Definition
Repairs or modifications to an aircraft, engine, propeller, or appliance that, if improperly performed, could appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that are not done according to accepted practices or by elementary operations. They are formally defined in 14 CFR Part 1 and listed in Part 43, Appendix A, and require specific approval, qualified personnel, and documentation on FAA Form 337.
Plain English
Big repairs or big changes to an aircraft — the kind that could affect how safely it flies if done wrong. Because of that, the FAA requires them to be done by approved people and recorded on a special form.
Context Anchor
Seen in maintenance records, logbooks, inspection discussions, and light-sport aircraft maintenance guidance.
Derivation
Major comes from a Latin word meaning “greater.” Repair means to restore something to proper condition. Alteration means a change. Together, the phrase points to work that is greater than a small fix or simple change because it can affect the aircraft’s safe flying condition.
Why Pilots Care
These repairs or changes must be documented and often require FAA Form 337 approval; without it the aircraft may be unairworthy and illegal to fly.
Intuition Check
“Major” does not just mean expensive or time-consuming. Here it means the work could affect the aircraft’s safe and legal flying condition, or it requires more than a simple routine procedure.
Example Sentence 1
Replacing a wing spar is a major repair, so it must be performed by a certificated mechanic and documented on FAA Form 337.
Example Sentence 2
Installing a new autopilot system was treated as a major alteration on the light-sport aircraft.