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

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.

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.

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?

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

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

"O. Sami Saydjari" wrote in message
...
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?


Center ATC is divided into Sectors. Each Sector has a series of altimeters
that they monitor. My Center Airspace has three low altitude sectors and 4
high/ultra high sectors that I work on a daily basis. The three low sectors
lie north of Atlanta, between Atlanta GA (ATL), Chattanooga TN (CHA),
Nashville TN (BNA),Knoxville TN (TYS) Asheville NC (AVL) and back to
Atlanta. Each of these low sectors has a set series of altimeter stations
that by facility SOP (standard operating procedure) the sector must monitor.
These stations have been determined by FAA management to be germain to the
safe operation of the sector.

Two of the sectors stack on top of each other, covering North Georgia from
the ground to FL230. These two sectors must monitor (by SOP) the ATL, CHA
and TYS altimeter settings. The other sector lies to the north in
Tennessee. This sector must monitor CHA, CSV (Crossville TN) and TYS
altimeter settings. When the three sectors are combined (as they often
are), then the combined sector must monitor ATL, CHA, CSV and TYS.

The way we monitor the altimeters is that we have them displayed in an
electronic box on the scope. As the weather updates at whatever station we
are monitoring, the altimeters update automatically. If one or more of the
local altimeters that belong to a sector dips below 29.92, then we are no
longer able to use FL180 for IFR separation. In ATC parlance, we say
"Eighteen is broken."

We are required to issue a local altimeter at least once to you as you
transit the sector. I try to use a little common sense on which one I
issue, but in the enroute environment, you are not always going to be flying
directly between or along altimeter stations that I am monitoring. An
example would be a flight from Chattanooga TN to Charelston SC. You have
departed CHA. You are exactly between TYS and ATL. When you check in with
ARTCC, which altimeter do I issue? CHA (which you just left) or one of the
other two which are not on your route? Many of us avoid these awkward
situations by monitoring additional stations. For example, if you depart
TYS heading up towards Cinncinnatti or points north, I will issue you the
London KY altimeter (LOZ) because I think it is stupid to issue you the TYS
altimeter when TYS is 30 miles behind you and LOZ is coming up. So I
monitor LOZ as well in my electronic list, just because...

Chip, ZTL

"Chip Jones" wrote in message =
link.net...
=20
"O. Sami Saydjari" wrote in message
...
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?

=20
Center ATC is divided into Sectors. Each Sector has a series of =
altimeters
that they monitor. My Center Airspace has three low altitude sectors =
and 4
high/ultra high sectors that I work on a daily basis. The three low =
sectors
lie north of Atlanta, between Atlanta GA (ATL), Chattanooga TN (CHA),
Nashville TN (BNA),Knoxville TN (TYS) Asheville NC (AVL) and back to
Atlanta. Each of these low sectors has a set series of altimeter =
stations
that by facility SOP (standard operating procedure) the sector must =
monitor.
These stations have been determined by FAA management to be germain to =
the
safe operation of the sector.
=20
Two of the sectors stack on top of each other, covering North Georgia =
from
the ground to FL230. These two sectors must monitor (by SOP) the ATL, =
CHA
and TYS altimeter settings. The other sector lies to the north in
Tennessee. This sector must monitor CHA, CSV (Crossville TN) and TYS
altimeter settings. When the three sectors are combined (as they =
often
are), then the combined sector must monitor ATL, CHA, CSV and TYS.
=20
The way we monitor the altimeters is that we have them displayed in an
electronic box on the scope. As the weather updates at whatever =
station we
are monitoring, the altimeters update automatically. If one or more =
of the
local altimeters that belong to a sector dips below 29.92, then we are =
no
longer able to use FL180 for IFR separation. In ATC parlance, we say
"Eighteen is broken."
=20
We are required to issue a local altimeter at least once to you as you
transit the sector. I try to use a little common sense on which one I
issue, but in the enroute environment, you are not always going to be =
flying
directly between or along altimeter stations that I am monitoring. An
example would be a flight from Chattanooga TN to Charelston SC. You =
have
departed CHA. You are exactly between TYS and ATL. When you check in =
with
ARTCC, which altimeter do I issue? CHA (which you just left) or one =
of the
other two which are not on your route? Many of us avoid these awkward
situations by monitoring additional stations. For example, if you =
depart
TYS heading up towards Cinncinnatti or points north, I will issue you =
the
London KY altimeter (LOZ) because I think it is stupid to issue you =
the TYS
altimeter when TYS is 30 miles behind you and LOZ is coming up. So I
monitor LOZ as well in my electronic list, just because...
=20
Chip, ZTL
=20

Chip, you didn't mention enroute altimeter settings being different from =
surface ones.
I've seen that condition in the Rocky Mountains, but I can't =
specifically recall
if I've ever seen it in the central and eastern U.S.
In the west, lapse rates can be such that an altimeter setting for an =
airport
can be noticeably different from the enroute value many thousands of =
feet higher.

The controller may have good reason to issue area altimeter settings =
which are
different from those reported at nearby airports.

John R. Copeland wrote:



Chip, you didn't mention enroute altimeter settings being different from surface ones.
I've seen that condition in the Rocky Mountains, but I can't specifically recall
if I've ever seen it in the central and eastern U.S.

Controllers issue altimeter settings from ground stations. Where else
would they come from?

In article ,
Newps wrote:

John R. Copeland wrote:



Chip, you didn't mention enroute altimeter settings being different from
surface ones.
I've seen that condition in the Rocky Mountains, but I can't specifically
recall
if I've ever seen it in the central and eastern U.S.

Controllers issue altimeter settings from ground stations. Where else
would they come from?

You get them off DUATS like the rest of us?