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.
Plain English
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.
Context Anchor
A pilot encounters this rule during taxi, before takeoff, during approach and landing, and in flight training whenever the instructor limits discussion so both people can focus on aircraft operation.
Derivation
Sterile' here is borrowed from the medical sense of 'kept free of contamination.' The flight deck is kept 'sterile' -- free of distractions -- during critical phases. The rule was created by the FAA in 1981 after several accidents were linked to crew distraction during low-altitude flight.
Why Pilots Care
Following the rule reduces the likelihood of missing critical cues or making errors when workload is highest.
Intuition Check
Do not read sterile as “clean from germs.” Here it means “free from unnecessary talk or activity.” The rule also does not mean total silence; it means only flight-related communication belongs during high-workload parts of the flight.
Example Sentence 1
Once they were cleared for taxi, the captain reminded the first officer that the sterile flight deck rule was now in effect.
Example Sentence 2
During the approach the first officer stopped casual conversation to observe the sterile flight deck rule.