Definition
Three parts of Title 14 of the Code of Federal Regulations that govern commercial air carrier operations in the United States. Part 121 covers scheduled and large unscheduled air carriers (most domestic and international airlines). Part 129 covers foreign air carriers operating into or out of the United States. Part 135 covers commuter and on-demand operations such as charter flights, air taxis, and many smaller passenger and cargo operators. Aircraft operated under any of these parts are held to stricter maintenance and inspection standards than aircraft flown under Part 91 (general aviation), and certain preventive maintenance allowances available to private pilots do not apply.
Plain English
These are the federal rules that commercial flying operations must follow — airlines, foreign carriers flying into the U.S., and charter or on-demand operators. Aircraft used in these operations have stricter maintenance rules than private aircraft.
Context Anchor
Seen in preventive maintenance discussions when deciding whether a pilot is legally allowed to perform simple maintenance on an aircraft.
Derivation
CFR stands for Code of Federal Regulations. Title 14 is the volume covering aeronautics and space. The three-digit numbers (121, 129, 135) are simply the part numbers within Title 14 — each part is a self-contained set of rules for a specific category of operation.
Why Pilots Care
Tells a pilot whether they may legally perform listed preventive maintenance items themselves or must use a certificated mechanic and approved procedures.
Intuition Check
Do not read “parts” here as aircraft parts or maintenance parts. Here it means numbered sections of federal aviation rules, and “121 129 or 135” means any one of those rule sections.
Example Sentence 1
Because the aircraft was operated under 14 CFR part 135, the owner could not legally change the oil himself — the work had to be done under the operator's approved maintenance program.
Example Sentence 2
Pilots flying Part 135 charter flights cannot perform certain preventive maintenance tasks that a Part 91 owner-pilot is allowed to do.