ATC Altimeter Settings

"Icebound" wrote in message
...
How would a station be more representative if it was/wasn't in a valley
or
on a hill top?



This is an obstruction-clearance issue in very cold weather.

If the altimeter setting came from a station in the valley 5000 feet in
true
height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height from
the
aircraft, the difference is likely less than 300 feet. (Source: Canadian
AIP)

If you are choosing flight altitudes without much margin for obstacle
clearance, you may want to ask for somewhat higher altitudes if you will
be
using settings from valley stations.


Mind telling me how that variation (of that magnitude) could come about?

"Matt Barrow" wrote in message
...

"Icebound" wrote in message
...
How would a station be more representative if it was/wasn't in a valley
or
on a hill top?



This is an obstruction-clearance issue in very cold weather.

If the altimeter setting came from a station in the valley 5000 feet in
true
height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height from
the
aircraft, the difference is likely less than 300 feet. (Source: Canadian
AIP)

If you are choosing flight altitudes without much margin for obstacle
clearance, you may want to ask for somewhat higher altitudes if you will
be
using settings from valley stations.


Mind telling me how that variation (of that magnitude) could come about?



Its been discussed here many times, but I will review:

We know that an altimeter setting by definition means that, set to the
station-determined value, our INDICATED altitude will match the REAL
ELEVATION, when we are parked on the threshold of the station. Bascially,
the "altimeter setting" which we set in the Kollsman window, is what
accomplishes that correction-for-station-elevation, so that we get MSL and
our INDICATED readout.

Beyond that, when we fly at some indicated altitude other than that exactly
equal to the station's, we are following a constant air pressure... a
pressure which is some fixed amount LESS (if we are higher) than the actual
air pressure at our station. The altimeter, converts this
pressure-difference (between the pressure at the station, and the actual
pressure at the airplane)... into an altitude readout. Adjusted, of course,
by adding the elevation of the station to get MSL... as I have already
mentioned, by means of the altimeter-setting adjustment.

But because the altimeter has no knowledge of the ambient temperature, it
makes this pressure-difference conversion assuming the "standard"
atmosphere. In our very cold conditions, the air is much denser that the
"standard", and thus the pressure levels are much closer together... in
other words: as we climb away from the altimeter-setting-station, the
pressure decreases much more, in 1000 feet of REAL altitude, than it does in
"standard" temperature conditions. Therefore our altimeter will read higher
than the REAL altitude, because it only knows about the lower pressure, and
nothing about the colder temperature. The altimeter reads higher than real,
real elevation is lower than indicataed, obstruction clearance may be an
issue.

Naturally, the further we climb from our altimeter-setting-station, the
greater the error. That is why we can reduce this error if we can get the
altimeter setting from the station closest to our real altitude.

Remember that this has nothing to do with assigned altitudes. Those are
flown according to the INDICATED altitude whatever it happens to be, based
on whatever approved official altimeter setting you have. The Canadian rules
specifically say:

quote:
IFR assigned altitudes accepted by a pilot shall not be adjusted to
compensate for cold
temperatures, i.e., if a pilot accepts “maintain 3 000”, an altitude
correction shall not be
applied to 3 000 ft.
:unquote

BUT IF there a concern about clearing obstacles or terrain, a knowledge of
the probable error between indicated and actual may cause you to file and/or
request a higher INDICATED altitude.

The actual numbers can be calculated from knowledge of "air constants", and
temperature-density relationships, etc., but since we fly in very cold
weather up here quite a bit, Transport Canada has done the math and
published some figures for us in the Canadian equivalent of the AIM.

Fore example, at 40 below, 5000 feet away from altimeter-setting-station,
error is published as 1210 feet. At 50 below it is 1500 feet. 1000 feet
away from altimeter-setting-station, error is published as 240 feet at 40
below, 300 feet at 50...

The comment is often made: "Well if the air is cold, and cold air is dense,
then the pressure that I am flying at should be higher", and not lower as I
have shown above. But that statement ignores the fact that air is
compressible and thus more of that cold density is BELOW you. The pressure
at your altitude is produced only by the weight of the air ABOVE you.

"Matt Barrow" writes:

"Icebound" wrote:
[...]
If the altimeter setting came from a station in the valley 5000 feet in
true height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height from
the aircraft, the difference is likely less than 300 feet. [...]

Mind telling me how that variation (of that magnitude) could come about?

If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. The effect
is that the lower you are AGL, the closer the calibrated & true
altitudes tend to become, because deviations from the standard
atmosphere become less significant within less tall columns of air.

- FChE

"Frank Ch. Eigler" wrote in message
...

"Matt Barrow" writes:

"Icebound" wrote:
[...]
If the altimeter setting came from a station in the valley 5000 feet
in
true height below the aircraft, the indicated height could differ from
true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height
from
the aircraft, the difference is likely less than 300 feet. [...]

Mind telling me how that variation (of that magnitude) could come about?

