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ATC Altimeter Settings



 
 
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  #1  
Old April 2nd 05, 01:35 AM
Bob Gardner
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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



  #2  
Old April 2nd 05, 01:54 AM
O. Sami Saydjari
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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




  #3  
Old April 2nd 05, 02:15 AM
Roy Smith
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"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.
  #4  
Old April 4th 05, 06:52 PM
Ross Richardson
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Roy Smith wrote:

"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.


And you do not think the controller is getting his information as
reported by that station's ATIS / AWOS/ ETC?

Ross

  #5  
Old April 4th 05, 06:58 PM
Steven P. McNicoll
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"Roy Smith" wrote in message
...

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.


Which could also be up to an hour old.


  #6  
Old April 2nd 05, 02:10 AM
Journeyman
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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?)
  #7  
Old April 3rd 05, 01:21 AM
Frank Ch. Eigler
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"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
  #8  
Old April 3rd 05, 06:00 AM
Icebound
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"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.


 




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