I have found references to the Obstacle Identification Surface (OIS), which is approximately a 152ft/NM slope from the end of the runway (1.43 degrees, 1:40 rise/run) and a 1:50 clearance plane noted in the illustration in this answer:

illustration in said answer

1:50 is 121.5ft/NM, so they clearly aren't one in the same.

Why are there both? What is the difference in purpose?

  • $\begingroup$ Are you sure the 2nd link is the right one? I don't see the 1:50 mention. $\endgroup$
    – ymb1
    Jun 23 '19 at 18:05
  • $\begingroup$ @ymb1 It's in one of the bubbles in the graphic of the accepted answer. $\endgroup$ Jun 23 '19 at 18:34

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