Questions on high altitude pressures

es330td wrote:
At low altitudes the rule of thumb is that a one thousand foot
increase in altitude results in a one inch of mercury decrease in
pressure; at least that is what happens when I turn the little knob on
my altimeter. Since commercial planes flight well into the FL300+
range, clearly that rule cannot hold at altitude as a plane taking off
at 29.92 inches would find itself in a vacuum at 29,900 feet above sea
level. Can someone with experience of high altitude flight expand on
my understanding of pressure variation?

Also, since the altimeter in the C182 I fly appears to incorporate
that rule of thumb, is it accurate at say, FL120, or is another kind
of altimeter needed for planes flying up there?

TIA

The relation between altitude and pressure is actually logrithmic, but
up to about 10,000 feet or so the rule of thumb is pretty close.

For a chart of altitude versus pressure up to 100,000 feet, see:

http://www.engineeringtoolbox.com/ai...ure-d_462.html

Everybodies altimeter is the same and the nonlinear nature of the
atmosphere is one of the reasons everyone sets the altimeter to
29.92 in the flight levels.


--
Jim Pennino

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