Definition
The lower altitude at which Class E (controlled) airspace begins in a given area. Depending on location, the base may be the surface, 700 feet above ground level (AGL), 1,200 feet AGL, or some other published altitude shown on aeronautical charts. Below the Class E base, the airspace is typically Class G (uncontrolled).
Plain English
The floor of Class E airspace — the altitude where controlled airspace starts in that area. Anything beneath that altitude (in that location) is uncontrolled airspace.
Context Anchor
Seen when reading airspace descriptions and sectional charts, especially around airports and transition areas.
Derivation
‘Base’ here uses its ordinary sense of ‘the bottom of something.’ It refers to the bottom edge of the Class E layer, in the same way you’d talk about the base of a cloud.
Why Pilots Care
Pilots must know the base to identify when they enter controlled airspace and the associated visibility and cloud clearance rules change.
Analogy
Think of Class E airspace like a layer that starts at a certain height. The Class E airspace base is the floor of that layer.
Intuition Check
Do not read “base” here as a base leg in the traffic pattern or as an airport base of operations. Here, “base” means the lower boundary where the Class E airspace begins.
Example Sentence 1
In areas marked by the shaded magenta line on the sectional, the Class E airspace base is 700 feet AGL.
Example Sentence 2
In the area around the airport, the Class E airspace base starts at 700 feet above the surface.