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Altimeter inaccurate



 
 
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  #1  
Old May 2nd 04, 07:50 PM
smf
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Default Altimeter inaccurate

My Altimeter is accurate when on the ground or at lower alt. When I climb up
to 7k and above and check it against GPS and approach controls read out it
is about 300+ feet off.

Is it the altimeter I'm having problems with?


Steve


  #2  
Old May 2nd 04, 07:54 PM
Dan Luke
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"smf" wrote:
Is it the altimeter I'm having problems with?


Probably not.

Your altimeter has no means to correct for non-standard temperature; GPS
altitude is unaffected by temperature.
--
Dan
C172RG at BFM


  #3  
Old May 2nd 04, 08:15 PM
John R. Copeland
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"smf" wrote in message =
news:jpblc.12364$bS1.6242@okepread02...
My Altimeter is accurate when on the ground or at lower alt. When I =

climb up
to 7k and above and check it against GPS and approach controls read =

out it
is about 300+ feet off.
=20
Is it the altimeter I'm having problems with?
=20
Steve
=20

It's possible you are not having any problems at all.
Barometric altimetry simply does not measure geometric height.
And most GPSs ignore barometric settings completely.

I'm unsure what you mean by "approach controls read out".
The controller can see only what your Mode C transponder sends.
Usually that reading is to the nearest 100 feet, and it derives from
a barometric sensor in your airplane, permanently set to 29.92 in.hg.

With a correct barometric setting, your altimeter should show the =
relevant
airport's elevation at its true height, but normally *not* any other =
elevation.
If your altimeter passes its 24-month 91.411 check, it's surely OK.
If it fails its check, though, you do have an altimeter problem.
---JRC---

  #4  
Old May 2nd 04, 08:41 PM
Bob Gardner
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Default

AIM 1-1-20 (a)(8) tells us not to rely upon GPS to determine aircraft
altitude.

Bob Gardner

"smf" wrote in message
news:jpblc.12364$bS1.6242@okepread02...
My Altimeter is accurate when on the ground or at lower alt. When I climb

up
to 7k and above and check it against GPS and approach controls read out it
is about 300+ feet off.

Is it the altimeter I'm having problems with?


Steve




  #5  
Old May 3rd 04, 03:40 AM
Judah
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Default

It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations and
does not actually measure geometric altitude above the ground, is it not
possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric altitude
is within the bounds of that tower that would have been 100' lower if it
also changed altitude with the temperature?


Just a thought...

"John R. Copeland" wrote in
:


"smf" wrote in message
news:jpblc.12364$bS1.6242@okepread02...
My Altimeter is accurate when on the ground or at lower alt. When I
climb up to 7k and above and check it against GPS and approach
controls read out it is about 300+ feet off.

Is it the altimeter I'm having problems with?

Steve

It's possible you are not having any problems at all.
Barometric altimetry simply does not measure geometric height.
And most GPSs ignore barometric settings completely.

I'm unsure what you mean by "approach controls read out".
The controller can see only what your Mode C transponder sends.
Usually that reading is to the nearest 100 feet, and it derives from
a barometric sensor in your airplane, permanently set to 29.92 in.hg.

With a correct barometric setting, your altimeter should show the
relevant airport's elevation at its true height, but normally *not* any
other elevation. If your altimeter passes its 24-month 91.411 check,
it's surely OK. If it fails its check, though, you do have an altimeter
problem. ---JRC---


  #6  
Old May 3rd 04, 04:24 AM
Teacherjh
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Default


It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations and
does not actually measure geometric altitude above the ground, is it not
possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric altitude
is within the bounds of that tower that would have been 100' lower if it
also changed altitude with the temperature?


IN theory yes, the altitude would be different from the indicated altitude.
However, you'd only be 100 feet away from the place where the altimiter setting
was derived, so the amount of error would be slight.