If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. The effect
is that the lower you are AGL, the closer the calibrated & true
altitudes tend to become, because deviations from the standard
atmosphere become less significant within less tall columns of air.

Ground stations are all AT GROUND LEVEL, whether the station is at 2000 MSL
or 6000 MSL.

"Icebound" wrote in message
...

"Matt Barrow" wrote in message
...

"Icebound" wrote in message
...
How would a station be more representative if it was/wasn't in a
valley
or
on a hill top?



This is an obstruction-clearance issue in very cold weather.

If the altimeter setting came from a station in the valley 5000 feet in
true
height below the aircraft, the indicated height could differ from true
height by as much as 1500 feet feet.

If it came from a station on the hill only 1000 feet in true height
from
the
aircraft, the difference is likely less than 300 feet. (Source:
Canadian
AIP)

If you are choosing flight altitudes without much margin for obstacle
clearance, you may want to ask for somewhat higher altitudes if you
will
be
using settings from valley stations.


Mind telling me how that variation (of that magnitude) could come
about?



Its been discussed here many times, but I will review:

We know that an altimeter setting by definition means that, set to the
station-determined value, our INDICATED altitude will match the REAL
ELEVATION, when we are parked on the threshold of the station. Bascially,
the "altimeter setting" which we set in the Kollsman window, is what
accomplishes that correction-for-station-elevation, so that we get MSL and
our INDICATED readout.

Beyond that, when we fly at some indicated altitude other than that
exactly
equal to the station's, we are following a constant air pressure... a
pressure which is some fixed amount LESS (if we are higher) than the
actual
air pressure at our station. The altimeter, converts this
pressure-difference (between the pressure at the station, and the actual
pressure at the airplane)... into an altitude readout. Adjusted, of
course,
by adding the elevation of the station to get MSL... as I have already
mentioned, by means of the altimeter-setting adjustment.

But because the altimeter has no knowledge of the ambient temperature, it
makes this pressure-difference conversion assuming the "standard"
atmosphere. In our very cold conditions, the air is much denser that the
"standard", and thus the pressure levels are much closer together... in
other words: as we climb away from the altimeter-setting-station, the
pressure decreases much more, in 1000 feet of REAL altitude, than it does
in
"standard" temperature conditions. Therefore our altimeter will read
higher
than the REAL altitude, because it only knows about the lower pressure,
and
nothing about the colder temperature. The altimeter reads higher than
real,
real elevation is lower than indicataed, obstruction clearance may be an
issue.

Naturally, the further we climb from our altimeter-setting-station, the
greater the error. That is why we can reduce this error if we can get the
altimeter setting from the station closest to our real altitude.

Remember that this has nothing to do with assigned altitudes. Those are
flown according to the INDICATED altitude whatever it happens to be, based
on whatever approved official altimeter setting you have. The Canadian
rules
specifically say:

quote:
IFR assigned altitudes accepted by a pilot shall not be adjusted to
compensate for cold
temperatures, i.e., if a pilot accepts “maintain 3 000”, an altitude
correction shall not be
applied to 3 000 ft.
:unquote

BUT IF there a concern about clearing obstacles or terrain, a knowledge of
the probable error between indicated and actual may cause you to file
and/or
request a higher INDICATED altitude.

The actual numbers can be calculated from knowledge of "air constants",
and
temperature-density relationships, etc., but since we fly in very cold
weather up here quite a bit, Transport Canada has done the math and
published some figures for us in the Canadian equivalent of the AIM.

Fore example, at 40 below, 5000 feet away from altimeter-setting-station,
error is published as 1210 feet. At 50 below it is 1500 feet. 1000 feet
away from altimeter-setting-station, error is published as 240 feet at 40
below, 300 feet at 50...

The comment is often made: "Well if the air is cold, and cold air is
dense,
then the pressure that I am flying at should be higher", and not lower as
I
have shown above. But that statement ignores the fact that air is
compressible and thus more of that cold density is BELOW you. The
pressure
at your altitude is produced only by the weight of the air ABOVE you.



You're bringing in a lot of irrelevant material. The original point was
"Area" altmiter settings and another point made that a station in
mountainous areas would be more accurate if it was located on the mountain
top at higher altitude than a station at lower altitude in the same area.

You're still not explaining how a ground station at, say, 6000' MSL would
be have a more accurate baro reading than one down in a nearby valley at,
say, 2000' MSL. That is the point of the thread.

The way I see it, if you have two stations, one on the ground at 350
feet MSL, and the other on the ground (on top of a mountain) at 5000
feet MSL, and they are both "nearby", then the one that is actually at
5000 feet MSL will give an altimeter setting that will be more accurate
for an airplane that's flying at 5000 feet MSL.

The altimeter setting from the 350' station will be correct at that 350'
elevation, but the altitude indicated by using that setting in the
window will be a guess (based on standard lapse rate and other such) for
an airplane at 5000 feet. Granted, usually a pretty good guess, but
altimeter altitude is still an indirect inference from other parameters
(pressure and such). If the actual atmosphere that day does not follow
the theoretical average curves, the actual altitude of an airplane that
is indicating 5000 feet using an altimeter setting from 350' will be
somewhat off.

