Definition
The portion of an en route, terminal, or approach obstacle clearance area that lies on either side of the primary area, where required obstacle clearance tapers from the full primary value at the inner edge to zero at the outer edge. Aircraft are expected to remain within the primary area; the secondary area provides a buffer that absorbs minor navigational deviations while still guaranteeing some obstacle clearance.
Plain English
It's the strip of protected airspace running alongside the main protected route. You get full obstacle clearance in the middle, and that clearance gradually shrinks to nothing as you move toward the outer edge.
Context Anchor
Seen in instrument procedure design and en route obstacle clearance discussions, especially when explaining how much obstacle protection exists beside an airway or route.
Derivation
"Secondary" comes from Latin secundus, meaning "following" or "second in order." It signals that this area is the supporting buffer that follows the primary protected area, not the main route itself.
Why Pilots Care
Knowing the secondary area exists lets pilots understand the full width of protected airspace and why even small lateral deviations can reduce their obstacle margin.
Grounding Statement
Picture the main route area as having full obstacle protection, with side areas where that protection gradually fades toward the outside edge.
Intuition Check
Do not read “secondary” as meaning optional or unimportant. In this context, it means the side area next to the primary area, where obstacle clearance is reduced.
Example Sentence 1
Course deviations during the en route segment should stay within the primary area, since obstacle clearance decreases steadily across the secondary area.
Example Sentence 2
If the aircraft drifts into the secondary area, obstacle clearance drops from 1,000 feet to as little as 500 feet at the outer edge.