Jose

--
(for Email, make the obvious changes in my address)
  #7  
Old May 3rd 04, 05:39 AM
Andrew Sarangan
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Default

Temperature variations will cause an error but it is very small. The AIM
has a table that lists these errors. At 200' AGL (ILS minimum), you will
get a 20' error if the temperature is -20 C. It becomes 60' when the
temperature is -50C (not a common temperature in the US except may be
Alaska). The error is larger at higher altitudes (as much as 300' at 1000
AGL when the temperature is -50C).





Judah wrote in
:

It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations
and does not actually measure geometric altitude above the ground, is
it not possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric
altitude is within the bounds of that tower that would have been 100'
lower if it also changed altitude with the temperature?


Just a thought...

"John R. Copeland" wrote in
:


"smf" wrote in message
news:jpblc.12364$bS1.6242@okepread02...
My Altimeter is accurate when on the ground or at lower alt. When I
climb up to 7k and above and check it against GPS and approach
controls read out it is about 300+ feet off.

Is it the altimeter I'm having problems with?

Steve

It's possible you are not having any problems at all.
Barometric altimetry simply does not measure geometric height.
And most GPSs ignore barometric settings completely.

I'm unsure what you mean by "approach controls read out".
The controller can see only what your Mode C transponder sends.
Usually that reading is to the nearest 100 feet, and it derives from
a barometric sensor in your airplane, permanently set to 29.92 in.hg.

With a correct barometric setting, your altimeter should show the
relevant airport's elevation at its true height, but normally *not*
any other elevation. If your altimeter passes its 24-month 91.411
check, it's surely OK. If it fails its check, though, you do have an
altimeter problem. ---JRC---



  #8  
Old May 3rd 04, 06:22 AM
John Bell
external usenet poster
 
Posts: n/a
Default

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations and
does not actually measure geometric altitude above the ground, is it not
possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric altitude
is within the bounds of that tower that would have been 100' lower if it
also changed altitude with the temperature?


Just a thought...


And a good thought it is. Try this:
http://www.aircraftbuyer.com/learn/train06.htm

John Bell
www.cockpitgps.com


  #9  
Old May 3rd 04, 12:55 PM
Ron Rosenfeld
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Posts: n/a
Default

On Mon, 03 May 2004 02:40:18 GMT, Judah wrote:

It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100'
clearance, right?), but does not account for temperature variations and
does not actually measure geometric altitude above the ground, is it not
possible that one would find oneself at an altitude that from a
barometric standpoint is legal and correct, but from a geometric altitude
is within the bounds of that tower that would have been 100' lower if it
also changed altitude with the temperature?


It is my understanding that in Canada there is a requirement to alter
minimums for certain approaches in very cold weather. No such requirement
exists (Part 91, at least) in the US.

I don't believe there have been any accidents in the US due to this
phenomenon.


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)
  #10  
Old May 3rd 04, 09:34 PM
John R. Copeland
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Posts: n/a
Default


"Ron Rosenfeld" wrote in message =
...
On Mon, 03 May 2004 02:40:18 GMT, Judah wrote:
=20
It begs an interesting question, though...

If an IAP is based on object clearance (in some cases, as low as 100' =


clearance, right?), but does not account for temperature variations =

and=20
does not actually measure geometric altitude above the ground, is it =

not=20
possible that one would find oneself at an altitude that from a=20
barometric standpoint is legal and correct, but from a geometric =

altitude=20
is within the bounds of that tower that would have been 100' lower if =

it=20
also changed altitude with the temperature?

=20
It is my understanding that in Canada there is a requirement to alter
minimums for certain approaches in very cold weather. No such =

requirement
exists (Part 91, at least) in the US.
=20
I don't believe there have been any accidents in the US due to this
phenomenon.
=20
Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)


Along that line, perhaps you've noticed that the new VNAV approaches
typically carry a note to the effect that Baro-VNAV is not authorized
at temperatures below some limiting number.
---JRC---

 




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