Definition
The protected airspace along an airway or en route segment within which terrain and obstacle clearance is guaranteed when the aircraft is flown at or above the published minimum en route altitude. It extends a specified distance either side of the airway centerline, with primary and secondary areas, and provides at least 1,000 feet of obstacle clearance in non-mountainous terrain and 2,000 feet in designated mountainous terrain.
Plain English
It is the corridor of sky around an airway that is checked for terrain and obstacles. As long as you stay inside that corridor and at or above the minimum altitude printed for that segment, you are guaranteed to clear everything below by a safe margin.
Context Anchor
Seen when studying how instrument routes and their minimum safe altitudes are designed and used during en route flight.
Derivation
En route comes from French and means “on the way.” Obstacle means something in the way, and clearance means safe space from it. Together, the phrase points to the protected “on-the-way” area where obstacle spacing is planned for an instrument route.
Why Pilots Care
It sets the lowest safe altitude a pilot can accept on a route segment, directly affecting terrain and obstacle avoidance.
Grounding Statement
Picture a protected corridor along the route; the minimum altitude is based on keeping the aircraft safely above the highest terrain or obstacle considered inside that corridor.
Intuition Check
Clearance here does not mean an air traffic control permission. It means planned vertical space between the aircraft and terrain or obstacles.
Example Sentence 1
As long as we stay on the airway at or above the MEA, we are inside the en route obstacle clearance area and guaranteed at least 1,000 feet above any terrain or obstacles.
Example Sentence 2
Obstacle surveys can expand the en route obstacle clearance area and raise the published minimum altitude.