Altimeter inaccurate

[smf]

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

[Dan Luke]

"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

[John R. Copeland]

"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---

[Bob Gardner]

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
>
>

[Judah]

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---
>

[Teacherjh]

>>
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)

[Andrew Sarangan]

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---
>>
>

[John Bell]

> 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

[Ron Rosenfeld]

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)

[John R. Copeland]

"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---

[Ron Rosenfeld]

On Mon, 03 May 2004 20:34:22 GMT, "John R. Copeland"
> wrote:

>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---

Yes, I have. The RNAV (GPS) Z RWY 1 at KPQI states Baro-VNAV not
authorized below -16°C (4°F).


Ron (EPM) (N5843Q, Mooney M20E) (CP, ASEL, ASES, IA)

[David Megginson]

Ron Rosenfeld wrote:

> 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.

You're correct about the Canadian regs, though I don't know how many
Canadian pilots remember to apply them. In any case, many parts of the
northern U.S. can see temperatures down to -35 degC or -40 degC just as much
as Canada can; when you combine that with the allowed +/-50 ft altimeter
error, you could be down below 100 ft when you think you're at 200 ft DH.

A more likely problem, though, would be early in an IAP, when you're still a
few thousand feet above the station elevation and have to clear a mountain
or a tower on a hill.


All the best,


David

[David Megginson]

Bob Gardner wrote:

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

Right -- ATC counts on the altimeter's temperature errors for traffic
separation (since every plane in the same area will share the same errors).
Our altimeters are pretty inaccurate at cruising altitudes, but as long as
we're all inaccurate in the same way, the system works.

In any case, I don't think that the poster was using his GPS for altitude --
he was just concerned that his altimeter might be wrong. I expect that this
will be a FAQ now that a lot of people have WAAS-enabled GPS's (they used to
think that the altimeter was right and the GPS altitude wildly inaccurate).


All the best,


David

[AJNOKC]

SOURCE DOD FLIGHT INFORMATION HANDBOOK
(https://164.214.2.62/dafif/dafif_0404_ed6/plan/fih.pdf)

3. TEMPERATURE ERRORS (AFFSA/AFFSA LTR)
a. Pressure altimeteres are calibrated to indicate true altitudes under
International
Standard Atmospheric (ISA) conditions. Any deviation from these
standard conditions will result in
an erroneous reading on the altimeter. This error becomes important
when considering obstacle
clearances in temperatures lower than standard since the aircraft’s
altitude is below the figure
indicated by the altimeter.
b. The error is proportional to the difference between actual and ISA
temperature and the
height of the aircraft above the altimeter setting source. Height
above altimeter source is
considered to be published HAT or HAA for the approach. The amount of
error is approximately 4
feet per thousand feet for each degree Celsius of difference.
c. Corrections will only be made for Decision Heights (DHs), Minimum
Descent Altitudes
(MDAs), and other altitudes inside, but not including, the Final
Approach Fix (FAF). The same
correction made to DHs and MDAs can be applied to the other altitudes
inside the FAF.

TEMPERATURE CORRECTION CHART (FEET)
AIRPORT
TEMP °C
0 0 20 20 20 40 40 40 40 60 80 90 110 120 140 180 240
300
-5 10 20 30 30 50 50 60 60 80 110 120 150 160 180 240 320
400
-10 20 20 40 40 60 60 80 80 100 130 150 180 200 230 300
400 500
-15 20 30 50 50 70 80 90 100 120 160 180 220 240 280 360
480 600
-20 20 40 60 60 80 100 100 120 140 180 210 250 280 320 420
560 700
-25 30 50 60 70 90 110 120 140 160 210 240 290 320 370 480
640 800
-30 40 60 60 80 100 120 140 160 180 240 270 330 360 410 540
720 900
-35 40 60 70 90 110 130 150 180 200 260 300 360 400 460 600
800 1000
-40 40 60 80 100 120 140 160 200 220 290 330 400 440 510 660
880 1100
-45 50 70 90 110 140 160 180 210 240 310 360 430 480 550 720
960 1200
-50 60 80 100 120 160 180 200 220 260 340 390 470 520 600 780
1040 1300
200 300 400 500 600 700 800 900 1000 1300 1500 1800 2000 2300 3000 4000
5000
HAT/HAA
NOTE: Must round interpoldated values to nearest 10 feet.



On Sun, 2 May 2004 13:50:02 -0500, smf > wrote:

> 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
>

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