On Nov 24, 5:40*pm, 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?

Heres an equation which gives the official ISA atmsopheric pressure
(mbar) as a function of altitude in km. you can put this equation in
excel and plot it yourself. When you plot it you will see the pressure
fall off pretty much linearly until about 15000 ft then it curves
upwards.

Press (mbar) = (101325*(1-6.5*altitude/288.15)^(9.80665*28.9644/
(8.31432*6.5)))/100

at 12000 ft the rule of thumb of 30 mb per 1000 ft gives you 653 mb
while the official ISA value from the above equation is 644 mb which
is pretty damn close. But then when you get to say 30000 ft there is
a big error in using the rule of thumb ( 113 mb vs 301 ISA)

Heres the reference if you want to look it up.
http://wahiduddin.net/calc/density_altitude.htm

Terry
PPL Downunder
sorry about the units. where metricated downunder.