Definition
The altitude or flight level assigned to an aircraft operating under Instrument Flight Rules during the en route, level-flight portion of a flight. Under uncontrolled conditions above 3,000 feet AGL, it must be selected according to magnetic course: odd thousands (plus 500 feet for VFR-on-top) for magnetic courses 0° through 179°, and even thousands for magnetic courses 180° through 359°, up to and including 18,000 feet MSL. Above 18,000 feet MSL (Class A airspace), flight levels are used following the same odd/even rule based on magnetic course. In controlled airspace, ATC may assign any altitude regardless of direction.
Plain English
The cruising height a pilot flies at when on an instrument flight plan. The rules say which heights are legal for which directions of travel, so aircraft going opposite ways are kept at different altitudes.
Context Anchor
Seen when planning or flying an IFR route, filing an IFR flight plan, reading an IFR clearance, or leveling off during the en route part of a flight.
Derivation
“Cruising” means moving steadily after the climb is complete, and “altitude” comes from a word meaning “height.” Together, the phrase points to the steady height used for the main traveling part of an IFR flight.
Why Pilots Care
Following the correct IFR cruising altitude keeps the aircraft separated from opposing traffic and satisfies regulatory requirements for safe, legal operations.
Grounding Statement
Once you are level in cruise under IFR, your altitude is not just a preference; it is part of the traffic system around you.
Intuition Check
“Cruising” does not mean any comfortable height you choose. Under IFR, the cruising altitude must match your clearance or the required IFR altitude rule for that route.
Example Sentence 1
On a magnetic course of 090°, the pilot selected 7,000 feet as the IFR cruising altitude.
Example Sentence 2
Westbound IFR flights select even IFR cruising altitudes such as 6,000 feet to maintain separation.