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What is meant by the OCH of a precision approach?
  • A
    The lowest height above the elevation of the relevant runway threshold (OCH), at which a missed approach shall be initiated to ensure compliance with the appropriate obstacle clearance criteria.
  • B
    The lowest altitude above the aerodrome elevation below which an aircraft cannot descend without infringing the appropriate obstacle clearance criteria.
  • C

    The lowest height or the elevation of the relevant runway threshold, if the threshold elevation is more than 2 m (7 ft) below the aerodrome elevation, below which an aircraft cannot descend without infringing the appropriate obstacle clearance criteria.

  • D

    The lowest height above the ground at which a missed approach must be initiated to ensure compliance with the appropriate obstacle clearance criteria.

ICAO doc. 8168 PANS-OPS Vol. I, Part II, Section 5, Chapter 1
1.6 - OBSTACLE CLEARANCE ALTITUDE/HEIGHT (OCA/H)
For each individual approach procedure an obstacle clearance altitude/height (OCA/H) is calculated in the development of the procedure and published on the IAC. In the case of precision approach and circling approach procedures, an OCA/H is specified for each category of aircraft listed in 1.4. Obstacle clearance altitude/height (OCA/H) is:
a) in a precision approach procedure, the lowest altitude (OCA) or alternatively the lowest height above the elevation of the relevant runway threshold (OCH), at which a missed approach shall be initiated to ensure compliance with the appropriate obstacle clearance criteria; or
b) in a non-precision approach procedure, the lowest altitude (OCA) or alternatively the lowest height above aerodrome elevation or the elevation of the relevant runway threshold, if the threshold elevation is more than 2 m (7 ft) below the aerodrome elevation (OCH), below which an aircraft cannot descend without infringing the appropriate obstacle clearance criteria; or
c) in a visual (circling) procedure, the lowest altitude (OCA) or alternatively the lowest height above the aerodrome elevation (OCH) below which an aircraft cannot descend without infringing the appropriate obstacle clearance criteria.

The different definitions for the OCA/H vary depending on the type of approach, but OCA is always based on QNH (mean sea level) and OCH is in reference to QFE (aerodrome elevation). It is important to note that the OCA/H is not always the same as Decision Altitude/Height (DA/H), but a DA/H will never be lower than the corresponding OCA/H.

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