"Robert Lyons" wrote in message
...
- Why would people be using two different altimeter settings anyway?

Well, the situation at hand is that one airplane is flying at or above
"flight level 180" while the other is flying below "flight level 180". By
definition, altitudes "in the flight levels" are referenced to standard
atmospheric pressure (29.92) even if non-standard conditions prevail. So
anyone flying at or above FL180 has their altimeter set to 29.92. However,
for all flying below the flight levels, the altimeter will generally be set
to the prevailing conditions (sea-level corrected, of course).

This is, in fact, the whole point with respect to the question at hand.

The reason for using a single altimeter setting for all airplanes at or
above FL180 is that most aircraft flying at that altitude travel so quickly
that it would be impractical for them to have to keep changing their
altimeter setting as they fly through different weather systems. They are
at a high enough altitude that the error between indicated and true altitude
won't cause any problem with terrain (at least in the US), which is the main
reason for using a local altimeter setting in the first place.

- Why would this problem affect just FL180 and FL200? Why not FL160, 220,
and every other altitude?

It affects all flight above FL180. There is no "FL160" in the US, and I'm
not sure why you got the impression that flights at FL220 are unaffected.

- Portions of other posts make it sound like there's something special
about calibrating ones' altimeter to 29.92, as if that's commonly done
in certain situations. Is there something different about that specific
number? or am I misinterpreting other posts?

29.92" is the pressure at sea level in standard atmospheric conditions. It
just happens to be a nice "middle-ground" number. Nothing special other
than that.

By the way, we typically reserve the word "calibrate" for an infrequent
operation in which the accuracy of the altimeter is ensured. Setting the
altimeter setting to a specific number in flight is generally referred to as
"setting the altimeter setting".

Pete