On Mar 1, 9:36*am, Nine Bravo wrote:
Thanks John.

I tend to think about it slightly differently. On the one hand I think about worst-case, inescapable sink for the conditions as measured by minutes times sink rate (result - a fixed number of feet that I am at risk of losing - I don't generally consider a string of sink occurrences - I assume one low probability sink patch is worse than multiple, higher probability sink patches and that multiple low probability events aren't likely enough to worry about). On the other hand I think about the probability of being able to find lift to recover after hitting a sink patch - which is a function of altitude above ground.

On very short final glides the constraint is the rate of sink (not much glide time left), on longer final glides the constraint is the probability of finding decent lift to get back up. The pinch point for me tends to be around 25 miles out - chances of finding lift are declining, odds of a long of stretch of sink still decent. This seems roughly consistent with your square root rule, though the math is different.

Your square root rule breaks down for me on very long final glides where I tend to optimize more around trying to transition away from climb and glide to cruise-climb in an effort to avoid thermal centering losses. This translates to an altitude buffer of maybe -1000 feet on a 50 mile "final glide" that you are trying to "bump-up" to +1000 feet by the time you get to 25 miles out. Not sure if/how that figures in your analysis.

9B

Good thoughts. But just to clarify, the article is not about final
glides -- how to do it efficiently. The article is about safety
margins -- how to do it with x percent chance of landing out.

John Cochrane