Questions on high altitude pressures

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

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

Remove .spam.sux to reply.

In article ,
wrote:

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.

Now make it even more interesting and convert the two main component of
the air we breath to their partial pressures.

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.

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.

Thanks for the answers.

es330td wrote
Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and
another that is right at actual altitude.

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.

Bob Moore

On Nov 24, 8:38*am, Robert Moore wrote:
es330td wrote

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.


Thanks for the reminder. I hope someday to have to care about that. :-)

es330td wrote

On Nov 24, 8:38*am, Robert Moore wrote:
es330td wrote

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.


Thanks for the reminder. I hope someday to have to care about that. :-)

Just remember.... on the way up, it's Transition Altitude, on the way
down, it's Transition Level. :-)

Bob Moore
ATP CFI
PanAm(retired)

"es330td" wrote in message
...
On Nov 24, 8:38 am, Robert Moore wrote:
es330td wrote

Just an extra point....above the transition altitude/level,
we fly a Flight Level, not an Altitude.


Thanks for the reminder. I hope someday to have to care about that. :-)

Just an extra, extra point.....there is no FL 120

Frank

es330td writes:

This brings up an interesting wrinkle then as GPS altitude info is not
dependent on external pressure so pilots must be careful to ignore
that info if available. As stated before, in Class A everyone sets
their altimeter to 29.92 so that as long as everyone is wrong together
everything is okay. Adding GPS info into the mix splits the groups
into two; one that is wrong together at 29.92 and another that is
right at actual altitude.

GPS is too inaccurate for most purposes in vertical positioning, anyway. It
is not designed to determine altitude with a high degree of accuracy, and can
easily be hundreds of feet off.