Questions on high altitude pressures

Wolfgang Schwanke wrote:

In theory you could use GPS altitudes when flying MSL/QNH setting,

No. Even with MSL/QNH setting, as you call it, the barometric altimeter
does *not* show the true geometric altitude. More precisely, it only
shows it in two cases:

1. When sitting on the ground at the reference altitude for the given
altimeter setting (QNH)

2. When the atmosphere happens to be an ICAO standard atmosphere.

Just to throw a little more fuel on this fire - GPS altitude is in
widespread use in the soaring world and has proven to be very useful
for computing height above ground and final glides to destinations.
It can also be used to give vertical motion, although not as
accurately as a pressure instrument.

The biggest difference (other than the previously discussed pressure
altitude vs height above the ellipsoid issue) is that while an
altimeter, when set to the correct pressure (mb or in hg) for QFE or
QNH, is stable in the short term but will become more innacurate over
time due to atmospheric pressure changes (not talking about flight
levels here), GPS is less accurate in the short term (altitude varies
with geometry, etc) but extremely stable over the long term.

I won't waste my time on argueing with Mxsmanic - he is really
clueless - but it sometimes amazes me how many "real" pilots have a
poor grasp on what altimeters really indicate!

Speaking of accuracy, anybody remember what the altitude tolerance of
an altimeter is? In military jets it's +- 75 feet or so - I have no
idea what it is for the non-TSO'd altimeter in my LS6. Which means
that as long as your GPS is tracking 4+ satellites with good geometry
and no SA, it's altitude (QNH) is probably better than what is shown
on your altimeter (but only if you know the difference between MSL and
HAE, which can be over 100' in many places...).

So - use your altimeter (set correctly) for IFR, and use your aviation
GPS to help avoid hitting the hard stuff!

Kirk

wrote:

So - use your altimeter (set correctly) for IFR, and use your aviation
GPS to help avoid hitting the hard stuff!

Actually, I use my eyes for that.

On Nov 28, 9:19*am, John Smith wrote:
wrote:
So - use your altimeter (set correctly) for IFR, and use your aviation
GPS to help avoid hitting the hard stuff!

Actually, I use my eyes for that.

Well, if you can see out the window, who cares about altitude anyway!

Now, at night, no moon, some haze, maybe over water....

Kirk

wrote:

So - use your altimeter (set correctly) for IFR, and use your aviation
GPS to help avoid hitting the hard stuff!

Actually, I use my eyes for that.

Well, if you can see out the window, who cares about altitude anyway!

Now, at night, no moon, some haze, maybe over water....

Wouldn't that be IFR?

On Nov 29, 3:30*am, John Smith wrote:
wrote:
So - use your altimeter (set correctly) for IFR, and use your aviation
GPS to help avoid hitting the hard stuff!
Actually, I use my eyes for that.
Well, if you can see out the window, who cares about altitude anyway!

Now, at night, no moon, some haze, maybe over water....

Wouldn't that be IFR?

Or really stupid night VFR...(which I realize is not legal in some
countries).

Of course, there has never been a case of a misset or misread
altimeter resulting in a pilot running a perfectly good airplane into
the ground, has there - especially with those horrible triple-pointer
altimeters! Much harder to do that with a GPS, especially if it has a
terrain database to give AGL height (well, you still have to read the
instruments...). Which is why most modern avionics setups have
exactly that arrangement. And it can be done cheap - $200 PDA with
GPS and some software does the same thing.

Kirk