Can someone explain me the difference between minimum obstacle clearance altitude and obstacle clearance altitude? I read definitions in Doc 8168 but i don't really see the difference. I would be grateful if someone explain where which type of altitude is used.

From ICAO Doc 8168:

Minimum obstacle clearance altitude (MOCA). The minimum altitude for a defined segment that provides the required obstacle clearance.

Obstacle clearance altitude (OCA) or obstacle clearance height (OCH). The lowest altitude or the lowest height above the elevation of the relevant runway threshold or the aerodrome elevation as applicable, used in establishing compliance with appropriate obstacle clearance criteria.

  • 3
    $\begingroup$ It seems clear to me: MOCA is for airway segments (i.e. enroute procedures), OCA/OCH is for airports (i.e. terminal procedures). $\endgroup$
    – randomhead
    May 15, 2022 at 17:17
  • $\begingroup$ @randomhead - this from Doc 8168: "4.3.2 Procedure altitudes/heights are at or above the minimum obstacle clearance altitude (MOCA) and are intended to place the aircraft in a position to support a stabilized prescribed descent gradient/angle on the final segment. MOCA (ICAO) seems to be a generic term applicable to all segments of flight. OCA and OCH are for IAPs . $\endgroup$
    – user22445
    May 15, 2022 at 17:55

1 Answer 1


Closely reading the definitions in the OP,

  • the MOCA exists with reference to a defined segment, while
  • the OCA or OCH exists with reference to a specified runway threshold or a specified aerodrome.

In other words, one applies to a line segment, while the other applies to a point. "This" segment has its particular MOCA, while the OCA/OCH would apply to whatever procedures are being designed for "this" runway or airport.


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