wrote in message
oups.com...
Well, I removed the innards and (independently) reset the reference
pointers to agree with the altitude hands at the correct Kollsman
setting. (I'm pretty handy at tinkering with delicate instruments.)

But I still don't really know what those little markers are for. If, as
you have suggested, they are used to set the altimeter to the correct
barometric pressure when the Kollsman window is blanked out, how does
one know where to set the markers? Let's say one is in an area where
the altitude-barometer combination there renders the Kollsman window
useless. If one is given the barometric pressure there, one would then
need a table or have to make a calculation to get the correct setting
for the reference markers. Does this sound correct?



Having read and re-read that article several times....

My latest take is that the correct position for them is
zero zero, when the Kollsman window set at 29.92.

From the article, they would appear to do nothing more than show the
estimated "difference" (not the "pressure altitude" as I first surmised, but
just the "difference").... the estimated difference between the *pressure*
altitude of the airport in standard-atmosphere conditions, compared to the
pressure altitude of the airport in the actual conditions of today. By
setting them to show that *difference*, your altitude readout will now show
the field elevation when you are on that field.

Left of zero would be negative (pressure altitude is less than standard,
i.e. actual pressure is higher than standard. Right of zero would be
positive, air pressure is lower than standard. ) The article mentions to
take care when setting negatives.... 900 would actually mean minus 100....


The only use of these markers would be if the station reported altimeter
setting in terms of this "pressure altitude variation", as depicted in the
diagram in the article. ( This seems to be something from a bygone era).
You would set the appropriate difference on the markers ( in the
appropriate plus or minus direction) and your altimeter would now be
adjusted correctly.....

Since pretty much everybody today reports the Kollsman value directly in
"inches of mercury" (or Millibars in Europe), you do that directly with the
Kollsman value, so the markers don't matter... you don't really care about
the "difference" any more.


So the only use today, would be... after setting the Kollsman window, you
could check a table .... or make a mental estimate.... of what the
difference should be ( about 100 feet per .10 inches of mercury away from
29.92).... Also, of course, that it is on the correct side of zero (left
side (negative) if greater than 29.92, right side if less than 29.92)....
and see that the markers correspond, as a double check.



After moving the markers to zero zero at 29.92, then setting your correct
altimeter setting, I would expect:

If, like 29.82, the indicators should show about plus 100, 29.72: plus200,
etc.
If, like 30.02: minus 100, 30.12: minus 200 , etc.