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I am having trouble figuring out the logic behind the way that obstacle clearance areas are constructed. Take, for example, a departure procedure where Initial Climb Area is defined in TERPS (section 14-1-6) as a trapezoid:

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Even as far as subsequent segments are concerned, their respective obstacle clearance areas seem to follow the same pattern, like this example from TERPS:

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Why is this happening? Is there a reason that at the beginning of each segment the obstacle clearance area "shrinks" and then splays outward? Especially when one considers SIDs with really lengthy segments, where near their end the aircraft's perpendicular distance from OCA's borders is huge. It would seem more reasonable to me if the obstacle clearance area was a rectangular region, as it is the case with STARs:

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Could someone point me in the right direction?

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    $\begingroup$ I'm no TERPS expert, but isn't the trapezoid defined by the "course accuracy lines" described in the example diagram (e.g. 4.5º for a VOR)? The accuracy of the VOR is reduced the farther you get from it, so the obstacle clearance area gets larger to accommodate that? $\endgroup$
    – Zach Lipton
    Nov 16, 2018 at 19:54
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    $\begingroup$ Same as Zach, I'm no expert on TERPS either, but I believe that they also have to take into consideration winds which could constantly be pushing you from any given direction, which requires an expanded obstacle clearance area if you are flying headings instead of courses. $\endgroup$
    – Lnafziger
    Nov 16, 2018 at 20:04
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    $\begingroup$ @Vector: Why not include the images inline in your post? Most of us won't bother to follow links and your question may suffer link rot in future. $\endgroup$
    – Transistor
    Nov 16, 2018 at 20:20
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    $\begingroup$ That's called "divergence" in ICAO documents and, for a departing obstacle clearance area, varies from 10° to 12.5° according to runway slope. See Annex 14, vol 1, §4.2.26. Reasons are one engine inop and wind. $\endgroup$
    – mins
    Nov 23, 2018 at 20:59
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    $\begingroup$ If you are flying a course, wind will not change your ground track since you have to adjust your heading in order to stay on course. If you are flying a heading, the wind will be a constant force pushing on the airplane, and could be coming from any direction so the "cone" of the obstacle clearance area has to expand as you get further from where the heading segment started. $\endgroup$
    – Lnafziger
    Nov 24, 2018 at 21:19

1 Answer 1

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Because the navigational beacons and the indicator instruments in the cockpit are such that the system is less precise at longer distances. Second the plane must find the initial fix or intersection while flying blind under IMC before becoming aligned and so the area acts like a funnel. In some areas the terrain may limit the radio beacon signal so a wedge is all that is available.

A rectangle isn't needed because planes can't make use of the inner corners and it would be both a wasted area and a hazard as a plane may be able to get in but not out. Directly over a VOR the signal is not useable and the turns in a route must be wide enough for a large-radius turn not a sharp angle precisely over the beacon. The political types like to minimize the zone of low flying aircraft noise and building restrictions on the ground dwelling tax paying voters. STARS are higher altitude and so the design is more forgiving and can be of simple rectangles.

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