Pressure above 31.00 in/Hg

[a-v-8er]

From the AIM 7-2-4:

"Cold, dry airmasses may produce barometric pressures in excess of 31.00 inches of Mercury, and many altimeters do not have an accurate means of being adjusted for settings of these levels..."

If I'm not mistaken the Kollsman only goes up to 31.00 in/Hg on most altimeters. Is this the reason why flight ops are not recommended or specific ATC instructions to remain on a setting of 31.00 are issued even though the pressure might be higher.

Same goes for abnormally low pressures (28.00 in/Hg and lower).

If flying with higher pressure you'll be higher than indicated by the altimeter if you're only able to have it set to 31.00 and vice versa for lower pressures than 28.00 when the altimeter stops there. But my question is why do some altimeters not have an accurate means of adjusting for these levels?

Is it as simple as a altimeter (Kollsman window) design or more complicated mechanics of how the altimeter behaves under these unusually high/low pressures?

You obviously fly every day in pressures lower than 28.00 in/Hg, however, at an altitude... so why do we have this limitation (or recommendation of not to fly under these circumstances)?

Sorry for rambling on too long here...

 

[Vladimir Lenin]

I think it may have to do with the "cold, dry" air (as per AIM). In colder weather, pressure levels are thinner, and altimeter indicated altitude may be higher than actual altitude, causing you obvious problems.

Just a thought, flame away

 

[80/20]

Many airlines and commercial operators solve this by using correction tables that are part of their manuals.
These tables are used in the following way:
1) Set QNH to max or min setting available on your altimeter
2) Use actual altimeter setting to find altimeter correction in feet
3) Subtract or add corrections to get aircraft altitude or corrections required to maintain altitude assigned by ATC.