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. On federal airways in the contiguous United States, MOCA assures acceptable navigation signal coverage only within 22 nautical miles of a VOR.
Plain English
The lowest altitude you are allowed to fly along a charted route segment that still keeps you safely above terrain and obstacles. Reliable VOR navigation signals are only guaranteed within 22 NM of the VOR.
Context Anchor
Seen on instrument enroute charts and in discussions of charted instrument altitudes for route planning and in-flight altitude decisions.
Derivation
Built from plain English: 'minimum' (lowest), 'obstruction clearance' (staying above obstacles), and 'altitude' (height). The name describes its job — it is the floor that clears obstacles on that segment.
Why Pilots Care
It lets a pilot fly lower than the usual MEA when needed for weather or fuel while still clearing all obstacles, provided the aircraft stays close enough to the VOR for reliable navigation.
Grounding Statement
MOCA answers the practical question: “How low can I fly this instrument route segment and still have protected obstacle clearance?”
Intuition Check
Do not read “clearance” here as permission from air traffic control. In this term, clearance means protected vertical space above obstacles; “minimum” means the lowest protected altitude, not the best or most comfortable altitude to fly.
Example Sentence 1
Picking up moderate icing at the MEA, the crew requested a descent to the MOCA for that segment to get into warmer air.
Example Sentence 2
Although the MEA was 8000 feet, the MOCA of 5500 feet allowed the aircraft to maintain terrain clearance on that airway segment.