Definition
Changes to an aircraft's airframe, powerplant, propeller, or appliances that are not listed in the aircraft specifications and that might appreciably affect weight, balance, structural strength, performance, powerplant operation, flight characteristics, or other qualities affecting airworthiness. Major alterations require approved data and must be recorded on FAA Form 337 by a certificated mechanic with appropriate authorization or an approved repair station.
Plain English
A significant modification to an aircraft that goes beyond what the manufacturer originally approved, and that could affect how the aircraft flies or holds together. Because of that, it has to be done using FAA-approved data and properly documented.
Context Anchor
You will see this term in maintenance records, aircraft logbooks, light-sport aircraft operating limitations, and discussions about modifying an aircraft after it leaves the factory.
Derivation
Major' comes from the Latin 'maior,' meaning 'greater.' 'Alteration' comes from the Latin 'alterare,' meaning 'to change.' Together it points to changes large enough to matter for safety — not minor tweaks.
Why Pilots Care
Determines whether a modification keeps the aircraft legal and airworthy or grounds it until proper approval is obtained.
Intuition Check
Do not read major alterations as meaning only large, expensive, or obvious changes. In FAA use, a change can be “major” because of what it can affect, even if the changed part looks small.
Example Sentence 1
Installing a different engine model than the one listed in the type certificate is a major alteration and requires FAA-approved data.
Example Sentence 2
After completing several major alterations on the light-sport aircraft, the pilot verified all approvals before the next flight.