Of course, the airplane that uses the 5000' station's altimeter setting
while at 5000 feet, and proceeds to land at the 350' high airstrip, will
likely find the indicated altitude once on the ground to be different
from 350' (by about the same amount)

Given this, it makes sense to me (depending on the accuracy needed and
the actual difference from standard lapse rate) that flights =through=
an area might use one setting, and flights =to= an area might use
another one - at least when setting up to land.

Jose
--
Get high on gasoline: fly an airplane.
for Email, make the obvious change in the address.

"Matt Barrow" writes:

[I wrote:]
If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. [...]

Ground stations are all AT GROUND LEVEL, whether the station is at
2000 MSL or 6000 MSL.

That's quite insightful, but irrelevant to the issue of *elevation* of
those stations.

- FChE

"Jose" wrote in message
om...
The way I see it, if you have two stations, one on the ground at 350
feet MSL, and the other on the ground (on top of a mountain) at 5000
feet MSL, and they are both "nearby", then the one that is actually at
5000 feet MSL will give an altimeter setting that will be more accurate
for an airplane that's flying at 5000 feet MSL.

How?


The altimeter setting from the 350' station will be correct at that 350'
elevation, but the altitude indicated by using that setting in the
window will be a guess (based on standard lapse rate and other such) for
an airplane at 5000 feet. Granted, usually a pretty good guess, but
altimeter altitude is still an indirect inference from other parameters
(pressure and such). If the actual atmosphere that day does not follow
the theoretical average curves, the actual altitude of an airplane that
is indicating 5000 feet using an altimeter setting from 350' will be
somewhat off.

Ummmm...air pressure is constant when corrected for altitude. otherwise they
would give altimeter setttings at various altitude, not a various locations.

Of course, the airplane that uses the 5000' station's altimeter setting
while at 5000 feet, and proceeds to land at the 350' high airstrip, will
likely find the indicated altitude once on the ground to be different
from 350' (by about the same amount)

Might you be able to point me to a difinitive paper on that rather than just
idle specualtion?

Given this, it makes sense to me (depending on the accuracy needed and
the actual difference from standard lapse rate) that flights =through=
an area might use one setting, and flights =to= an area might use
another one - at least when setting up to land.

Again, could you point me to an authoritative reference?

I keep hearing people running on about that they would be different, but
nothing substantial offered as evidence and certainly nothing I've heard of
in 25 years of flying (but I might have had a deprived career).

--
Matt
---------------------
Matthew W. Barrow
Site-Fill Homes, LLC.
Montrose, CO

"Frank Ch. Eigler" wrote in message
...

"Matt Barrow" writes:

[I wrote:]
If you run through the full "true altitude" calculation discussed
during early ground school, you'll see that there is a term that
relates to the elevation of the measurement station. [...]

Which has nothing to do with how accurate they would be -- it's all
corrected out.


Ground stations are all AT GROUND LEVEL, whether the station is at
2000 MSL or 6000 MSL.

That's quite insightful, but irrelevant to the issue of *elevation* of
those stations.

Which you haven't answered either...or any of the other four or five people
who've responded. Only the same "well, maybe it might be....".

"Matt Barrow" wrote in message
...


You're bringing in a lot of irrelevant material. The original point was
"Area" altmiter settings and another point made that a station in
mountainous areas would be more accurate if it was located on the mountain
top at higher altitude than a station at lower altitude in the same area.

You're still not explaining how a ground station at, say, 6000' MSL would
be have a more accurate baro reading than one down in a nearby valley at,
say, 2000' MSL. That is the point of the thread.





There is NO issue of "altimeter-setting accuracy" nor "barometer accuracy".

Both stations have accurate barometers, and are reading their station
pressure correctly and accurately. Both altimeter-settings are "accurate"
in so far as the settings have been properly determined according to the
rules, for each individual station's actual-air-pressure and each individual
station's actual-measured-elevation.

But as we all know, setting our altimeter to an "accurate altimeter setting"
does not mean that the INDICATED altitude matches the TRUE altitude. In
almost never ever does, because the real atmosphere is almost always
different from the "standard" for which altimeters are calibrated.

So there is always a discrepancy between INDICATED and TRUE altitudes. In
most cases this does not matter, because the discrepancy is the same for
everybody.

Somebody in this thread asked WHY this discrepancy was greater if using a
valley station's setting, as opposed to a hill station's setting. (He may
have used the word "accuracy", but his meaning was: "why is my INDICATED
altitude going to be closer to my TRUE altitude when I use the
hill-station's-altimeter setting as opposed to the valley's?")

My entire tirade was to try to explain why that is so... and just to warn,
that in very cold weather, this means that you are flying much lower (TRUE
altitude) than INDICATED. If you are not paying attention to
indicated-vs-true discrepancies, you may choose an INDICATED altitude which
may put you below a comfortable margin of terrain/obstacle clearance.