Originally Posted by aterpster
In the U.S. an unrestricted VGSI must clear all obstacles 1 degree below the commissioned angle, out to 4 n.m, and with a 15 degree splay.
Just out of curiosity, I projected this out to the see how much clearance would be afforded by this. I used the spot which JC identified on Google earth which everyone mocked him for. Turns out that a obstacle clearance plane of 2.2degrees would give a clearance of about 65 feet to an airplane exactly on a 3.2 degree PAPI, 3700 ft from the threshold. That's remarkably close to the clearance from the terrain which JC figured using Google earth. Granted, the first point of impact was quite a bit further out and lower than this, so this is only peripherally related to the accident. However it is interesting to note how little clearance you might have over a half mile from the threshold and perfectly centered on the PAPI.