Definition
The minimum vertical buffer that must exist between an aircraft on a published instrument procedure or airway and the highest obstacle in the protected area beneath or alongside that route. The specific value varies by segment type — for example, 1,000 feet over non-mountainous terrain and 2,000 feet over designated mountainous terrain on en route segments — and is built into the published minimum altitude for that segment.
Plain English
It's the safety gap of empty air the FAA insists you keep between your aircraft and the ground or anything sticking up from it. The published altitude on a chart already includes this gap, so flying that altitude keeps you safely above the highest obstacle in the area.
Context Anchor
Seen in instrument procedure and route altitude discussions, especially when reading about MOCA and other published minimum altitudes.
Derivation
“Clearance” comes from the idea of being clear of something. In this aviation use, it means physical separation from terrain or obstacles, not an air traffic control permission.
Why Pilots Care
It directly determines the altitudes published on charts to keep aircraft safely above terrain and obstacles during IFR flight.
Grounding Statement
Picture the published minimum altitude as including a safety space above the highest obstacle in that part of the route.
Intuition Check
Do not read “clearance” here as ATC permission. In required obstacle clearance, it means vertical space between the aircraft and obstacles.
Example Sentence 1
The MOCA guarantees the required obstacle clearance within 22 nautical miles of the VOR, but only provides reliable navigation signal closer in.
Example Sentence 2
As the approach segment narrows, the required obstacle clearance is reduced to allow lower altitudes closer to the runway.