"Andrew Gideon" wrote in message
online.com...
Mike Rapoport wrote:

Many, if not most, turbine airplanes have this but it requires an airdata
computer system. You can set it to arrive at a particular point at a
specific altitude. You need an airdata system as long as you are
climbing
or descending to a pressure altitiude. WAAS could only guide you to a
GPS
altitude.

Perhaps I'm missing something basic, but don't we specifically adjust the
kollsman window to convert values provided by a barometric altimeter to a
true altitude (at least below 18,000')? Of course, you're quite right
that
the GPS would need "correction" to yield the pressure altitude used above
18,000 (which I didn't consider because I've never flown there myself
{8^).

- Andrew
You are right and as Julian points out, I should use the term barometric
altitude. Barometric altitude (with the correct kollsman setting) is only
correct if the temperature lapse rate is 2C/1000'. It is also affected by
airflow over mountains and can be off by 1,000' in extreme conditions (I've
read). My point is that there is no way to "convert" GPS altitude to
barometric altitude since the barometric altitude is subject to a host of
errors. Everybody needs to be on the same "system" and that is barometric.

Mike
MU-2