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ATC Altimeter Settings



 
 
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  #1  
Old April 1st 05, 06:13 PM
O. Sami Saydjari
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Default ATC Altimeter Settings

When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC
region, they say something like, "Midway altimeter is xx.xx." I have
some questions.

1. So, do they pick some fix for the region and give everyone (those
below FL 180) the altimeter of that fix? Do they break up into
sub-regions and give everyone in that sub-region that same fix altimeter
setting?

2. At what point after departing an airport do you change altimeter
setting to the one supplied by ATC? Similarly, on approaching an
airport to land, at what point do you switch over to the altimeter
setting of the airport (from that given by ATC).

The reason I ask is that sometimes, the altimeter settings can be quite
different. Especially if there is VFR traffic around an airport, it
seems that there is some risk if everyone entering the pattern (or
approaching the pattern) does not share the same altimeter setting.

Just curious.

-Sami
N2057M, Piper Turbo Arrow III


  #2  
Old April 1st 05, 07:22 PM
Roy Smith
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Default

O. Sami Saydjari wrote:
When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC
region, they say something like, "Midway altimeter is xx.xx." I have
some questions.

1. So, do they pick some fix for the region and give everyone (those
below FL 180) the altimeter of that fix? Do they break up into
sub-regions and give everyone in that sub-region that same fix altimeter
setting?


You should get the altimeter setting for the airport nearest your
location which has weather reporting.

2. At what point after departing an airport do you change altimeter
setting to the one supplied by ATC?


I update my altimeter setting whenever ATC gives me a new one.

Similarly, on approaching an airport to land, at what point do you
switch over to the altimeter setting of the airport (from that given
by ATC).


If ATC has given me a setting for my destation, I'll just use that.
If not, then I'll set my altimeter whenever I get the ATIS/AWOS.

The reason I ask is that sometimes, the altimeter settings can be quite
different.


What do you consider "quite different"? Unless there's some severe
weather going on, it's unusual to have to change the setting more than
a couple of 1/100's at a time.

Especially if there is VFR traffic around an airport, it
seems that there is some risk if everyone entering the pattern (or
approaching the pattern) does not share the same altimeter setting.


Risk of what? If somebody's 50 feet high or low in the pattern, it
doesn't make much difference. As long as it's VFR, see and avoid is
the rule of the day. The real reason for having sensitive altimeters
is for instrument work.
  #3  
Old April 1st 05, 07:37 PM
O. Sami Saydjari
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Default



Roy Smith wrote:

O. Sami Saydjari wrote:

When entering an ATC region, we often get an altimeter setting, usually
at some fix. So for example, when entering the Chicago center ATC
region, they say something like, "Midway altimeter is xx.xx." I have
some questions.

1. So, do they pick some fix for the region and give everyone (those
below FL 180) the altimeter of that fix? Do they break up into
sub-regions and give everyone in that sub-region that same fix altimeter
setting?



You should get the altimeter setting for the airport nearest your
location which has weather reporting.


So, if ATC gives you a setting on initial contact out of the airport you
just took off from, you use the airport's setting, not ATC's? When do
you cut over to ATC's? Or do you just listen to every AWOS/ASOS enroute
and change to the closest one?



2. At what point after departing an airport do you change altimeter
setting to the one supplied by ATC?



I update my altimeter setting whenever ATC gives me a new one.


See above. What I am asking is when to change over?


Similarly, on approaching an airport to land, at what point do you
switch over to the altimeter setting of the airport (from that given
by ATC).



If ATC has given me a setting for my destation, I'll just use that.
If not, then I'll set my altimeter whenever I get the ATIS/AWOS.


So, ATC expects you to change your altimeter whenever you first hear the
ATIS at your destination? That could be 50 miles out. Seems a little odd.



The reason I ask is that sometimes, the altimeter settings can be quite
different.



What do you consider "quite different"? Unless there's some severe
weather going on, it's unusual to have to change the setting more than
a couple of 1/100's at a time.


I have seen differences of 0.5 inches....that is alot!



Especially if there is VFR traffic around an airport, it
seems that there is some risk if everyone entering the pattern (or
approaching the pattern) does not share the same altimeter setting.



Risk of what? If somebody's 50 feet high or low in the pattern, it
doesn't make much difference. As long as it's VFR, see and avoid is
the rule of the day. The real reason for having sensitive altimeters
is for instrument work.


