Reply #96: I'll give it my best shot.

[trof]

This may be TMI (Too Much Information).

Also, with the advent of radio and radar altimeters, not quite a crucial as it used to be.

Some organization, DOD, AAC/AAF, FAA, Bureau of Standards?...whatever...a long time ago established a "standard day" as having a barometric pressure of 29.92 inches of mercury. That was used as a standard to establish Mean Sea Level (MSL) at zero feet. Barometric altimeters measure feet above (or below...Death Valley) MSL, and for a long time barometric altimeters were the only thing we had.

However..."standard days" are few and far between. Barometric pressure (as measured at the surface) usually varies daily and even hourly. The pressure (baro) altimeter has a little knob and window so you can change the setting to coincide with the surface pressure. Most altimeters were manufactured by the Kollsman company, so we call this the "Kollsman window".

OK, let's say that the runway elevation at destination airport is 600' above MSL. On a standard day the Kollsman window setting on the ground would be 29.92. But it's not a standard day and a guy at the airport has measured the pressure at 30.15. When you begin your descent you reset your Kollsman window to 30.15 so that you will match the actual pressure and accurately reflect your actual altitude above the field. When you land, your altimeter should read 600', NOT zero. All altitudes on instrument approaches are (were? I've been gone a while.) given in height above MSL. If you have a 200' minimum approach altitude going into a 600' MSL airport, it will be shown as 800'. If you are taking off, you also set the local pressure so you'll have an accurate readout in the climb.

If I remember correctly you reset your altimeter at 18,000 feet. So, below 18,000 you reflect local pressure, but ABOVE 18,000 you want all aircraft to use the SAME setting for traffic separation to avoid collisions. 29.92 above FL180, local setting below.

That's about the best explanation I can give off the top of my head.
Was this just a test of an aging aviator's gray matter and you already knew the answer?
;-)