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Questions on high altitude pressures



 
 
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  #51  
Old November 27th 08, 11:03 PM posted to rec.aviation.piloting
Jose Jimenez
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Default Questions on high altitude pressures

Frank Barchi wrote:

Just an extra, extra point.....there is no FL 120


You might not be aware of it, but there's actually life (and flying)
outside of the USA.
  #52  
Old November 27th 08, 11:08 PM posted to rec.aviation.piloting
Jose Jimenez
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Default 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.
  #53  
Old November 28th 08, 04:51 PM posted to rec.aviation.piloting
[email protected][_2_]
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Default Questions on high altitude pressures

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
  #54  
Old November 28th 08, 05:19 PM posted to rec.aviation.piloting
John Smith
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Default Questions on high altitude pressures

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.
  #55  
Old November 28th 08, 09:21 PM posted to rec.aviation.piloting
a[_3_]
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Posts: 562
Default Questions on high altitude pressures

Given the nature of this thread, some of you will enjoy this video,

http://www.grayeagles.org/video.htm

and others will let the lettering on the side of this rebuilt Mustang,
named "Feburary", ruin the entire experience.

The video is worth watching, especially this close to Veteran's Day.
  #56  
Old November 29th 08, 02:18 AM posted to rec.aviation.piloting
[email protected][_2_]
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Default Questions on high altitude pressures

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
  #58  
Old November 29th 08, 04:52 PM posted to rec.aviation.piloting
[email protected][_2_]
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Default Questions on high altitude pressures

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
  #60  
Old December 3rd 08, 12:09 PM posted to rec.aviation.piloting
Bertie the Bunyip[_24_]
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Default Questions on high altitude pressures

Mxsmanic wrote in
:

es330td writes:

This brings up an interesting wrinkle then as GPS altitude info is
not dependent on external pressure so pilots must be careful to
ignore that info if available. As stated before, in Class A everyone
sets their altimeter to 29.92 so that as long as everyone is wrong
together everything is okay. Adding GPS info into the mix splits the
groups into two; one that is wrong together at 29.92 and another that
is right at actual altitude.


GPS is too inaccurate for most purposes in vertical positioning,
anyway. It is not designed to determine altitude with a high degree
of accuracy, and can easily be hundreds of feet off.



You#re aan idiot.


Bertie
 




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