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 and other direct routes for which a MOCA is designated, this altitude also assures acceptable navigation signal coverage only within 22 nautical miles of a VOR.
Plain English
The lowest altitude you are allowed to fly on a given route segment that still keeps you safely above terrain and obstacles for the whole segment. On VOR-based routes, the navigation signal is only guaranteed reliable when you are within 22 nautical miles of the VOR.
Context Anchor
Seen on instrument enroute charts and in route planning when choosing or checking the altitude for a specific route segment.
Derivation
The phrase is built from three plain words. 'Minimum' (Latin minimus, smallest) sets the floor. 'Obstruction clearance' means staying clear of things that stick up — terrain, towers, buildings. 'Altitude' is the height above sea level. The name tells you exactly what it does: the lowest height that still clears obstructions.
Why Pilots Care
It permits a lower altitude than the MEA in some areas, which can help with fuel, icing avoidance, or terrain, while still guaranteeing obstacle clearance.
Grounding Statement
MOCA is a published altitude floor for a route segment: below it, obstacle protection is no longer assured.
Intuition Check
Clearance here does not mean permission from air traffic control. It means enough vertical space between the airplane and terrain or obstacles.
Example Sentence 1
The MEA on that segment was 6,000 feet, but the MOCA was 4,500 feet, giving us a lower option once we were within 22 NM of the VOR.
Example Sentence 2
Even though the MOCA kept them clear of terrain, the crew climbed to the MEA to restore reliable navigation signal reception.