Plain English Meaning

During the busiest and most dangerous parts of a flight, pilots are not allowed to do anything that isn't directly related to flying the airplane safely. No casual conversation, no eating, no non-essential radio chatter -- just flying.

Full Definition

A federal regulation (14 CFR 121.542 and 135.100) that prohibits flight crewmembers from performing any non-essential duties or activities during critical phases of flight, defined as all ground operations involving taxi, takeoff, and landing, and all other flight operations conducted below 10,000 feet, except cruise flight.

Context Anchor

A pilot encounters this rule during taxi, before takeoff, during approach and landing, and in whenever the instructor limits so people can focus on operation.