Definition
The section of Title 14 of the Code of Federal Regulations, Part 91 (General Operating and Flight Rules), titled 'Altimeter settings.' It specifies which altimeter setting a pilot must use when operating an aircraft, including the requirement to set 29.92 inches of mercury when operating at or above 18,000 feet MSL, and to use the current reported setting from a station along the route within 100 nautical miles when operating below 18,000 feet MSL.
Plain English
This is the rule that tells pilots what number to put in the altimeter window. Below 18,000 feet, you use the local pressure setting from a nearby station. At or above 18,000 feet, everyone sets 29.92 so all aircraft are using the same reference.
Context Anchor
Seen in FAA handbooks, instrument flying discussions, altitude assignments, and explanations of when to use a local altimeter setting versus 29.92 at high altitude.
Derivation
CFR stands for Code of Federal Regulations — the published rules of the U.S. federal government. 'Part 91' is the chapter covering general flight rules, and '91.121' is the specific section number for altimeter settings. The numbering simply locates the rule within the larger regulation.
Why Pilots Care
Correct altimeter settings prevent altitude deviations, ensure proper vertical separation, and maintain required terrain clearance.
Grounding Statement
The altimeter depends on air pressure, so the setting in its window tells it which pressure reference to use when showing altitude.
Intuition Check
Do not read this as a page number or a checklist step. “Part 91” is a group of federal operating rules, and “91.121” is the specific rule section about altimeter settings.
Example Sentence 1
Climbing through 18,000 feet, the crew reset the altimeters to 29.92 in accordance with 14 CFR 91.121.
Example Sentence 2
While level at 8,000 feet on an IFR flight plan, the crew updated the altimeter to the reported setting from a station 40 miles away per 14 CFR Part 91.121.