Definition
The section of the Federal Aviation Regulations that governs scheduled and on-demand operations of large air carriers in the United States, including domestic, flag, and supplemental operations. It sets the rules for airline-level operations covering crew training, maintenance, dispatch, flight time limits, and operational equipment requirements.
Plain English
The set of FAA rules that scheduled airlines must follow when operating their flights. If you fly for a major or regional airline, this is the rulebook that governs how the airline is run and how its flights are conducted.
Context Anchor
Seen when FAA material is explaining requirements that apply to airline-style operations, including how certain routes and procedures may be approved or used.
Derivation
CFR stands for Code of Federal Regulations -- the official compilation of US government rules. Title 14 of the CFR covers Aeronautics and Space, and within Title 14, each numbered Part covers a specific area. Part 121 is the part dedicated to scheduled airline operations.
Why Pilots Care
Compliance determines training, dispatch, and procedural standards that directly affect how Part 121 crews plan and execute instrument flights.
Intuition Check
Do not read “Part 121” as an aircraft part or a training stage. Here, “Part” means a numbered section of federal aviation rules.
Example Sentence 1
Airlines like Delta and United operate their scheduled flights under 14 CFR Part 121.
Example Sentence 2
Crew rest and duty time limits under 14 CFR Part 121 apply to this supplemental cargo flight.