"Chip Jones" wrote in message =
link.net...
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"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?

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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.
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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.
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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."
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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...
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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.