Definition
A cruising altitude rule under VFR (and IFR in uncontrolled airspace) that assigns specific altitudes based on magnetic course. When flying above 3,000 feet AGL on a magnetic course of 0° through 179°, pilots use odd thousands plus 500 feet (e.g., 3,500; 5,500; 7,500). On a magnetic course of 180° through 359°, pilots use even thousands plus 500 feet (e.g., 4,500; 6,500; 8,500). The rule separates opposing traffic vertically by direction of flight.
Plain English
A simple rule that tells pilots which altitude to fly based on the direction they are heading. Aircraft going roughly east fly at one set of altitudes, and aircraft going roughly west fly at a different set. This keeps planes coming from opposite directions from being at the same altitude.
Context Anchor
Seen when planning cross-country flights, choosing a cruise altitude, or teaching altitude selection for traffic separation.
Derivation
‘Hemispheric’ comes from the idea of dividing the compass into two halves (hemispheres) — one for eastbound courses (0°–179°) and one for westbound courses (180°–359°). Knowing the term refers to halves of the compass makes the rule easier to picture: which half of the compass am I flying in?
Why Pilots Care
Following these altitudes prevents mid-air collisions by maintaining consistent vertical separation between aircraft on opposing routes.
Analogy
It is like using different lanes of a road for opposite directions of travel. The altitude pattern gives aircraft a built-in way to stay out of each other’s path vertically.
Intuition Check
Do not read “hemispheric” as meaning the Northern or Southern Hemisphere of the Earth. Here it means which half of the compass your course falls in.
Example Sentence 1
Heading west on a magnetic course of 270°, the pilot leveled off at 6,500 feet to comply with standard hemispheric altitudes.
Example Sentence 2
ATC cleared the IFR flight to 8000 feet, the correct even altitude under standard hemispheric altitudes for its westbound course.