Definition
The altitude or flight level assigned to an aircraft operating under Instrument Flight Rules (IFR) during the en route, level-cruise portion of flight. In controlled airspace, the IFR cruising altitude is the one assigned by ATC. In uncontrolled airspace, the pilot selects an altitude in accordance with the IFR cruising altitude rule (14 CFR 91.179), which is based on magnetic course: odd thousands plus any assigned increment for courses 0° through 179°, and even thousands for courses 180° through 359°.
Plain English
The level you cruise at when flying IFR. ATC normally tells you what it is. If you are flying IFR in airspace where ATC is not controlling you, you pick the level using a rule based on the direction you are flying.
Context Anchor
You will see this term in IFR flight planning, ATC clearances, en route charts, and discussions of what altitude to maintain after departure and during cruise.
Derivation
“Cruising” comes from the idea of traveling steadily rather than climbing, descending, or maneuvering. “Altitude” comes from a word meaning height. “Flight level” is a standardized way to express height so aircraft at higher altitudes use the same pressure reference.
Why Pilots Care
Correct choice prevents mid-air conflicts and keeps the flight legal under IFR rules.
Intuition Check
“Cruising” does not mean the pilot may choose any comfortable height. Under IFR, the cruising altitude or flight level must match the clearance, the rules, and the airspace being used. “Flight level” does not mean just any altitude. It means an altitude expressed using the standard pressure setting, such as Flight Level 190 for 19,000 feet on that reference.
Example Sentence 1
After leveling off at the IFR cruising altitude of 9,000 feet, she set cruise power and began the en route checklist.
Example Sentence 2
Westbound IFR traffic was assigned flight level 240 as its cruising level through the sector.