I am studying Obstacle Clearance Surface construction using FAA's Performance Base Navigation manual 8260.58B as my guide. I am confused about the rules concerning aircraft's permitted altitude in relation to the OCS slope and the OCS 3D surfaces.

On page 79, a sample image for LPV/GLS final approach case is provided. enter image description here

This is a 3D surface, for which the manual states:

3-4-4. Obstacle clearance surface. The primary area OCS consists of the W and X surfaces. The Y surface is an early missed approach transitional surface. The W surface slopes longitudinally along the final approach track, and is level perpendicular to track. The X and Y surfaces slope upward from the edge of the W surface perpendicular to the final approach track [see figure 3-4-3]. Obstacles located in the X and Y surfaces are adjusted in height to account for perpendicular surface rise and evaluated under the W surface.

Based on some calculations I made, from the pilot's point of view the aircraft's cross section seems to intersect with the Y OCS, even if the aircraft's theoretical position is not (it is always above the W surface since it is required by the OCS + ROC slope).

In other words, I would like to know whether the first case (please see left image below) is correct or it should always be like the second case (right image below).

enter image description here

Could someone point me in the right direction?


Since I have found some relevant information in ICAO's 8168 Volume I document, I would like to quote the passage below from Chapter 1:


1.3.1 Where track guidance is provided in the design of a procedure, each segment comprises a specified volume of airspace, the vertical cross-section of which is an area located symmetrically about the centre line of each segment. The vertical cross-section of each segment is divided into primary and secondary areas. Full obstacle clearances are applied over the primary areas reducing to zero at the outer edges of the secondary areas (see Figure I-2-1-2).

1.3.2 On straight segments, the width of the primary area at any given point is equal to one-half of the total width. The width of each secondary area is equal to one-quarter of the total width.

1.3.3 Where no track guidance is provided during a turn specified by the procedure, the total width of the area is considered primary area.

1.3.4 The minimum obstacle clearance (MOC) is provided for the whole width of the primary area. In the secondary area, MOC is provided at the inner edges reducing to zero at the outer edges (see Figure I-2-1-2).

enter image description here

The above figure depicts the assumed lowest flight path in such a way that the cross-section does not intersect with secondary area's OCS surface. However, I am not sure if it is just an example for demonstration, and the aircraft is supposed to be anywhere inside the canyon-like 3D OCS.


1 Answer 1


The OCS is the edge of the safe flight 'corridor.' It defines a safe volume of airspace for the approach. Since it's normal to have obstacles off to the side of the approach path the OCS typically rises as you move away from the approach centerline.

Your Case 1 is the most correct description of an aircraft flying the approach. The green 'floor' normally will extend greater than one 'dot' on either side of the localizer (or localizer look-alike) and is at least one 'dot' below the glide path. That is the reason a missed approach should be executed if you deviate more than one dot from the approach path. At the zero deviation level for glide path (your blue dashed line in Case 1 assuming the a/c is on path) the intersection with the yellow (x surface) should be beyond the full scale left/right deviation.

Essentially the OCS defines a canyon that you can safely fly through to the runway. So yes, the extension of the aircraft's cross section line can intersect the x and y surfaces, but the aircraft itself should not.

  • $\begingroup$ Gerry thank you for your thorough answer. Could you also clarify the reason behind the use of 3D surfaces, instead of surfaces belonging to the same (horizontal) plane? Provided that OCS surfaces represent aircraft's uncertainity regarding its position, using a canyon-like surface seems to imply that a deviation from centerline will probably result in a higher altitude and thus the aircraft will be safe. I have also updated my answer a bit with information found in the related ICAO manual. $\endgroup$ Commented Dec 17, 2020 at 8:41
  • $\begingroup$ I think you are approaching this from the wrong angle.OCS is not suggesting that deviation from centerline will result in higher alt; it merely allows for somewhat higher obstacles to be present as long as they are far enough from centerline. This might (if choose to see it that way) imply that the more certain you are of your position the more you can safely descend. $\endgroup$
    – Radu094
    Commented Dec 17, 2020 at 11:25

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