On Friday, July 24, 2020 at 10:15:32 PM UTC+1, wrote:

Snip:
" Judging clearance is a skill that can take a while to learn. Some mountains lack features that indicate scale. Mountains with trees, roads, vehicles etc offer scale that people are used to. Bare rocks don't.

Phil Plane
Omarama

A geometrical observation about judging distance from a mountain slope:

If you travel at 60 knots ground speed in a straight line along a mountain slope (or past any point or mast etc) then for every second that it takes for a chosen reference point to pass from 45 degrees ahead to directly opposite then you (not the wing tip) are 100 feet from the reference point. At 90 knots it would be 150 feet and so on. This applies in any plane so it works for crossing over ridges as well.

Obviously you can't judge this exactly but it is surprisingly easy to get in the ball park and I am sure it is the relative rate of motion that experienced mountain pilots intuitively use. I figured it out during my first trip to the Pyrenees after I thought I was flying close to a steep featureless rocky ridge and was overtaken on the inside by a rather small looking Duo Discus. I realised that I needed a way to calibrate my inexperienced judgement where there were no size reference features.

John Galloway