Definition
Altitudes or flight levels expressed in whole thousand-foot increments, such as 5,000 feet, 6,000 feet, FL250, or FL260. "Cardinal Altitudes or Flight Levels" specifically refers to these even-thousand values used as standard reference points for IFR cruising altitudes and altitude assignments.
Plain English
Altitudes that land exactly on a thousand-foot mark — 3,000, 4,000, 5,000, and so on — rather than odd numbers like 4,500 or 7,200.
Context Anchor
Used in ATC and AIM wording when referring to standard whole-thousand altitude assignments.
Derivation
"Cardinal" comes from the Latin cardinalis, meaning "principal" or "hinge" — the main points something turns on. Just as the cardinal directions (N, S, E, W) are the principal compass points, cardinal altitudes are the principal, whole-thousand reference points on the altitude scale.
Why Pilots Care
Flying at the correct cardinal altitude keeps you separated from aircraft coming the opposite way and satisfies the regulatory requirement for direction-based cruising.
Intuition Check
Cardinal does not mean north, east, south, and west here. It means the main whole-thousand altitude values.
Example Sentence 1
ATC cleared the flight to climb to a cardinal altitude of 8,000 feet.
Example Sentence 2
Westbound VFR traffic selects even cardinal altitudes plus 500 feet to maintain separation.