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.

Not nearly as helpful as some of the other posts, but to give you a
sense of how things change, the pressure at 18,000 feet is about half
of that on the surface (30 inches of Hg on the surface, 15 at 18,000
feet). It's halved again at twice that altitude (7.5 inches at
36,000) and halved again at twice that altitude (3.7 inches at 72,000
feet).




On Nov 24, 1:40*am, es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. *Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. *Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA