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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 |
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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 Remove .spam.sux to reply. |
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Questions on high altitude 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. |
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Questions on high altitude pressures
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. |
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Questions on high altitude pressures
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 |
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Questions on high altitude pressures
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. :-) |
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Questions on high altitude pressures
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) |
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Questions on high altitude pressures
"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 |
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Questions on high altitude pressures
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. |
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