Definition
The lowest altitude (OCA) or the lowest height above the elevation of the relevant runway threshold or airport elevation (OCH) used in establishing compliance with appropriate obstacle clearance criteria during an instrument approach procedure. OCA/H is published on approach charts as the minimum altitude or height at which an aircraft remains safely clear of all obstacles within the protected area of the procedure.
Plain English
The lowest altitude or height a pilot can fly during a published instrument approach and still be guaranteed to clear every obstacle in that approach's protected airspace.
Context Anchor
Seen in instrument approach procedure design, approach minimums discussions, and obstacle clearance criteria.
Derivation
Obstacle comes from a Latin idea meaning something standing in the way. Clearance means having enough free space to pass safely. Altitude comes from Latin altus, meaning high, while height is the simpler idea of how far something is above a chosen surface. Together, the term points to the required vertical space above anything that could be hit.
Why Pilots Care
It directly determines the lowest safe altitudes shown on approach charts and protects against controlled flight into terrain.
Intuition Check
Do not read clearance here as an ATC permission. It means physical space above obstacles. Also, altitude and height are not the same reference: altitude is measured from sea level, while height is measured from a stated surface such as the runway or airport.
Example Sentence 1
The approach chart listed an OCA of 540 feet and an OCH of 320 feet above the runway threshold.
Example Sentence 2
After crossing the final approach fix the crew descended no lower than the obstacle clearance height until the runway was in sight.