ATC Altimeter Settings

"Sriram Narayan" wrote:
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.

Are you saying that the mode-c altitude your transponder was reporting
differed by 100-150 ft from your indicated altitude? If that's the case,
your indicated altitude is considered the correct one.

Mitty wrote:


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.

Yes, everybody's mode C reports a 29.92 altitude that is corrected by ATC.

According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. Increase the distance to 1000
miles and the windspeed will be 40 mph after three hours. Don't know about
you, but 80 mph surface winds are beyond my meager skills. I think your .5
inch figure is overstated.

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

OK, I am convinced that 0.5 inches was overstated.

So are we are concluding that it is best to switch to ATC altimeter
setting as soon as they give them to you, and that differences from
local airport settings, except when you are on approach to your
destination (where you use that airport's setting).

Still, I am curious to hear from a controller, how they pick their
altimeter settings for their region of control. Is it always the same
airport? Is the fix somewhere in the center of their region.

Just curious.

-Sami


Bob Gardner wrote:

According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. Increase the distance to 1000
miles and the windspeed will be 40 mph after three hours. Don't know about
you, but 80 mph surface winds are beyond my meager skills. I think your .5
inch figure is overstated.

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

In article , Bob Gardner wrote:
According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. Increase the distance to 1000
miles and the windspeed will be 40 mph after three hours. Don't know about
you, but 80 mph surface winds are beyond my meager skills. I think your .5
inch figure is overstated.

I had a 0.05 difference about 30 miles apart last weekend and thought that
was unusually high.

Morris (what's an order of magnitude among friends?)

"O. Sami Saydjari" wrote:
So are we are concluding that it is best to switch to ATC altimeter
setting as soon as they give them to you

That's what I'm concluding. You?

and that differences from local airport settings, except when you are on
approach to your destination (where you use that airport's setting).

I'll always favor an altimeter setting ATC gives me over anything I hear on
the ATIS. The ATIS could be up to an hour old; the controller is looking
at the most current reading right now.

"Bob Gardner" writes:

According to "The Weather Book," by USA Today weather editor Jack Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. [...]

Does that refer to surface or upper winds?

[...] I think your .5 inch figure is overstated.

Well, today it's not hard to find two places on the continent with a
0.50 difference in altimeter settings: any place under the big storm
in the north east, and another place far enough, like KORD. That's
only a couple hours' flight in our bugsmashers.


- FChE

"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

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

"Bob Gardner" writes:

According to "The Weather Book," by USA Today weather editor Jack
Williams,
a one-half-pound pressure difference between places 500 miles apart will
accelerate still air to 80 mph in three hours. [...]

Does that refer to surface or upper winds?

[...] I think your .5 inch figure is overstated.

Well, today it's not hard to find two places on the continent with a
0.50 difference in altimeter settings: any place under the big storm
in the north east, and another place far enough, like KORD. That's
only a couple hours' flight in our bugsmashers.



Yes, Detroit to JFK would be just about .5 this evening.

That translates to 500 feet down-error, even if you ignore the "fly toward
the low, look out below" mantra and never adjust the altimeter. Allowing
200 feet altitude-maintenance error, you will still have 300 feet buffer
from the adjacent flight level.

Unless, of course, someone is flying in the opposite direction ALSO ignoring
the mantra, in which case his error will be UP, while yours is DOWN, and
interesting things may result.

So know your weather situation before takeoff, and get QNH updates
frequently along the route in such extreme situations.

"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.