Definition
On a federal airway or other en route segment, the airspace that lies on either side of the primary obstacle clearance area, extending outward from the edge of the primary area to the outer boundary of the protected airspace. Within the secondary area, required obstacle clearance tapers from the full primary value at the inner edge down to zero at the outer edge.
Plain English
The strip of protected airspace just outside the main centered route corridor. Obstacles in this outer strip don't need as much clearance as obstacles directly under the route, and the required clearance gradually reduces to nothing at the very edge.
Context Anchor
Seen in instrument procedure design and en route obstacle clearance discussions, especially when explaining how protected route widths are built around an instrument route.
Derivation
"Secondary" comes from the Latin secundus, meaning "following" or "second in order." The secondary area follows the primary area outward — it's the next ring of protection beyond the main one.
Why Pilots Care
Knowing the boundaries helps pilots understand the safety margins available if they drift slightly off the centerline of an airway.
Intuition Check
Secondary does not mean unprotected or unimportant here. It means the side area outside the primary area, where obstacle protection tapers down toward the outer edge.
Example Sentence 1
Because the airway runs through hilly terrain, the chart designer had to check obstacles in both the primary and secondary obstacle clearance areas before publishing the MEA.
Example Sentence 2
A brief excursion into the secondary obstacle clearance area is still safe but with reduced margins.