Definition
Three sets of FAA regulations that govern commercial air carrier operations. Part 121 covers scheduled airline operations (passenger and cargo) by U.S. carriers using larger aircraft. Part 129 covers operations within the United States by foreign air carriers. Part 135 covers commuter and on-demand operations, such as charter flights, air taxi, and smaller scheduled commuter service. Aircraft operated under these parts are held to stricter maintenance, inspection, and personnel standards than aircraft operated under Part 91 (general aviation).
Plain English
These are the rule sets that apply to commercial flying. Part 121 is for the major airlines, Part 129 is for foreign airlines flying into the U.S., and Part 135 is for charter and smaller commuter operators. They all carry tighter maintenance rules than private flying.
Context Anchor
Seen when reading FAA maintenance rules, especially the limits on what preventive maintenance a pilot may perform.
Derivation
CFR stands for Code of Federal Regulations -- the official collection of U.S. federal rules. Title 14 is the volume that contains all aviation rules. The 'part' numbers simply divide that volume into topic areas. Knowing this makes it easier to read citations: '14 CFR Part 135' just means 'Title 14, the aviation volume, section 135.'
Why Pilots Care
Pilots and operators under these parts must follow the specified maintenance standards to keep the aircraft legal to fly and to avoid regulatory violations.
Intuition Check
Do not read “parts” here as aircraft components. In “14 CFR parts 121, 129, or 135,” a part is a numbered group of federal aviation rules.
Example Sentence 1
Pilots may perform preventive maintenance on aircraft they own, but not on aircraft operated under 14 CFR parts 121, 129, or 135.
Example Sentence 2
A charter operator checks whether the flight falls under 14 CFR part 135 before applying the required inspection program.