Definition
Odd cardinal altitudes are the standard reference altitudes ending in 1,000 feet that are odd-numbered, such as 3,000, 5,000, 7,000, 9,000 feet, and so on. They are used as the basis for the cruising altitudes assigned to aircraft on magnetic courses from 0 through 179 degrees under both VFR and IFR rules.
Plain English
These are the round-number altitudes ending in odd thousands of feet (3,000; 5,000; 7,000; 9,000, etc.). Pilots flying generally eastbound use these as their cruising altitudes.
Context Anchor
Seen in ATC altitude assignments, cruising altitude rules, and Pilot/Controller Glossary references to odd and even altitude groups.
Derivation
Cardinal' comes from the Latin cardinalis, meaning 'principal' or 'pivotal' — the same root as 'cardinal directions.' In aviation it signals the main, evenly spaced reference altitudes the system is built around, rather than altitudes that fall between them.
Why Pilots Care
Ensures proper vertical separation between opposite-direction traffic on airways and routes.
Grounding Statement
If the altitude is a clean thousand-foot level and the number of thousands is odd, it is an odd cardinal altitude.
Intuition Check
Odd does not mean unusual here. It means the altitude’s thousand-foot number is an odd number, such as 5, 7, or 29.
Example Sentence 1
Flying east at a magnetic course of 090 degrees, the pilot selected 7,000 feet, an odd cardinal altitude appropriate for that direction.
Example Sentence 2
ATC cleared the aircraft to an odd cardinal altitude to maintain separation from westbound traffic flying even altitudes.