Definition
FL 450 is a flight level designation indicating an altitude of 45,000 feet referenced to the standard altimeter setting of 29.92 inches of mercury. It marks the upper limit of the jet route system in the United States; jet routes operate from 18,000 feet MSL up to and including FL 450.
Plain English
A flight level of 45,000 feet, measured against a fixed pressure setting that all aircraft use at high altitude. It is the ceiling of the high-altitude jet route structure.
Context Anchor
Seen in high-altitude IFR clearances, route descriptions, and discussions of the upper structure of the National Airspace System.
Derivation
FL stands for Flight Level. The number that follows is the altitude in hundreds of feet, so FL 450 means 45,000 feet. Flight levels are used instead of MSL altitudes above 18,000 feet because every aircraft sets the same standard pressure (29.92), keeping vertical separation consistent regardless of local weather.
Why Pilots Care
It keeps high-speed jet traffic properly separated on airways and jet routes while allowing efficient use of favorable winds and weather.
Intuition Check
Do not read FL 450 as exactly 45,000 feet above the ground or necessarily exactly 45,000 feet above sea level. It means the 45,000-foot level based on the standard altimeter setting.
Example Sentence 1
Jet routes extend from 18,000 feet MSL up to and including FL 450.
Example Sentence 2
Strong tailwinds at FL 450 allowed the aircraft to cover the route faster than planned.