Plain English Meaning
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.
Full 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.
Context Anchor
Seen in high-altitude IFR clearances, route descriptions, and discussions of the upper structure of the .