Grandpa, what’s the farthest you can see?

[DeanoP]

In the old days at Nav school, we were taught:

Radio Range Line of sight formula. = 1.22 X Sq. Root of Aerial Height + 1.22 X Sq.Root of A/C height.

Thus assuming you are looking at the sea level horizon at 24000ft. Line of sight = 189 nautical miles.

For terrain, substitute terrain height for Ae Ht. eg ground elevation 3000ft, A/C height 24000ft.
You can expect to see the top of the terrain at:
67 nm (terrain height)+ 189nm (A/C height) = 256nm