At 15:23 07 March 2012, John Cochrane wrote:
On Mar 1, 9:36=A0am, Nine Bravo wrote:
Thanks John.

I tend to think about it slightly differently. On the one hand I think
ab=
out 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
losin=
g - 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
wo=
rry about). On the other hand I think about the probability of being able
t=
o 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
g=
lide time left), on longer final glides the constraint is the probability
o=
f finding decent lift to get back up. The pinch point for me tends to be
ar=
ound 25 miles out - chances of finding lift are declining, odds of a long
o=
f stretch of sink still decent. This seems roughly consistent with your
squ=
are 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
gli=
de to cruise-climb in an effort to avoid thermal centering losses. This
tra=
nslates 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


Very good thoughs both John and Andy, I tend to think of it more like a
funnel. The further out the closer I can be to my actual glide ratio and
the closer in i need way bigger margin. The funnel idea makes me put the
"art of final glide" into a logrithmic equation in my head and is instantly
scalable and movable to alternate landing sites. Especially flying out west
where most of the airports that I fly at do not have any safe landing spots
close by and the last few miles need to have extra high safety margins.
Thanks for sharing

CH