From: elmore@virga.ucar.edu (Kim Elmore)
Subject: Re: Temperature Related Altimeter Errors
Organization: Research Applications Program/NCAR, Boulder, CO
Date: Tue, 28 Jul 1992 15:58:42 GMT

In article <yvmmzy#.rkaplan@netcom.com> rkaplan@netcom.com (Richard Kaplan) writes:

     Concerning altimeter settings there seems to be some confusion
about just how it's done in that some feel that temperature affects
the altimeter setting.  For example:

>Except for temperature effects.

	Altimeter settings that yield field elevation are *strictly* a
function of pressure.  The accuracy of the pressure-to-altitude
conversion that is performed goes to pot if the density profile (which
is a function of the temperature profile) is different from what the
altimeter was designed for.

>>I thought that all minimum altitudes specified by the FAA for IFR use
>>(MDA, MOCA, etc.) included adequate safety margins even when pressure
>>altitude is lower than tapeline altitude on very cold days.  Anybody
>>know for sure?
>
>Take a look at the IFR article I quoted from this month.
>They give an example where there is DANGEROUSLY little
>margin.
>
>Even if there were reasonable margin, what do we gain by
>not adjusting for temperature effects?
>
>What better way can there be to define an altimeter
>setting than operationally, i.e. whatever it takes to get
>the altimeter to read field elevation?

	It will, regardless of the temperature, but once off of the
ground (on an approach, for example) it will indicate that you are
highre than you actually are.

	Here's a hypothetical graph:

	|            /<---- standard atmosphere (calibration built into
	|           /            *                          altimeters)
   P	|          /          *
   R   	|         /        * <---- colder-than-standard atmosphere (what
L  E |	|        /      *             an altimeter would have to use to
O  S |	|       /    *                properly indicate altitude)
G  S |	|      /  *
   U v	|     /*
   R	|   */^
   E	|*  / |
	|  /  |---- field elevation
	| /
	|__________________________
                ALTITUDE --->


	Note that the two curves intersect at the field elevation.
The *slope* of the curves is a function of the temperature (and so
density) distribution of the atmosphere (lapse rate); they will be
straight lines only of the lapse rate is constant.  However, *where*
they intersect is a function of the "altimeter setting".  The "/" line
is essentially moved back and forth horizontally by adjusting the
Kollsman window, *but its slope cannot be adjusted*.  So, we adjust
the horizontal position of the standard atmosphere curve (the / line)
such that it intersects the actual atmosphere pressure-altitude curve
(the * line) at the field elevation.

	Heck, I probably should have drawn a picture two posts ago.
Sorry!


					Kim Elmore
					N5OP, PP ASEL/Glider 2232456
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