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