Definition
The lowest published altitude in effect between fixes on VOR airways, off-airway routes, or route segments that meets obstacle clearance requirements for the entire route segment. MOCA assures acceptable navigation signal coverage only within 22 nautical miles of a VOR.
Plain English
The lowest altitude you can legally fly along a charted route that still keeps you safely above terrain and obstacles. The catch is the navigation signal is only guaranteed to work within 22 NM of the VOR.
Context Anchor
Seen on instrument en route charts and in discussions of IFR route design, especially when comparing the lowest safe altitude for a route segment with navigation signal coverage.
Derivation
From the words 'minimum,' 'obstruction,' 'clearance,' and 'altitude.' The key word is 'obstruction' -- this altitude is built around what's sticking up from the ground (mountains, towers, etc.), not around radio reception range.
Why Pilots Care
It guarantees the minimum safe altitude needed to avoid obstacles when flying under instrument rules, even if a higher altitude is normally used.
Grounding Statement
Think of MOCA as the published floor for obstacle clearance on that route segment.
Intuition Check
MOCA is not a promise that navigation signals will be usable everywhere on the entire segment. Its main job is obstacle clearance; VOR signal coverage is only assured within 22 nautical miles of the station.
Example Sentence 1
ATC cleared us to descend to the MOCA of 4,800 feet, which kept us above the ridgeline as we approached the VOR.
Example Sentence 2
The chart showed the MOCA at 4500 feet for that airway segment, allowing a lower altitude than the MEA.