Definition
The required vertical buffer between an aircraft's lowest permitted altitude on a given segment of an airway or instrument procedure and the highest obstacle within that segment's protected area. On a typical Victor airway, the MOC is 1,000 feet in non-mountainous terrain and 2,000 feet in designated mountainous terrain. MOC is a design value used by procedure designers when establishing minimum altitudes such as the MEA, MOCA, and MIA; it is not an altitude the pilot reads directly off the chart.
Plain English
The set amount of space that must always exist between your lowest legal altitude on a route and the tallest thing sticking up below you. It's a built-in safety cushion baked into the published minimum altitudes.
Context Anchor
Seen in instrument procedure, route, and minimum altitude discussions, especially when explaining why a published altitude may not be flown below.
Derivation
Minimum comes from a Latin word meaning “smallest.” Obstruction means something that blocks or stands in the way. Clearance means open space. Together, the term points to the smallest required open vertical space above anything that could be in the airplane’s path.
Why Pilots Care
Prevents controlled flight into terrain by establishing safe minimum altitudes over obstacles.
Grounding Statement
Think of MOC as the protected vertical gap between the airplane and the tallest object beneath the route or procedure.
Intuition Check
Clearance here does not mean permission from air traffic control; it means vertical space above obstructions. Minimum does not mean a comfortable extra margin; it means the least protected space the procedure is designed to provide.
Example Sentence 1
The MEA on this airway segment was set high enough to provide the required MOC over the mountainous terrain below.
Example Sentence 2
Charts list the MOC so crews can choose altitudes that keep them well above every obstruction.