Well, good point, except that if you are descending through a cloud deck
into VFR conditions. Also, if you change your altimeter 50 miles out
and I change mine 5 miles out, it seems like their is some distance from
the airport where we are using different settings, and that seems like a
bad thing to me.
  #4  
Old April 1st 05, 07:56 PM
Roy Smith
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Default

O. Sami Saydjari wrote:
You should get the altimeter setting for the airport nearest your
location which has weather reporting.


So, if ATC gives you a setting on initial contact out of the airport you
just took off from, you use the airport's setting, not ATC's?


I think I may have confused you with some sloppy wording. What I
meant was, "The altimeter setting ATC gives you will mostly likely be
for the airport nearest your current location".

The bottom line is anytime ATC gives me an altimeter setting, I reset
my altimeter to whatever the controller gives me.

Or do you just listen to every AWOS/ASOS enroute
and change to the closest one?


Well, I do that too. Maybe not *every* one, but on a long flight, I
will certainly tune in the occassional ATIS as I go by airports. It's
also a good way to stay aware of weather trends; if the altimeter
settings keep going down, you know you're flying into worsening weather.

What do you consider "quite different"? Unless there's some severe
weather going on, it's unusual to have to change the setting more than
a couple of 1/100's at a time.


I have seen differences of 0.5 inches....that is alot!


0.5 inches is a HUGE altimter setting change. Are you sure about
that?
  #5  
Old April 1st 05, 08:59 PM
A Lieberman
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Default

On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote:

0.5 inches is a HUGE altimter setting change. Are you sure about
that?


I have to agree with Roy here.

I just finished coming back from Ohio to Mississippi, and I experienced a
very large altimeter setting adjustments in my journey southbound north of
Bowling Green KY, but I sure did not experience .50 inches adjustments en
route. I did end up .25 inch less over the whole trip though.

If I remember correctly on one ATC hand off, I went from 29.96 to 29.88
which lowered my altitude by 80 feet. This really alarmed me as I sure
don't like deviating more then 20 feet from assigned altitude. Needless to
say, I made a quick climb to my assigned altitude!

There was a rather steep pressure gradient even in my trip I just did this
week, but as Roy says, .5 inch altimeter change in a flight range distance
in planes I fly is probably indicative of weather I wouldn't / shouldn't be
flying in anyway.

Allen
  #6  
Old April 1st 05, 09:19 PM
O. Sami Saydjari
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Default

Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point
substantially. Even a 0.25 inch change is substantial, and if there is
not a standard of when one changes settings, it seems like a risk,
albeit small.

My experience is that ATC does not give me settings of nearby airports.
For example, I recall entering Chicgo Center airspace from the south.
I was just north of Lafayette, Indiana. The controller gave me the
altimeter setting for Chicago Midway. That is a long way away.
Lafayette airport was at least 0.1 inches different. I used ATC's
setting, because I figure they must have some standard reference point
in a region, or sub-region, so that all the planes are at least on the
same relative setting.

-Sami
N2057M, Piper Turbo Arrow III

A Lieberman wrote:

On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote:


0.5 inches is a HUGE altimter setting change. Are you sure about
that?



I have to agree with Roy here.

I just finished coming back from Ohio to Mississippi, and I experienced a
very large altimeter setting adjustments in my journey southbound north of
Bowling Green KY, but I sure did not experience .50 inches adjustments en
route. I did end up .25 inch less over the whole trip though.

If I remember correctly on one ATC hand off, I went from 29.96 to 29.88
which lowered my altitude by 80 feet. This really alarmed me as I sure
don't like deviating more then 20 feet from assigned altitude. Needless to
say, I made a quick climb to my assigned altitude!

There was a rather steep pressure gradient even in my trip I just did this
week, but as Roy says, .5 inch altimeter change in a flight range distance
in planes I fly is probably indicative of weather I wouldn't / shouldn't be
flying in anyway.

Allen

  #7  
Old April 1st 05, 09:50 PM
A Lieberman
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Posts: n/a
Default

On Fri, 01 Apr 2005 14:19:56 -0600, O. Sami Saydjari wrote:

Sami

Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point
substantially. Even a 0.25 inch change is substantial, and if there is
not a standard of when one changes settings, it seems like a risk,
albeit small.


The .25 inch change was over 684 NM. Unless you are crossing a cold front
or a rather intense low pressure, you are not too likely to experience a
significant pressure change to the point that you describe.

My experience is that ATC does not give me settings of nearby airports.
For example, I recall entering Chicgo Center airspace from the south.
I was just north of Lafayette, Indiana. The controller gave me the
altimeter setting for Chicago Midway. That is a long way away.
Lafayette airport was at least 0.1 inches different.


