Definition
A repair or alteration to an aircraft, engine, propeller, or appliance that, if improperly done, could appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness, or that is not done according to accepted practices or cannot be done by elementary operations. Major repairs and alterations must be performed by appropriately authorized personnel and recorded on FAA Form 337.
Plain English
A repair or change to the aircraft that is significant enough to affect how safely it flies. Because the stakes are high, it must be done by qualified people and documented on a specific FAA form so there is a permanent record of the work.
Context Anchor
Seen during preflight review of aircraft maintenance records, especially when checking whether the airplane is legally and safely ready to fly.
Derivation
Major' comes from the Latin maior meaning 'greater.' The regulatory use here keeps that sense — these are the greater repairs and alterations, the ones large enough in consequence that they need extra oversight and a formal paper trail.
Why Pilots Care
Major repairs or alterations can change the aircraft's operating limitations or handling qualities, so pilots must verify they are properly documented and airworthy before flight.
Intuition Check
Do not assume “major” means the job was large, costly, or took a long time. Here, “major” means the work could significantly affect the aircraft’s safety, legality, structure, performance, or handling.
Example Sentence 1
Before the flight, the pilot reviewed the aircraft logbooks and confirmed that the recent wing skin repair was documented as a major repair on Form 337.
Example Sentence 2
During the records review, the pilot noted a previous major repair or alteration to the engine mount and confirmed it was approved.