Definition
A sloping surface, on each side of an airway or route segment, that defines the obstacle clearance applied within the secondary area. Required obstacle clearance starts at 500 feet above the highest obstacle at the inner edge of the secondary area (where it meets the primary area) and slopes downward to zero at the outer edge of the secondary area.
Plain English
An imaginary tilted surface running along each side of a route. Directly next to the protected center of the route, terrain and obstacles must be cleared by 500 feet, but that required clearance gradually reduces to nothing as you move outward to the edge of the protected zone.
Context Anchor
Seen when studying how FAA instrument routes are protected from obstacles outside the main route centerline area.
Derivation
"Secondary" comes from Latin secundus, meaning "following" or "second in order" — it is the area beside the primary protected zone. "Plane" here is used in the geometric sense of a flat surface, not an aircraft. Together, the term describes the second, outer sloped surface used to guarantee obstacle clearance along a route.
Why Pilots Care
It maintains safe separation from terrain and obstacles in the outer portions of protected airspace while allowing a wider total area than the primary zone alone.
Analogy
Think of a roof sloping down from the main protected area toward the outside edge. The closer an obstacle is to the main route, the more vertical room is required above it; near the outside edge, that protected room has tapered away.
Grounding Statement
Picture a route as a flat-topped roof down the middle (the primary area) with sloped eaves on each side (the secondary planes) that taper down to the gutter at the outer edge.
Intuition Check
Secondary does not mean unimportant here; it means the side area next to the primary protected area. Plane does not mean aircraft here; it means an imaginary flat surface used for measuring clearance.
Example Sentence 1
Because the aircraft was flown near the edge of the airway, it was operating beneath the lower portion of the secondary obstacle clearance plane, where the protection above terrain is reduced.
Example Sentence 2
During the en route segment the aircraft stayed within the limits where the secondary obstacle clearance plane provided at least 500 feet of clearance.