# Why does obstacle clearance area splay outward instead of being a rectangle?

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:

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

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:

Could someone point me in the right direction?

• 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? Nov 16, 2018 at 19:54
• 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. Nov 16, 2018 at 20:04
• @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. Nov 16, 2018 at 20:20
• 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.
– mins
Nov 23, 2018 at 20:59
• 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. Nov 24, 2018 at 21:19