What day were you flying? If you go to
http://www.hpc.ncep.noaa.gov/dailywxmap/frame.html, you can retrieve
historical weather maps to verify what you say. Was it windy that day?
Was there a low pressure passing through. If so, then yes, it is possible
to experience a .10 inch change over a short range.

I used ATC's
setting, because I figure they must have some standard reference point
in a region, or sub-region, so that all the planes are at least on the
same relative setting.


I always use ATC's settings. I figure their scope reads my transponder,
and my transponder readout is dependent on what I put in for my altimeter
setting. I sure don't want an altitude deviation based on something
different then what ATC tells me.

Like Roy said, if you are arriving at an airport VFR, the altimeter is not
quite as important as when you are arriving at an airport under instrument
conditions.

It's see and avoid for VFR, and if your altimeter is only .01 off, that
only puts you approximately 10 feet off pattern altitude. .10 would put
you 100 feet off pattern altitude, and again, see and avoid would be the
ruling factor.

It becomes more critical on an instrument approach, as you want to be as
close as you can for an accurate altimeter setting for missed approaches
purposes at MDA or DH.

Allen
  #8  
Old April 1st 05, 10:16 PM
Sriram Narayan
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Default



"A Lieberman" wrote in message
...
On Fri, 01 Apr 2005 14:19:56 -0600, O. Sami Saydjari wrote:

Sami

Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point
substantially. Even a 0.25 inch change is substantial, and if there is
not a standard of when one changes settings, it seems like a risk,
albeit small.


The .25 inch change was over 684 NM. Unless you are crossing a cold front
or a rather intense low pressure, you are not too likely to experience a
significant pressure change to the point that you describe.


The only time I had seen a significant change in altimeter setting was when
crossing over from the central valley in California (Bakersfield sector) to
the LA basin (Socal). I remember my altitude was off by 100-150ft which
really bothered me and I could do nothing about it since I switched to the
altimeter setting provided when given to me. One of them was certainly off
for sure. Also, the radio comm was weak in the transition area. The Socal
controller was not too happy.



  #9  
Old April 1st 05, 10:28 PM
Mitty
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Default



I always use ATC's settings. I figure their scope reads my transponder,
and my transponder readout is dependent on what I put in for my altimeter
setting.


IIRC the Mode C is pressure altitude, usually from a separate transducer.
  #10  
Old April 1st 05, 10:28 PM
Bob Gardner
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Default

AIM 7-2-3 says your altimeter can be in error by 74 feet without a trip to
the shop, so at least that much slop is built into the system. The ATC
separates IFR from IFR by miles, not feet (OK, feet vertically, but opposite
direction traffic is separated by at least 1000 feet vertically).

If my preflight weather analysis showed a pressure difference of .5 inches
from one area to another but within my range, I would seriously consider
going somewhere else.

One-tenth of an inch is 100 feet...I would not get ulcers over it.

Bob Gardner


"O. Sami Saydjari" wrote in message
...
Well, I thought it was 0.5 inches, but I am not absolutley certain...it
was a while back. But, I do not think this changes my point
substantially. Even a 0.25 inch change is substantial, and if there is
not a standard of when one changes settings, it seems like a risk, albeit
small.

My experience is that ATC does not give me settings of nearby airports.
For example, I recall entering Chicgo Center airspace from the south. I
was just north of Lafayette, Indiana. The controller gave me the
altimeter setting for Chicago Midway. That is a long way away. Lafayette
airport was at least 0.1 inches different. I used ATC's setting, because
I figure they must have some standard reference point in a region, or
sub-region, so that all the planes are at least on the same relative
setting.

-Sami
N2057M, Piper Turbo Arrow III

A Lieberman wrote:

On 1 Apr 2005 13:56:42 -0500, Roy Smith wrote:


0.5 inches is a HUGE altimter setting change. Are you sure about
that?



I have to agree with Roy here. I just finished coming back from Ohio to
Mississippi, and I experienced a
very large altimeter setting adjustments in my journey southbound north
of
Bowling Green KY, but I sure did not experience .50 inches adjustments en
route. I did end up .25 inch less over the whole trip though.

If I remember correctly on one ATC hand off, I went from 29.96 to 29.88
which lowered my altitude by 80 feet. This really alarmed me as I sure
don't like deviating more then 20 feet from assigned altitude. Needless
to
say, I made a quick climb to my assigned altitude!

There was a rather steep pressure gradient even in my trip I just did
this
week, but as Roy says, .5 inch altimeter change in a flight range
distance
in planes I fly is probably indicative of weather I wouldn't / shouldn't
be
flying in anyway.

Allen



 




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