AIM 7-2-3 says your altimeter can be in error by 74 feet without a trip to
the shop, so at least that much slop is built into the system. The ATC
separates IFR from IFR by miles, not feet (OK, feet vertically, but opposite
direction traffic is separated by at least 1000 feet vertically).

If my preflight weather analysis showed a pressure difference of .5 inches
from one area to another but within my range, I would seriously consider
going somewhere else.

One-tenth of an inch is 100 feet...I would not get ulcers over it.

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