6: Conformity Statement                                                  │
  │ - Certifies that the work was performed in accordance with 14 CFR Part 43      │
  │ - Signed by the person or repair station PERFORMING the work                   │
  ├─────────────────────────────────────────────────────────────────────────────────┤
  │ Block 7: Approval for Return to Service                                        │
  │ - Certifies that the work is APPROVED for return to service                     │
  │ - Signed by an IA, Part 145 Repair Station, Manufacturer, or FAA Inspector    │
  ├─────────────────────────────────────────────────────────────────────────────────┤
  │ Block 8: Description of Work Accomplished (Page 2)                             │
  │ - Detailed narrative describing the repair/alteration                          │
  │ - Explicit citations of APPROVED DATA used                                     │
  │ - Weight and balance revision calculations (if applicable)                     │
  └─────────────────────────────────────────────────────────────────────────────────┘

Form 337 Distribution Rules

• Standard Rule (2 Copies):
  1. Original executed Form 337: Given to the aircraft owner or operator, who must retain it permanently with the aircraft records under 14 CFR § 91.417.
  2. Duplicate copy: Must be mailed or submitted electronically to the local FAA Flight Standards District Office (FSDO) within 48 hours after the aircraft is approved for return to service.
• Extended-Range Fuel Tank Rule (3 Copies):
  • When an extended-range fuel tank is installed in the passenger compartment or baggage compartment, the person approving the work must execute an additional copy.
  • The third copy must be placed and kept on board the aircraft (in addition to the owner original and the FSDO copy submitted within 48 hours).
• Repair Station Major Repair Exception:
  • Under 14 CFR Part 43 Appendix B, a certificated Part 145 repair station performing a major repair may release the aircraft using a detailed work order/customer invoice in lieu of completing FAA Form 337, provided the work order contains all required technical references and a copy is retained for at least 2 years.

3. FAA Field Approvals and Engineering Authorizations

When a major repair or major alteration does not have pre-existing approved data (such as a manufacturer STC or AD), the technician must obtain engineering approval:

Methods of Securing Approved Data

1. FAA Field Approval (Block 3): An FAA Aviation Safety Inspector (ASI) inspects the technical data and/or the physical alteration and signs Block 3 of FAA Form 337, conferring approved data status for that specific serial-number aircraft.
2. Designated Engineering Representative (DER): A DER approves technical engineering data using FAA Form 8110-3 (Statement of Compliance with Airworthiness Standards). The Form 8110-3 serves as approved data to support the Block 8 entry on Form 337.
3. Organization Designation Authorization (ODA): An ODA issues FAA Form 8100-9 approving major repairs or alterations on behalf of the FAA.