Definition
The lowest altitudes at which a Special Use Airspace (SUA), such as a Military Operations Area (MOA) or Restricted Area, becomes active. Aircraft operating below the published altitude floor are outside the active airspace and are not subject to its restrictions; aircraft at or above that altitude are within it.
Plain English
The bottom edge of a block of special airspace. Below this height, you are clear of it. At or above it, you are inside it.
Context Anchor
Seen on charts and in written descriptions of special use airspace, where pilots must know whether their planned altitude goes under, through, or above the area.
Derivation
"Floor" is borrowed from the everyday meaning -- the lowest surface of a room. In airspace, it refers to the lowest altitude of a defined volume. The plural "floors" is used because a single SUA often has several stacked sections, each with its own lower limit.
Why Pilots Care
Pilots must know altitude floors to plan routes that avoid penetrating active special use airspace from below.
Analogy
Think of the airspace like a room in the sky. The altitude floor is the room’s floor; below it, you are not inside that room.
Intuition Check
Do not read floor as a safe minimum altitude or a suggested cruising altitude. Here it means the lower vertical boundary where that airspace begins.
Example Sentence 1
The pilot checked the altitude floors of the MOA and chose a cruise altitude 500 feet below the lowest one.
Example Sentence 2
Check the chart for the altitude floors of the restricted area before filing your IFR flight plan.