wrote on 7/20/2016 7:53 AM:

3. Managing final glides. This was my original focus, in particular
in the Western U.S. where both altitudes and temperatures can be high
(leading to greater differences between observed pressure altitudes
and GPS altitudes) and final glides are often much longer. The latter
can increase the odds that, owing to both distance and time, a pilot
may transition from an area with lift to an area where there is less
or no lift, making it difficult to salvage a final glide that is
falling short at the end.

Pressure altitude is conservative in the sense that it often under
reads the geometric altitude and--at least out West--will therefore
provide a cushion against unforeseen sink. But I already have an
explicit arrival height safety margin. Layering that with an
uncertain additional cushion isn't where I wanted to go.

That said, the GPS vs. pressure altitude cushion has the virtue that
it tends to increase with altitude. Final glide computers I've used
allow entering an arrival (safety) altitude of X feet, without regard
to MC setting or altitude or length of glide. There are ways to make
it proportional (e.g., % bugs or % risk) but using pressure vs. GPS
altitude can do the same thing. Of course, the differences will vary
from day to day so that introduces more uncertainty into an area that
already has plenty of it. I'd rather nail the altitude (subject to
the uncertainties of GPS determination) and factor in the safety
factor(s) explicitly myself.

Using GPS would remove the altitude uncertainty, but still leaves the
two most important ones: wind and lift/sink.

I really noticed the effect of wind while flying at Parowan in June: my
vario nav page used the vario's "live wind" (updated several times a
minute) it computes from it's sensors, while the flight computer used
the wind obtained while circling. When 30-50 NM from the goal, I saw
variations in "altitude required" of as much as 2000'.

--
Eric Greenwell - Washington State, USA (change ".netto" to ".us" to
email me)
- "A Guide to Self-Launching Sailplane Operation"

https://sites.google.com/site/motorg...ad-the-guide-1
- "Transponders in Sailplanes - Dec 2014a" also ADS-B, PCAS, Flarm

http://soaringsafety.org/prevention/...anes-2014A.pdf