Definition
A change to an aircraft's airframe, powerplant, propeller, or appliance that is not listed in the aircraft specifications and that may 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 and cannot be done by elementary operations. Major alterations must be approved using FAA Form 337 and performed by appropriately certificated personnel.
Plain English
A significant modification to the aircraft or its parts -- one big enough that it could change how the aircraft flies, how strong it is, or how it performs. These changes require formal FAA paperwork and must be done by qualified people.
Context Anchor
Seen when reviewing aircraft maintenance records, logbook entries, FAA Form 337 records, or airworthiness documents before flight.
Derivation
From Latin 'alterare' (to change) via Old French. 'Major' simply marks it as the more consequential category, distinct from a 'minor alteration' which has lesser impact and lighter paperwork.
Why Pilots Care
Pilots must confirm proper approval and documentation for any major alteration because it can change operating limitations, performance, or the validity of the airworthiness certificate.
Intuition Check
Do not read major alteration as simply “a big modification.” In FAA use, the key question is whether the change could affect the aircraft’s approved safe condition or requires more than ordinary standard work.
Example Sentence 1
During the preflight document check, the pilot looked through the logbooks for any FAA Form 337 entries that would indicate a major alteration had been performed on the airframe.
Example Sentence 2
Replacing the original engine with a different model created a major alteration that required completing Form 337.