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The minimum obstacle clearance (MOC) value to be applied in the primary area of a mountainous area during the en-route phase of an IFR flight is:
  • A
    Always 1 000 ft.
  • B
    If variation in terrain elevation is less than 5 000 ft: 1 500 ft, if variation is greater than 5 000 ft: 2 000 ft.
  • C
    Always 2 000 ft.
  • D
    If variation in terrain elevation is less than 5 000 ft: 2 000 ft, if variation is greater than 5 000 ft: 1 500 ft.

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

A MOCA is determined and published for each segment of the route. The MOCA provides the required clearance above obstacles contained inside the obstacle clearance areas. Charting accuracies are taken into account when establishing minimum altitudes by adding both a vertical and a horizontal tolerance to the depicted objects on the chart.

According to ICAO Doc 8168, PANS-OPS, Part I, Section 3 - Enroute Procedures - OBSTACLE CLEARANCE:

  • The minimum obstacle clearance (MOC) value to be applied in the primary area for the en-route phase of an instrument flight rules (IFR) flight is 300 m (1 000 ft).
  • In mountainous areas, this shall be increased depending on the variation in terrain elevation as follows:
Variation in Terrain Elevation MOC
3000 ft - 5000 ft (900 m - 1500 m) 1500 ft (450 m)
Greater than 5000 ft (1500 m) 2000 ft (600 m)

The MOC to be applied outside the primary area is normally equal to half the value of that applied in the primary area. Where this is found to be too constraining an alternative method is to use a value which reduces from the full MOC at the edge of the primary area to zero at the outer edge of the secondary area.

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