Definition
14 CFR 91.121 is the Federal Aviation Regulation that governs altimeter settings. It requires pilots operating an aircraft to maintain the cruising altitude or flight level by reference to an altimeter set to a specific reference: when below 18,000 feet MSL, the current reported altimeter setting of a station along the route within 100 nautical miles of the aircraft (or, if no such station, the nearest available); when at or above 18,000 feet MSL, set to 29.92 inches of mercury (the standard altimeter setting).
Plain English
This is the rule that tells pilots how to set their altimeter. Below 18,000 feet, you set it using the local pressure reading from a nearby weather station. At or above 18,000 feet, everyone sets their altimeter to the same standard value (29.92) so that all aircraft up there are referencing the same thing.
Context Anchor
Seen in IFR altitude discussions, especially when changing between altitudes below 18,000 feet and flight levels at or above 18,000 feet.
Derivation
This is a regulation citation. “91” means 14 CFR Part 91, the main set of operating rules for many civil aircraft operations. “.121” is the specific section number for altimeter settings.
Why Pilots Care
Accurate altimeter settings maintain proper vertical separation from other aircraft and terrain clearance during IFR operations.
Intuition Check
Do not read 91.121 as an altitude, frequency, or page number. Here it is a rule citation: Part 91, section 121, covering altimeter settings.
Example Sentence 1
Climbing through 17,500 feet on the way up to FL230, the pilot reset the altimeter to 29.92 in compliance with 91.121.
Example Sentence 2
During the IFR flight, the crew verified compliance with 91.121 using the latest METAR altimeter setting.