Definition
Geographic regions designated by the FAA where terrain elevation changes by more than 3,000 feet within a horizontal distance of approximately 10 nautical miles. These designated areas are depicted on FAA charts and trigger higher obstacle clearance requirements for instrument procedures, minimum en route altitudes, and off-route obstruction clearance altitudes.
Plain English
Specific parts of the country the FAA has officially marked as mountainous because the ground rises and falls steeply. Flying in these areas requires extra altitude buffers to keep aircraft safely above terrain.
Context Anchor
Seen in instrument procedure design criteria, route planning, and discussions of minimum safe altitudes near high terrain.
Derivation
Mountainous comes from mountain, ultimately from a Latin word meaning a mountain or large hill. The ending -ous means “full of” or “having,” so mountainous points to land that has mountains or mountain-like terrain.
Why Pilots Care
Designated mountainous areas trigger higher MEAs and approach minima to guarantee safe terrain clearance when flying on instruments.
Intuition Check
Do not read mountainous areas as simply any place where mountains are visible. In this FAA context, it means terrain areas where special obstacle-clearance rules may apply.
Example Sentence 1
Because the route crossed a designated mountainous area, the controller assigned a higher minimum en route altitude than the surrounding flatland segments.
Example Sentence 2
Procedure designers apply extra obstacle clearance buffers inside mountainous areas.