The short answer
FAA Form 337 documents a major repair or major alteration to an airframe, engine, propeller, or appliance. One copy goes to the FAA and one stays with the aircraft's permanent records, making 337s a key part of an aircraft's paper trail.
When a 337 is required
Repairs and alterations classed as "major" — those that might appreciably affect weight, balance, structural strength, performance, or flight characteristics, or that aren't done according to accepted practices — require Form 337 and approval by an IA (or use of FAA-approved data such as an STC).
The FAA keeps 337s on file, and copies are retrievable with the aircraft's records from the FAA — a standard step in researching an unfamiliar aircraft's history. Missing or inconsistent 337s for known modifications are a classic records red flag.
Related terms and reading
Keep every hour and inspection straight
Maggneto tracks engine times, inspections, and ADs from your actual logbooks — so terms like these become numbers you can act on. Browse the full maintenance glossary.