Definition
A reference to 14 CFR Part 121, the section of the Federal Aviation Regulations that governs scheduled air carrier operations — primarily the major and regional airlines that operate large aircraft on published schedules. Part 121 sets the rules for crew qualifications, training, flight and duty limits, maintenance, dispatch, and operational procedures for these carriers.
Plain English
The set of FAA rules that the airlines fly under. If you fly for a scheduled airline carrying passengers or cargo, your operation is run according to Part 121.
Context Anchor
Seen in FAA handbooks and procedures when different kinds of operators are grouped by the rules they fly under, such as Part 91, Part 121, or Part 135.
Derivation
‘Part’ here refers to a numbered section of Title 14 of the Code of Federal Regulations (14 CFR), which contains the FAA's rules. ‘121’ is simply the number assigned to the section covering scheduled air carriers. Knowing this helps because pilots often hear ‘Part 91,’ ‘Part 135,’ and ‘Part 121’ — each is a different rule set for a different kind of flying.
Why Pilots Care
Determines the specific training, certification, equipment, and procedures that airline pilots and operators must meet.
Intuition Check
Do not read “Parts 121” as aircraft parts or pieces of an airplane. Here, “Part 121” means a numbered section of the FAA regulations.
Example Sentence 1
After building hours as a flight instructor, she moved on to a regional airline flying under Part 121.
Example Sentence 2
A pilot must complete Parts 121 training before flying for a major carrier.