Definition
A federal regulation, found in Title 14 of the Code of Federal Regulations Part 121, Section 542, that prohibits flight crewmembers of scheduled air carriers from performing non-essential duties or activities during critical phases of flight. Critical phases of flight include all ground operations involving taxi, takeoff, landing, and all other flight operations conducted below 10,000 feet, except cruise flight. This rule is the formal basis of what is commonly called the 'sterile cockpit rule.'
Plain English
It is the FAA rule that says airline pilots must keep the cockpit free of distractions and non-flying conversation during the busiest, most accident-prone parts of a flight — taxi, takeoff, landing, and any flying below 10,000 feet that isn't cruise.
Context Anchor
Seen in discussions of controlled flight into terrain, crew discipline, airline procedures, and the sterile cockpit rule.
Derivation
The reference reads as a location address. '14 CFR' means Title 14 of the Code of Federal Regulations, which is the volume of federal law covering aviation. 'Part 121' is the chapter governing scheduled air carriers (the airlines). '121.542' points to section 542 within that part. Reading it as 'Title 14, Part 121, Section 542' makes the citation system make sense.
Why Pilots Care
Following this rule reduces distraction and lowers the chance of an accident during the busiest and most demanding parts of any flight.
Intuition Check
Do not read “Part 121” as an aircraft part. Here, “Part” means a numbered section of federal aviation rules, and 121.542 points to one specific rule inside that section.
Example Sentence 1
The captain reminded the first officer that, under 14 CFR 121.542, they should hold non-essential conversation until they were established in cruise above 10,000 feet.
Example Sentence 2
During the descent the captain reminded the first officer of 14 CFR Part 121.542 before beginning nonessential conversation.